Plant diseases and treatments

HostsAmong perennials, garden phlox and bee balm first, then aster, delphinium, lungwort, rudbeckia and coneflower. Among woody plants, common lilac is the classic host in Quebec (Japanese tree lilac and Peking lilac are much less affected, which is worth knowing before you buy), along with purple-leaved ninebark, hydrangea, mock orange, elderberry, viburnum, roses, crabapple and serviceberry. It isn't the same fungus everywhere: Podosphaera species attack the rose family — P. pannosa on roses, P. leucotricha on crabapple — and P. physocarpi on ninebark, while lilac falls to Erysiphe syringae.

Not to be confused withDowny mildew, which instead produces a purplish-grey down under the leaf and angular spots bounded by the veins, in cool, wet weather. Astilbe foliage scorched at the edges isn't powdery mildew either: that's a lack of water.

PreventionSpace plants generously at planting, divide phlox and bee balm clumps every three or four years, and thin out the centre of shrubs so the air can pass through. Water at the base, in the morning, never on the foliage, and go easy on nitrogen, which produces soft, very susceptible foliage. Choose cultivars known to be less susceptible, cut back the foliage in the fall and put it in the garbage, and on roses remember that the fungus overwinters in the buds and canes: spring pruning matters more there than raking.

TreatmentSulphur is the only product available to home gardeners that still acts on powdery mildew once it is established, because it works through its vapours — which requires heat: spray on a day that will reach 20 to 28 °C, never below 15 °C, where the application is wasted, never above 30 °C, where the foliage burns, and leave 30 days between sulphur and a horticultural oil, in either order. Never use it on viburnum, which labels exclude by name and whose foliage burns, and always test it on three or four leaves 48 hours ahead before treating an entire clump. A leaf that has already turned yellow won't turn green again: what you are saving is the foliage that is still healthy. Before you buy, read the label — your plant and powdery mildew must be named on it, and the label alone governs — and check your municipality's bylaw, as several cities go further than the province.

HostsAmong perennials, rudbeckia and coneflower are the most affected, along with phlox, bee balm and chrysanthemum. Among woody plants, dogwood, hydrangea and lilac, and among trees birch and poplar, which lose their leaves early in late summer. The species changes with the host: Septoria cornicola on dogwood, Cercospora hydrangeae on hydrangea.

Not to be confused withFoliar nematode, whose brown spots form bands and wedges sharply bounded by the veins, never circles. Simple scorching of the leaf margins from a lack of water, which starts around the edge of the leaf rather than as isolated spots.

PreventionIt all comes down to the fall cleanup: cut back and remove the foliage — it goes neither into the compost nor into mulch, because the fungus overwinters in it. Fresh mulch in spring keeps soil from splashing up onto the lower leaves, and watering at the base in the morning, without wetting the foliage, cuts out a good share of infections. Space and divide plants so the foliage dries quickly after rain.

TreatmentYou have to be clear about timing: a contact product only protects leaves that are still healthy, so the cue isn't the first spot but the return of rain on young foliage, around mid-June — if you discover the spots in August, nothing will change the current year. On a bed affected last year, copper octanoate can be justified at that point, but limit yourself to two or three applications, never as a yearly routine: copper builds up in the soil and harms earthworms, and it easily marks young foliage in cool, wet weather, which makes a test on a few leaves worthwhile. Frankly, for a disease that doesn't kill, picking up the foliage in the fall gives better results than a spray program. Read the label before you buy — your plant and your disease must be named on it — and check your municipality's bylaw.

HostsHollyhock above all — it's the perennial that rusts most consistently in Quebec — then bee balm, bellflower, aster and rudbeckia. Among woody plants, roses, especially old plants in sheltered spots. Each host has its own: Puccinia malvacearum on hollyhock, P. menthae on bee balm, Coleosporium campanulae on bellflower, Phragmidium species on roses.

Not to be confused withGymnosporangium rusts, which make orange gelatinous galls on juniper and orange spots on crabapple, are a completely different disease with an alternating cycle. The bright orange rust sometimes seen on a daylily bought the same year isn't this one either: that fungus doesn't survive our winters; it arrives on plants grown in greenhouses down south and disappears after the first hard frost.

PreventionSpace, divide, water at the base in the morning, and never let the foliage stay wet overnight. Cut back and remove all the foliage in the fall, into the garbage and not the compost. For bellflower, be aware that its rust completes its cycle on pine: don't plant it at the foot of a pine. And don't count on the advertised resistance of modern bee balms, which was bred against powdery mildew and guarantees nothing against rust.

TreatmentOn a plant that rusted last year, a sulphur program has to start when the leaves emerge in spring, not in July: started once the pustules are visible, it catches nothing up and only protects the healthy foliage higher up. Be honest with yourself about hollyhock — no product sold to home gardeners keeps it clean all season; strip the lower leaves in July and treat the plant as a biennial to be replaced. Sulphur needs 20 to 28 °C to work, burns above 30 °C and requires a test on a few leaves first, and both your plant and the disease must appear on the label, which alone governs; also check your municipality's bylaw.

HostsBearded iris (Iris germanica) is by far the most affected, to the point that it's almost a rite of passage on an old clump. Siberian iris and blue flag iris catch it too, but much more lightly, and almost never warrant any attention. Older books call it Mycosphaerella macrospora, Didymellina macrospora or Heterosporium gracile: it's the same fungus.

Not to be confused withThe brown tips of late season, normal on an iris, which show no ringed oval spots. The water-soaked streaks and holes of the iris borer, which point instead to a soft rot of the rhizome.

PreventionCut back and discard all the foliage in the fall, and rake up the old leaves and debris between the rhizomes: the fungus overwinters there and nowhere else. Keep the tops of the rhizomes exposed and in full sun, with no mulch and no perennials flopping over them, and divide the clump every three or four years so the air can circulate.

TreatmentThe only treatment that really works is cleanup: cut back and remove the foliage in the fall, and remove the worst-affected leaves during the season. Copper octanoate can legally be used if leaf spots of ornamental plants appear on its label, but I know of no efficacy data on this particular fungus — the successful trials involved products that are no longer permitted here: think of it as an uncertain extra, not as the treatment. If you insist, start when the leaves are 15 to 20 cm tall in spring, before any spots, because once the spots are out it won't win anything back; and check the label, which alone governs, as well as your municipality's bylaw.

HostsPeony above all, both herbaceous and tree peonies. Also lily, columbine, lupine, delphinium and coral bells, and on roses with very double flowers, buds that rot without opening. On peony the agent is B. paeoniae, on lily B. elliptica; everywhere else it's B. cinerea, which isn't picky.

Not to be confused withA late frost, which blackens the shoots all at the same time and across the whole clump, whereas botrytis takes one here, one there, days apart. Peony red spot, which by contrast strikes after flowering and doesn't make the shoots collapse.

PreventionCut peony stems back to ground level in the fall and take them out of the garden, into the garbage: the fungus overwinters in old stems and on the surface of the crown. Don't put mulch against the stems, stake early so the clump doesn't flop to the ground after rain, and remove spent flowers as you go. A peony planted too deep and shaded in the morning stays wet too long every day.

TreatmentOn a collapsed shoot, cut at ground level — the lesion is already at the base; don't dig into the crown and don't cut below soil level, you would only open the door to rots; on a single browned bud, cut the stem about ten centimetres lower, disinfect the pruners between each cut and take the debris out of the garden. Expect almost nothing from spraying: copper is weak against botrytis and washes off with every rain, exactly when the disease strikes, and there is no home-garden product I can say really targets this fungus in Quebec. The real gain is in the fall cleanup, staking and cutting affected shoots right away.

HostsHerbaceous peonies, especially old cultivars and big clumps that have been in place for a long time. Tree peonies are affected more rarely.

Not to be confused withBotrytis blight, which is exactly the opposite on the calendar: botrytis strikes before flowering and makes the shoots collapse, while red spot arrives afterwards and leaves the stems standing. A simple reddish fall colouring, which is uniform rather than in distinct spots.

PreventionCut back and remove all the foliage in the fall, into the garbage rather than the compost, because the fungus overwinters in old stems and debris on the surface of the crown. Space the clumps, stake them so the foliage doesn't drag on the ground, and water at the base.

TreatmentThere is no treatment to apply, and that's just fine: no product sold to home gardeners has been shown to work against this fungus, and the plant doesn't really suffer from it. A fall cleanup done seriously two or three years in a row visibly lowers the pressure. If an old clump is particularly marked every year, divide it and replant the healthiest divisions in a more open, sunnier spot.

HostsHosta first, where the disease is well known as petiole rot. Also delphinium, phlox, coneflower, iris and columbine, and in fact a long list of other perennials. Agroathelia delphinii was long called Sclerotium delphinii: that's the name you'll find it under in many books.

Not to be confused withBotrytis blight, which isn't fatal and leaves no round grains. Simple water stress or a late frost, which don't attack the base of the petioles. The useful reflex: scratch at the base and look for sclerotia.

PreventionKeep the crown clear: no packed mulch against it, no soil added on top, no watering that leaves the base soaked. Plant at the right level, divide big clumps, and never put a hosta in a hollow where water collects. When buying, look at the base of the petioles of a potted plant before bringing it home.

TreatmentNo product sold to home gardeners reaches this fungus: this is a case where you have to dig the plant out. Remove it with its root ball and a good ten centimetres of soil around it, and put everything in the garbage along with the mulch from that area, which also contains sclerotia — don't rake it toward neighbouring clumps and don't put it back in spring. Sclerotia survive three to five years in the soil and this fungus attacks many species: no plant is guaranteed in that spot, so keep it clear and dry and watch what you put back there.

HostsMostly bearded irises, whose rhizome sits at the soil surface. Siberian and blue flag irises, with fibrous roots, are affected much less often. The bacterium is Pectobacterium carotovorum subsp. carotovorum, which older references call Erwinia carotovora; Dickeya species sometimes accompany it.

Not to be confused withIris leaf spot, which spots and browns the leaves but leaves the rhizome firm and has no smell. Simple rotting of old leaves at ground level, which rakes away and leaves a hard rhizome.

PreventionBearded iris wants its rhizome half exposed, in full sun, in soil that drains quickly: no mulch, no soil, no perennials flopping over it, and no high-nitrogen fertilizer. Divide in July or August in dry weather rather than in spring, and do a complete cleanup of old leaves in the fall. Watch for the iris borer, which opens the door to the bacterium: no insecticide sold to homeowners reaches the larva, which is inside the leaf, so in May and June squeeze the leaves that show water-soaked streaks and holes between two fingers, and remove them.

TreatmentNo product, copper included, has any effect here: the bacterium is in the rhizome and a spray on the leaves never reaches it — the treatment is the knife, the sun and drainage. Dig up the rhizome, trim it with a clean knife back to firm white flesh, disinfect the blade between each cut, then let the wound dry in the sun for a few hours before replanting elsewhere, in dry, well-drained soil, and not in the same hole. The soft flesh you removed and the old leaves go in the garbage, never in the compost, where the bacterium survives very well.

HostsHostas only. Some cultivars, including 'Sum and Substance', 'Gold Standard' and 'Striptease', come up often in reported cases.

Not to be confused withA cultivar's normal variegation, which is regular, crisp and identical on every leaf, whereas the virus gives an irregular pattern that bleeds along the veins and differs from one leaf to the next. A change in variegation linked to age or light, which doesn't pucker the leaf blade.

PreventionThe virus is transmitted only through sap, on tools, during division or when a leaf is pulled off. Disinfect the knife with bleach diluted to 10%, one minute of contact, and not with rubbing alcohol, which isn't reliable against this type of virus — rinse and oil the blade afterwards, as bleach attacks steel. Examine every new hosta before adding it to a collection, and be wary of plants swapped between gardeners, which are the main route of spread.

TreatmentThere is nothing to spray: a virus can't be cured. Dig up the plant with all its roots, search the hole well so as not to leave a piece that would regrow, and put everything in the garbage, never in the compost. Since the virus doesn't live in the soil and has no vector, you can replant a hosta in the same spot as soon as the plant and its roots are out; the only real risk left is the tool.

HostsPurple coneflower at the top of the list, then aster, rudbeckia, coreopsis, blanket flower and chrysanthemum. Carrots and lettuce in the vegetable garden are affected by the same organism.

Not to be confused withEchinacea purpurea 'Doppelgänger', also sold as 'Doubledecker', which naturally produces a second rosette of petals above the cone: in that plant, these are real, well-formed petals and every plant does the same, whereas aster yellows produces greenish leafy shoots and only affects a plant here and there. A simple stress or deficiency, which yellows the foliage without deforming the flowers.

PreventionInspect the bed once a week in summer and pull out the first plant that goes wrong without waiting: as long as it stays in place, the leafhoppers feeding on it leave with the phytoplasma and carry it to the others. Weeds along the edges do serve as a reservoir, but in a home garden the carrier leafhoppers mostly fly in from very far away: weeding three metres of border won't protect you much, and it certainly isn't worth a herbicide.

TreatmentThere is no treatment, and it has to be said plainly: no fungicide or insecticide cures or protects a plant here. Don't count on insecticidal soap against leafhoppers either — it kills the ones it wets, has no residual effect, and transmission happens within a few minutes of feeding by insects that arrive the next day. The only step that counts is to pull out the affected plant as soon as the diagnosis is clear, roots included, and put it in the garbage rather than the compost.

HostsHosta and coral bells above all, but also astilbe, brunnera, Japanese anemone and hellebore — often one after another in the same bed.

Not to be confused withFungal leaf spots, which form circles with a halo and cross the veins. Sunscald on a badly placed hosta, which pales and whitens the centre of the leaves without following the veins, and the clean holes made by slugs.

PreventionThe nematode swims in a film of water to move from one leaf to another: water at the base, in the morning, never on the foliage, and space clumps so everything dries quickly. Clean up dead leaves in the fall, and inspect every new or gifted plant against the light before putting it in a healthy bed.

TreatmentNo product sold to home gardeners reaches this nematode, which lives inside the leaf — there's no point buying a fungicide, it isn't a fungus. The affected plant is dug out with its roots and goes in the garbage; above all, it doesn't get divided or given away, because that's exactly how the problem travels from one garden to another. If a clump means a lot to you, remove every marked leaf as you go and stop watering the foliage completely: it doesn't cure, but it slows things down.

HostsDaylilies, with big differences between cultivars: some stay clean all season next to a completely marked clump.

Not to be confused withTip burn from drought or from de-icing salt along a street, which browns the tip evenly with no yellow streak along the midrib. Daylily rust, which makes powdery orange pustules and doesn't overwinter in Quebec: you only see it on plants bought the same year.

PreventionCut back and remove all the old foliage in late fall or very early in spring: the fungus overwinters in it, and that's the step that changes the most. Water at the base without splashing, divide big clumps so the air can circulate, and disinfect the division knife, which carries the disease from one clump to another.

TreatmentThere's no product to buy: no fungicide sold to home gardeners targets this disease in Canada, and nobody can honestly recommend one to you. What works is cleanup — remove marked leaves as you go during the season, cut down and discard all the foliage in the fall, into the garbage and not the compost. If a clump is badly affected every year despite everything, replace it with a cultivar that stays clean in your garden: the difference between cultivars is the real long-term solution.

HostsThe whole Rosa genus, but with huge differences from one cultivar to another: hybrid teas and floribundas are the most affected, while several rugosa hybrids are affected very little. Beware of the shortcut: 'hardy' doesn't mean 'resistant', and two roses from the same Canadian series planted side by side can end the summer one spotless, the other completely bare. Its asexual stage is called Marssonina rosae.

Not to be confused withRose downy mildew, whose purple spots have a clean outline and start at the top of the plant; rose anthracnose, whose small spots have a grey centre that eventually falls out; a sulphur or fertilizer burn, which leaves spots without a yellow halo where the product ran; and the normal ageing of the lower leaves in late summer.

PreventionPick up and discard all fallen leaves, in the fall as well as during the season: that's where the fungus overwinters, and they shouldn't go in the compost. Water at the base, never on the foliage and never late in the day; space the plants, thin out the centre of the bush and mulch to limit spores splashing up from the soil. Avoid too much nitrogen, which produces soft foliage, and trust what you see in your own bed rather than a cultivar's reputation, because the fungus has several races and a rose considered resistant twenty-five years ago can get spotted today.

TreatmentA leaf that is already spotted won't heal, and no curative systemic fungicide is offered in a home-garden format in Quebec: the step that pays off most is still picking up every infected leaf, and replacing a cultivar that goes bare every year is a perfectly legitimate treatment. As a complement, calcium polysulphide (lime sulphur) applied after pruning, on bare wood with the plant fully dormant, between 4 and 25 °C, cleans up the lesions on the canes but does nothing against the inoculum left on the ground; it's a caustic product, so wear goggles and gloves and protect the concrete, brick, paint and aluminum around it, never mix it with a horticultural oil or apply them less than two weeks apart, and never mix it with an acidic product. During the season, sulphur protects the leaves that are still healthy, but only if you keep up the program from bud break to September — one or two sporadic sprays protect nothing — so try it first on a few leaves, avoid it above 30 °C and on open flowers, and check that your rose and this disease are named on the label, which alone governs, then that your municipality's bylaw allows it, as several cities, including Montréal and Québec City, go further than the province.

HostsClimbing roses, rambler roses and species roses are the most prone to it, which directly targets the hardy climbers planted here. It is also seen, more rarely, on hybrid teas and shrub roses. It is also found under the name Elsinoe rosarum.

Not to be confused withBlack spot, whose spots are larger, truly black, with a fringed margin and ringed by a yellow halo, and which don't turn into holes. Damage from sucking insects, which leaves holes without a clear coloured border.

PreventionSpace and thin out so the foliage dries quickly after rain, water at the base and pick up fallen leaves in the fall. Prune out and discard old canes carrying spots, because the fungus also overwinters on the wood.

TreatmentHonestly, there isn't much to do, and that's just fine: the plant hardly suffers from it and the disease recedes on its own as soon as the weather dries out. Cleaning up fallen leaves and affected canes is enough in the vast majority of cases. If the pressure is high two years in a row, sulphur can be used as a preventive on foliage that is still healthy, provided your rose and this disease are named on the label, which alone governs, and your municipality's bylaw allows it.

HostsAll roses, but in Quebec gardens it is rarely seen: almost always on a plant bought in a pot and already infected in the greenhouse, or on roses planted close together in a shady, confined corner. The same organism moves between roses and cultivated brambles, so avoid putting a rose bed right up against raspberries and blackberries.

Not to be confused withBlack spot, whose spots have a fringed margin and a yellow halo and progress from the bottom up; powdery mildew, whose old French name, 'mildiou poudreux', keeps the confusion going; and a cold snap or too much water, which reddens the foliage diffusely rather than in defined spots. Watch out for copper itself, too, which can mark rose foliage with very similar purple spots.

PreventionInspect the foliage of a new rose before planting it, looking especially at the underside of the upper leaves. Space the plants, thin out the centre, water at the base and in the morning, and avoid shady corners without air circulation. Pick up fallen leaves, knowing that the organism also survives in stems and debris.

TreatmentSulphur does nothing here, since its mode of action doesn't affect this group of organisms, and no curative systemic fungicide is offered in a home-garden format in Quebec. Prune out and discard the affected shoots, not just the leaves: the organism persists in the stems and that's where it starts again the following year, so a plant bought already infected that relapses every spring deserves to be replaced. Copper octanoate is the only home-garden active ingredient that acts on this group, but it is itself phytotoxic on roses in cool, wet weather, which are precisely the conditions for downy mildew: test it on a few leaves, stick to the lowest rate, limit yourself to a few applications per season because copper builds up in the soil, and check that your rose and this disease are named on the label, which alone governs, then that your municipality allows it.

HostsMostly woody plants of the rose family: roses, especially grafted ones, apple and crabapple, pear, cherry, plum, raspberry; also euonymus, poplar and willow. Infection often happens during production, through a wound, and the gall only becomes visible a year or two later in the garden. The bacterium is sometimes called Rhizobium radiobacter.

Not to be confused withThe normal swelling of a graft union, which is regular and goes all the way around the stem instead of being bumpy and haphazard. The healing callus of an old wound, smooth and the same colour as the bark. The root nodules of certain nitrogen-fixing plants, which have nothing to do with a disease.

PreventionBuy plants whose crown and roots you can examine, and refuse those with a suspicious swelling. Avoid wounding the crown, since the bacterium enters through wounds, whether from pruners, a shovel or a string trimmer. Disinfect your tools between plants and don't replant a rose or a fruit tree in exactly the same spot as a plant dug out for this reason.

TreatmentThere is no curative treatment: an established gall doesn't go away, and cutting it out with a knife only opens a new way in. On a weak young plant, the right decision is to dig it out, bag it and put it in the garbage rather than the compost, so as not to leave a reservoir of bacteria in the bed. On an established plant that is still growing properly, keep it, water and feed it reasonably and keep an eye on it; if it's a good-sized tree, have a certified arborist assess its soundness and its future before deciding anything.

HostsGrafted roses, especially those that come from elsewhere: hybrid teas, grandifloras, tree roses. Own-root roses, including most of the hardy Canadian series, are rarely involved. It isn't a single virus but a complex: mainly Prunus necrotic ringspot virus (PNRSV) and apple mosaic virus (ApMV).

Not to be confused withAn iron or magnesium deficiency, which yellows the tissue between the veins evenly and over the whole plant, not in fanciful patterns on a few leaves. Herbicide damage drifting from the lawn, which twists and stretches the leaves instead of patterning them. The normal variegation of a few cultivars.

PreventionThere's nothing to do in the garden: these viruses are not transmitted by insects, pruners or the soil, but only by grafting during production. So everything happens at purchase: examine the spring foliage of a grafted rose and, whenever possible, choose an own-root rose.

TreatmentThere is no treatment: a rose with a virus keeps it for life, and no spray or fertilizer will clean it up. The good news is that you don't have to dig it out to protect your other roses, since it won't infect them. Keep it if it blooms well, replace it if it's struggling, and never take scions from it.

HostsRoses of all types. The main reservoir in North America is the wild multiflora rose, very abundant in southern Ontario and the northeastern United States, much less present here. According to the EPPO database updated in 2024, the virus is present in Canada with reports in Ontario and Manitoba; there is no data for Quebec. The virus doesn't travel on its own: it is carried from rose to rose by a tiny mite, the eriophyid Phyllocoptes fructiphilus.

Not to be confused withDamage from an auxin-type herbicide drifting from a neighbouring lawn, which twists and stretches the foliage without multiplying the thorns or forming a witches' broom. Suckers coming from below the graft union, which have different foliage but are still normal shoots. Reddish juvenile growth, perfectly normal, which turns green in two or three weeks, whereas the virus-infected shoot stays red.

PreventionInspect the new shoots of an imported rose during its first year: that's the whole point of this entry. Pull out wild multiflora roses growing near you and space your roses, since the mite that carries the virus moves from plant to plant and is carried by the wind.

TreatmentThere is no treatment, either for home gardeners or for professionals, and pruning out the affected branch doesn't save the plant. No product sold to gardeners controls the eriophyid mite that carries the virus, which lives sheltered under the bud scales: insecticidal soap changes nothing and has no proven effect on this disease. Dig out the whole rose, roots included, because a sucker coming from a forgotten root would still be infected, bag it and put it in the garbage, disinfect your tools and watch the neighbouring roses for two seasons; the virus doesn't survive in the soil, so you can replant once the roots are completely removed, and if you think you're seeing this symptom in Quebec, photograph it and report it to MAPAQ (Quebec's agriculture ministry) or the CFIA.

HostsA rose planted in the exact spot where a rose was dug out. The same phenomenon exists in fruiting members of the rose family, apple after apple or cherry after cherry, which is why it is also called 'soil sickness'.

Not to be confused withA simple planting failure — hole too small, root ball not teased apart, crown buried too deep or drought in the first year — which should be ruled out first. Root rot in poorly drained soil, which causes outright wilting and soft black roots rather than roots that are merely stunted.

PreventionThe simplest thing is not to replant a rose where there was one: move it over a metre or two, or put something else in that spot. If you have no choice, replace the soil in the hole over about 60 cm wide and 30 to 40 cm deep with soil taken from elsewhere in the garden, carefully removing all the old roots.

TreatmentThere is no product against this problem, and its exact cause still hasn't been worked out: it's thought to be an imbalance in soil life and a build-up of small root parasites, not a disease that can be treated. An established plant that is languishing rarely recovers; better to dig it up in the fall or the following spring, redo the soil in the hole and start again with a new plant. A generous addition of compost and good mulch help it establish, without solving the problem on their own.

HostsMostly common lilac (Syringa vulgaris), and especially white-flowered cultivars. Chinese lilac (S. × chinensis) and Meyer lilac (S. meyeri) can be affected, while Japanese tree lilac (S. reticulata) and Hungarian lilac (S. josikaea) are much less susceptible. The same bacterium also attacks ornamental Prunus, mountain ash and crabapple.

Not to be confused withIt is almost always taken for simple frost damage, and the two go together: the frost opens the door, then the bacterium gets in. It has nothing to do with fire blight, which spares lilac and only attacks pome fruits of the rose family, such as crabapple, mountain ash and serviceberry.

PreventionGo light on nitrogen, and only once the frosts are over: shoots full of nitrogen are the first to burn. Don't prune late in summer, which pushes soft new growth before winter, and avoid wetting the foliage when watering. Disinfect your pruners between each plant; the bacterium travels on the blades.

TreatmentThere is no curative treatment: it all comes down to sanitation pruning, preferably done in late summer or during dormancy, in truly dry weather, because in May and June every fresh wound made in wet weather is one more way in — at that time of year, only cut what is clearly dead and threatens the framework. Cut 15 to 20 cm below the edge of the blackening, into healthy wood, disinfect the blade for a good thirty seconds between each cut and put the debris in the garbage rather than the compost. Copper-based products only reduce bacterial pressure without eliminating it, since copper-resistant Pseudomonas strains are common, and copper itself burns young foliage in cool, wet weather, which are exactly the conditions that make you want to spray: test it on one branch, wait 48 hours, stick to two or three applications per season, and only buy it if your lilac and this disease are named on the label, which alone governs — also checking your municipality's bylaw, as several cities, including Montréal and Québec City, are more restrictive than the province.

HostsServiceberry (Amelanchier canadensis, A. × grandiflora, A. alnifolia), chokeberry (Aronia melanocarpa), cotoneaster, hawthorn (Crataegus), mountain ash (Sorbus), ornamental pear and flowering quince (Chaenomeles). Its asexual form, Entomosporium mespili, gave the disease its name.

Not to be confused withIt gets confused with Gymnosporangium rust, whose spots are bright orange and larger, with small hair-like outgrowths on the underside of the leaf. Here, the spots stay small, red and smooth.

PreventionPick up and discard all fallen leaves in the fall: they are the source of the next spring's infection, and this is by far the step that gives the best results. Mulch to stop splashing, water at the base and never with a sprinkler, especially not in the evening. Plant serviceberries in the sun, well spaced, and thin out the branches in the centre so the foliage dries quickly after rain.

TreatmentThere is no curative product for home use, and picking up all the leaves in the fall remains the main lever. A preventive copper program, started at bud break and repeated every 7 to 14 days and after every driving rain during the rainy May–June period, only makes sense on a plant under 2.5 to 3 m that you can wet completely on both sides of the leaves: on a mature serviceberry or mountain ash, full coverage is impossible and spraying protects nothing. Sulphur has no efficacy data against this particular disease, and copper builds up in the soil, where it harms earthworms and aquatic life, so two to three applications per season at most, away from ditches and ponds, only if your shrub and this disease are named on the label — several home-garden labels only name roses and flowers — and after checking your municipality's bylaw.

HostsPagoda dogwood (Cornus alternifolia) is the only known host of golden canker. Flowering dogwood (C. florida), prone to anthracnose elsewhere in North America, is hardly hardy in Quebec; kousa dogwood (C. kousa), hardy to zone 5 and commonly planted here, is not very susceptible. Older publications call it Cryptodiaporthe corni.

Not to be confused withIt is often blamed on winter damage or a lack of water, but neither one turns the bark a clear yellow-orange on a single branch.

PreventionThis fungus takes advantage of stressed trees. Plant pagoda dogwood in cool part shade, the way it grows in the forest, and not in reflected full sun. Mulch, water deeply during summer droughts and protect the trunk from lawn mower and string trimmer knocks, which are ways in.

TreatmentNo home-garden product reaches this fungus, which lives in the bark: don't let anyone sell you a treatment. Spot the yellowed branches in summer, mark them with a ribbon, then cut them in late winter or very early spring in dry, cold weather, 15 to 20 cm below the lesion into healthy wood, disinfecting the blade for a good thirty seconds between each cut; put the branches in the household garbage and not in the compost, the green waste bin or a fire, since open burning requires a municipal permit in Quebec and is prohibited during SOPFEU fire restrictions. A tree supported by good mulch and regular watering lives a long time with the disease, which only becomes fatal if the canker reaches the main trunk; if that happens, or if the branches to be cut are out of your reach, call in a certified arborist.

HostsHydrangea (Hydrangea), viburnum (Viburnum opulus var. americanum, V. opulus), rhododendron and azalea, elderberry (Sambucus), weigela, forsythia, dogwood (Cornus), holly (Ilex) and shrubby cinquefoil (Dasiphora fruticosa, syn. Potentilla fruticosa). Other fungi produce the same picture depending on the shrub, including Neofusicoccum species and Nectria cinnabarina; there's no need to tell them apart, as the course of action is the same.

Not to be confused withIt gets confused with Phytophthora root rot, except that in that case the whole shrub declines at once, not a single branch. Winter damage or a string trimmer wound also leave a dead branch, but without sunken bark or brown wood spreading upward.

PreventionThese fungi are opportunists: they only get into stressed plants, and the real remedy is to remove the stress. Water deeply and all at once during summer droughts rather than a little every day, keep 8 cm of mulch without piling it against the stems, don't plant too deep and protect the base from string trimmer knocks. Fertilize in a balanced way, without too much nitrogen.

TreatmentNo product reaches these fungi: they lie dormant inside the wood and only show up when the plant weakens, so no dormant treatment, lime sulphur included, has efficacy data against them. Prune in late winter or very early spring in dry weather, never in late summer or fall, when wounds heal poorly and Nectria gets in precisely that way: go 15 to 20 cm below the lesion, then look at the cut surface, and if the wood is still stained brown in the centre, go down another 10 cm and cut again until you get an entirely white cross-section, because Botryosphaeria often extends 15 to 30 cm beyond the visibly affected part. Disinfect the blade between each cut, discard the debris in the garbage, and then put all your energy into deep watering and mulch: that's where what happens next is decided.

HostsPoplar, trembling aspen and willow first, then birch and mountain ash. On Prunus — cherry, plum, peach, chokecherry, purple-leaf sand cherry — closely related species of the same genus cause Leucostoma canker. In every case, it's stressed, frost-cracked or wounded plants that get caught. On poplar and willow it is Cytospora chrysosperma, which many books still call Valsa sordida; on Prunus, C. cincta and C. leucostoma.

Not to be confused withOn Prunus, the gum is reminiscent of bacterial canker, and only a laboratory test can really tell them apart. A similar picture exists on spruces, but it is caused by other species of the same genus and produces flows of white resin rather than amber gum.

PreventionThis fungus only settles on weakened wood: water deeply during droughts, mulch without piling the mulch against the trunk, and protect the bark from lawn mower and string trimmer knocks. Avoid planting poplar and willow in dry, exposed soil, where they live under constant stress. On young Prunus, protect the trunk from winter sun and frost, which crack the bark and open the way.

TreatmentNo home-garden product has proven efficacy against cankers, and neither this canker nor Leucostoma canker appears on the labels of the lime sulphur products sold here: everything is mechanical and cultural. Cut the affected branches in late winter, in dry, cold weather, well below the lesion into healthy wood, disinfecting the blade between each cut, and put the wood in the garbage without chipping it into mulch or leaving it to dry at the foot of the tree. Don't apply any sealant or wound dressing on the cut: they aren't fungicides, they trap moisture against the wood and interfere with natural healing; and if the canker is on the trunk or the branches are out of your reach, hand the job over to a certified arborist.

HostsBoxwood (Buxus sempervirens, B. microphylla) and hardy hybrids such as 'Green Velvet', 'Green Gem' and 'Green Mountain', which make up most of the boxwoods sold here. The fungus was long named after its asexual form, Volutella buxi; a closely related species, P. foliicola, produces the same picture.

Not to be confused withThe real trap is boxwood blight, which is present in Quebec and much more serious: it produces crisp black streaks on the green stems and massive leaf drop within a few weeks, whereas Volutella works in patches and shows salmon-pink cushions. The two are often found on the same plant, Volutella moving in on top, and any doubt deserves laboratory confirmation. Volutella is also confused with simple winter burn and with the box tree moth, which leaves webbing and green caterpillars.

PreventionBoxwood suffers from winter first, and the fungus moves in afterwards. Water generously in October before the ground freezes, shield it from the winter sun and the northwest wind with a burlap screen, and mulch. Every spring, open up the plant by hand and remove the dead leaves that have piled up in the centre, which hold moisture and shelter the fungus; prune early in the season, never after early August, and never in wet weather.

TreatmentThere is no curative treatment. In dry weather, open up the plant, remove all the dead leaves from the centre and prune the affected wood back to green wood, disinfecting the pruners between cuts and throwing everything in the garbage. If you're set on a copper program, it's only worthwhile applied to new growth at bud break, before the spots show — on a plant that is already bronzed in June, it's too late — with two or three applications per season at most, and only if boxwood and this disease are named on the label, which alone governs; also check your municipality's bylaw, as several cities, including Montréal and Québec City, are more restrictive than the province. Don't use sulphur on boxwood: it burns the evergreen foliage and this use doesn't appear on the labels.

HostsAll boxwoods (Buxus), hardy hybrids included. Sweet box and pachysandra can carry the disease while showing almost nothing, and act as a reservoir.

Not to be confused withVolutella blight also produces bronzed foliage, but it is slower, works in straw-coloured patches and shows salmon-pink cushions, never crisp black streaks on green stems. Winter burn mostly scorches the side exposed to sun and wind and doesn't mark the stems. If in doubt, have the diagnosis confirmed by a laboratory before digging anything out.

PreventionBuy your boxwoods from a reliable source and keep a new plant apart for a few weeks before adding it to an existing border. Avoid wreaths and garlands of cut boxwood, which are a documented way in, and never work in a wet boxwood hedge. Space the plants, water at the base and remove the dead leaves from the centre of the shrubs every spring.

TreatmentNo home-garden fungicide controls this disease, and the right response is to act fast: dig out the affected plant with as many roots as possible, carefully pick up the fallen leaves and put everything in the household garbage, never in the compost or the green waste bin. Don't replant boxwood in the same spot, since the microsclerotia survive for years in debris and soil; another small edging shrub will do very well. Then disinfect your tools, gloves and boots and wash your clothes, because you are the one carrying the disease from one plant to another and from one garden to another.

HostsMaples, and by far mostly Norway maple and its purple cultivars, then silver maple, red maple and sugar maple. Rhytisma acerinum is found here only on Norway maple. Three species share our maples: R. americanum, R. acerinum and R. punctatum.

Not to be confused withRhytisma punctatum is seen on the same native maples as well as on mountain maple and striped maple, but it produces a constellation of 1 to 2 mm dots rather than a patch. Don't confuse it with sooty mould either, a black, sooty deposit that does wipe off with a finger.

PreventionIt all happens in the dead leaves, the only place where the fungus overwinters. Rake them up in the fall and send them to municipal collection or the garbage rather than the backyard compost, which doesn't heat up enough; shredding them with the lawn mower also helps. Since the spores travel several dozen metres, your raking won't solve everything if the neighbourhood has Norway maples.

TreatmentThere's nothing to spray, and that's good news: no home-use fungicide sold in Quebec names tar spot on its label, and in any case no useful coverage is possible on the crown of a mature maple. The disease is purely cosmetic: it doesn't reduce growth and doesn't kill the tree, even after several years in a row.

HostsAsh, oak, maple and linden, each with ITS own fungus: ash anthracnose doesn't move to oak or maple, and an affected tree doesn't infect a neighbour of a different species. Sycamore, rarely planted in Quebec, also has its own anthracnose (Apiognomonia veneta), which is particularly severe. The fungus changes with the species: Discula fraxinea on ash, Apiognomonia errabunda on oak and beech, Aureobasidium apocryptum on maple.

Not to be confused withA late frost burns all the foliage at once, evenly and over the whole crown, whereas anthracnose progresses in spots and mostly in the lower part of the tree. On maple, anthracnose produces diffuse brown spots in June, while tar spot produces raised black patches in August.

PreventionRake up fallen leaves in the fall and prune out dead twigs in winter; that's where the fungus overwinters. An open crown dries faster after rain, and a tree that is well watered during summer droughts easily takes a spring defoliation in stride.

TreatmentOn a mature tree, there's nothing to do and nothing to treat: the disease is unpleasant to look at but has no consequence for the tree's health, and the tree grows new foliage in July. On a young tree defoliated three springs in a row, copper octanoate at bud break can be considered, provided your tree species and anthracnose appear on the product label — the label alone governs — and never as a yearly routine: two or three applications per season at most, away from ditches and bodies of water, because copper builds up in the soil. Also check your municipality's bylaw before buying, as several cities are more restrictive than the province.

HostsMostly American elm, then slippery elm and, more rarely, Siberian elm. American elm selections ('Princeton', 'Valley Forge', 'Jefferson') and hybrids of Asian origin ('Accolade', 'New Horizon', 'Triumph') are tolerant, which isn't the same as immune.

Not to be confused withVerticillium wilt produces the same branch-by-branch decline, but it doesn't affect elm the same way and the streaks in the sapwood are olive green rather than brown. Simple water stress or a root cut during construction work can also brown a branch, but without discolouring the sapwood.

PreventionDon't prune an elm while the bark beetles are flying, roughly from April 1 to October 1; several municipalities set specific dates in their bylaws, so check with yours, and aim for December to February. All felled, dead or diseased elm wood must be debarked, chipped, buried or taken to a designated site before April 1, before the bark beetles emerge — never use elm as firewood. The disease also spreads through root grafts between elms up to about fifteen metres apart: before cutting down a diseased elm, an arborist can sever these roots with a trench to protect the neighbouring trees.

TreatmentNo home-garden product exists, and this isn't a job for an amateur: preventive fungicide injections into the trunk are done by an arborist or an agronomist, on a tree that is still healthy and of great value, and are repeated every two or three years. Therapeutic pruning of a flag is the only pruning allowed during the season: it must be done without delay, in dry weather, by an arborist, with the wood destroyed or debarked the same day — if you can't ensure that immediate disposal, don't prune, and report the tree to your municipality. Beyond about 10% of the crown affected, pruning no longer saves anything; several cities oversee detection and wood disposal and sometimes offer financial assistance.

HostsNorway maple first, then silver maple and red maple, catalpa, smoke tree, staghorn sumac (Rhus typhina) and other Rhus, lilac, ash and many perennials. Conifers and monocots (grasses, daylilies) are the only ones truly out of reach; oak, birch, beech, ginkgo, honey locust, hawthorn, crabapple and linden have no or very low susceptibility and are good candidates for replacement.

Not to be confused withOn elm, Dutch elm disease is what to suspect first (brown streaks rather than olive green). Armillaria and a crown buried or girdled by its own roots cause a slower, more general decline, with no sectoral discolouring of the sapwood.

PreventionThe fungus lives in the soil as microsclerotia that survive about ten years: where a tree has died of Verticillium wilt, don't replant a susceptible host; choose a conifer, an oak, a birch or a honey locust instead. Don't compost prunings or affected perennials, because microsclerotia make it through backyard compost and end up in your beds.

TreatmentThere is no product, home or commercial, against Verticillium wilt: it all comes down to keeping the tree comfortable. Water deeply during droughts, mulch the root zone without piling mulch against the trunk, stop all nitrogen fertilizing and remove dead branches; some trees live many years this way. Disinfecting the saw is a good general habit, but it doesn't change the course of the disease, which enters through the roots; cutting down a large tree is a job for a certified arborist.

HostsOrnamental crabapples and apple trees, with huge differences in susceptibility from one cultivar to another: 'Royalty' and 'Hopa' lose their leaves every year, while 'Adirondack', 'Prairifire', 'Royal Raindrops', 'Donald Wyman', 'Purple Prince' and Malus sargentii hold up well in Montérégie. 'Thunderchild' has a good reputation in the dry climate of the Prairies, but it needs watching here.

Not to be confused withGymnosporangium rust produces bright orange spots, not olive ones. Entomosporium leaf spot on serviceberry and ornamental pear produces small round spots with a black centre dot, more sharply outlined than the fuzzy velvet of scab.

PreventionIt all happens in the fallen leaves, the only source of infection the following spring: rake them all up in the fall and send them to municipal collection, whose industrial composting gets hot enough, or to the garbage, or shred them with the lawn mower so they break down before spring — only backyard compost is to be avoided. Thinning pruning in winter helps the foliage dry faster. The real solution remains choosing the right cultivar at the time of purchase.

TreatmentDon't expect anything from spraying the trunk and branches in winter: the inoculum isn't on the bark, it's in the leaf litter on the ground, and raking is what attacks it. The only useful aerial treatment is wettable sulphur to protect the new leaves, from bud break to late June, every 7 to 10 days and after every rain, wetting both sides of the leaves — a realistic program only on a young tree under 3 m that can be covered completely, since partial coverage protects nothing, and one that remains only partly effective on a very susceptible cultivar. Make sure your tree and scab are named on the product label, which alone governs, and check your municipality's bylaw before buying, as several cities are more restrictive than the province.

HostsMostly pome fruits of the rose family: crabapple, apple, pear, mountain ash, serviceberry, hawthorn and cotoneaster. Spirea and bramble are occasional, minor secondary hosts. Lilac, despite a bacterial blight that looks similar, isn't involved: it isn't in the rose family and it isn't the same bacterium.

Not to be confused withA late frost also blackens flowers and young shoots, but all at once, all over the tree and with no shepherd's crook or oozing. Anthracnose produces spots on leaves that, for their part, end up dropping.

PreventionCut out nitrogen and avoid severe pruning, which produce the soft, water-filled shoots the bacterium needs to get in. Choose cultivars known to be resistant when buying; that's the most effective long-term measure. If you live near an orchard, an affected ornamental crabapple is a source of inoculum for your apple-grower neighbours: pruning out cankers in winter then becomes a matter of being a good neighbour.

TreatmentBy default, don't prune during the season; wait for dormancy and then remove each canker 30 to 60 cm below the edge of the dead wood. Only cut in summer if the blackening is moving down toward the trunk or a main limb: in dry weather, cutting into wood at least two years old, deliberately leaving a 10 to 15 cm stub that you'll remove in winter, and disinfecting the tool with 70% alcohol between each cut, because every summer wound is a new way in. The debris goes in the household garbage the same day, never in the compost or the green waste bin, and don't burn it, since open burning is prohibited or requires a permit in most municipalities; as for copper octanoate during flowering, it's a partially effective extra, out of reach for an amateur on a tree several metres tall and phytotoxic on members of the rose family in bloom — for a garden crabapple, stick to dormant pruning.

HostsThe native American beech, heavily affected in forests and residential woodlots in southern Quebec, including those of the Richelieu valley. On a purple or weeping European beech standing alone in a yard, away from any forest, the disease remains rare, because the scale insect needs a continuous beech population. These fungi only get in thanks to the beech scale, Cryptococcus fagisuga: without it, there is no disease.

Not to be confused withWhitish or grey-green lichens on the bark are flat, firmly attached and perfectly harmless, whereas the scale insect's felt is cottony and squashes under your finger. Simple sunscald on the bark causes a wound on the southwest side, with no cotton wool or red dots.

PreventionDon't wound the bark of a beech: no lawn mower or string trimmer at its base, no nails or rope, and a wide ring of mulch rather than lawn. When clearing land, avoid suddenly exposing a beech that grew up in the understorey to sun and wind. Inspect the bark every fall; that's when the cotton wool is most visible.

TreatmentThere is no fungicide: you can only act on the scale insect, and only on a young beech whose bark you can reach. Gently brush off the felt with a soft brush and soapy water — this brushing isn't optional, it's the condition for success, because insecticidal soap or horticultural oil penetrates the protective wax poorly — then apply in late summer or early fall, when the young crawlers are moving, after checking that the scale insect and beech are named on the label. A large beech that is already cankered is a job for a certified arborist: the risk of breakage makes it a dangerous tree to be around and to cut down.

HostsAll Prunus: chokecherry and its widely sold cultivar 'Schubert', European bird cherry, purple-leaf sand cherry (Prunus × cistena), flowering almond (P. triloba), and ornamental and fruiting plums and cherries. Wild cherries along property edges and in hedgerows are the reservoir that reseeds the whole area.

Not to be confused withInsect galls are round, smooth and regular, whereas black knot is elongated, cracked and matte black. An ordinary canker hollows out and sinks the bark instead of making it swell.

PreventionInspect your Prunus every winter, when everything is visible, and cut down affected wild cherries along the edge of your property, or you'll be pruning the same branches every year. When buying, examine the young plant: a swelling, even green and discreet, should make you turn the plant down.

TreatmentPruning is the only treatment, and there is no home-use fungicide that names black knot. Cut 15 to 20 cm below each knot, into healthy wood, during dormancy and before bud break, disinfecting the tool with 70% alcohol between each cut. Take the prunings off your property the same day, to the garbage or your municipal ecocentre — never into the compost, never in a pile at the back of the yard, where a cut knot keeps releasing spores all the following spring; burning is prohibited or requires a permit in most municipalities.

HostsAll ashes in the genus Fraxinus: white ash, green ash, black ash and their cultivars. White fringetree (Chionanthus virginicus) is occasionally attacked. Mountain ash, despite its name, isn't an ash and isn't affected, and neither is lilac. It's an insect, not a fungus or a bacterium: no fungicide has any effect on it.

Not to be confused withVerticillium wilt kills one branch at a time, without galleries or D-shaped holes. A severe drought or a root cut during construction work also thin the crown, but produce neither water sprouts on the trunk nor split bark.

PreventionNever move ash wood or firewood from one region to another; that's how the insect travelled, and federal restrictions also govern the transport of ash wood. If you replant, choose something other than an ash, and keep in mind that a healthy, well-watered ash holds out a little longer, nothing more.

TreatmentThere's nothing you can spray yourself and no home-garden product has any effect: the only practical treatment in Quebec is an injection of azadirachtin into the trunk, done by a certified arborist, on a tree in full leaf and actively transpiring, that is, from mid-June to late August, and generally repeated every two years. It only makes sense on an ash that is still healthy, having lost less than a third of its crown, and it is a commitment for the life of the tree. Beyond that, cutting it down is the only way out, and it's a job for a professional: a dead ash dries out quickly, becomes brittle and comes apart dangerously.

HostsColorado blue spruce is by far the most affected, followed by Norway spruce, white spruce and black spruce. Fir and Douglas fir can occasionally be affected; hemlocks have their own Rhizosphaera species, which is a different disease. A second fungus, Stigmina lautii, often accompanies it on the same needles.

Not to be confused withSpider mites, which make the foliage dull and flecked with yellow, with a little fine webbing — shake a branch over a white sheet of paper to see them. Cytospora canker, which kills the whole branch at once and leaves runs of white resin. And the normal shedding of old needles on the inside, which is gradual and never reaches the tips of the twigs.

PreventionIt's a disease of stagnant air and foliage that stays wet: space the plants, clear the bottom of the crown and avoid planting blue spruces in tight rows. Never water the foliage or water late in the day; water at the base, early in the morning. Pick up the needles that fall in autumn and put them in the garbage; they carry the next spring's inoculum.

TreatmentThere is no curative treatment: an infected needle will never turn green again, and no home-use fungicide names this disease in Quebec — the reference products are commercial class and require a permit and an applicator certificate. On a spruce taller than three or four metres, no gardener's sprayer reaches the crown and partial coverage achieves nothing, so the real choice is between having the tree assessed by a licensed company and accepting the decline while planning a replacement with a less susceptible species, such as Serbian spruce or Korean fir. Never apply horticultural oil to a blue spruce: it permanently removes the waxy bloom.

HostsAgeing Colorado blue spruce above all, then Norway spruce and white spruce. More rarely larch, fir, Douglas fir and hemlock; pines have their own Cytospora species. Its asexual form, Cytospora kunzei, gave the disease its common name.

Not to be confused withRhizosphaera needle cast, which makes the needles drop without killing the branch and without resin. Drought or salt damage, which affects a whole side of the tree rather than a single branch.

PreventionIt's above all a stress disease: it settles on trees that lack water, have been wounded by the lawn mower or string trimmer, or are planted close together in compacted soil. Water deeply during summer droughts, especially young plants and trees near an asphalt driveway, and mulch around the base to keep the mower away. Never prune a spruce in wet weather.

TreatmentNo fungicide fixes a canker; it all comes down to pruning. Remove the affected branches in dry, cold weather, cutting at the branch collar without leaving a stub, and wipe the blade thoroughly with 70% alcohol between each cut, because a quick pass disinfects nothing; put the wood in the garbage rather than composting it, chipping it into mulch or burning it, and don't apply any sealant or wound dressing on the cut: these products aren't fungicides and they trap moisture against the wood. If the cankers are on the trunk or on large main limbs, have the tree assessed by a certified arborist.

HostsEastern white cedar (thuja), in hedges as well as specimens, and creeping or columnar junipers. False cypresses too. Pestalotiopsis, an opportunist, can settle on other conifers that are already weakened. The species involved are mainly Kabatina thujae and K. juniperi, as well as Phomopsis juniperovora.

Not to be confused withAbove all, the cedar leafminer, very widespread in Quebec: it also produces scattered brown tips, but the affected tip is hollow and translucent when held up to the light, with a tunnel and sometimes a small caterpillar inside. Also physiological browning and salt damage, which are uniform and face one side only.

PreventionPrune early in the season and in dry weather, never during a week of rain: every cut is a way in. Water at the base with a soaker hose, never with a sprinkler that wets the foliage, and avoid over-fertilizing with nitrogen, which produces soft, very receptive shoots. In a new hedge, leave 60 to 75 cm between plants so the air can circulate.

TreatmentNo home-use fungicide names these fungi on cedar or juniper in Quebec, and in any case Kabatina gets in through wounds in the fall: a spring spray would come after the battle. The treatment is therefore entirely sanitary — prune each affected twig 10 to 15 cm below the brown area, into healthy wood, in dry weather, disinfecting the tool, and put the debris in the garbage. Done properly, this pruning takes care of the vast majority of cases.

HostsEastern white cedar, especially young plants and tight, poorly ventilated hedges; western red cedar too. Mature, open-grown plants get away with a bit of browned foliage, nothing more.

Not to be confused withKabatina or Phomopsis tip blight, where it's the whole twig tip that dies and turns grey, not isolated scales. Normal fall browning, which also affects the inside but makes all the old scales drop at the same time, with no cushions or craters.

PreventionIt all depends on air circulation: space plants out at planting, don't crowd a hedge against a wall or a solid fence, and don't let the lower branches drag in damp mulch. Water at the base, never on the foliage, and especially not in the evening. Pick up the browned foliage that has fallen at the base and put it in the garbage.

TreatmentNo home-use fungicide names this disease in Quebec; it is managed by airflow and sanitation, not by spraying. Prune to open up the inside of the hedge, remove the affected foliage and put it in the garbage. On a badly affected young plant that stays bare for two seasons in a row, it's often faster to replace it than to wait.

HostsEastern white cedar in all its forms, in hedges as well as specimens. The same mechanism exists in juniper, yew, hemlock and pine, which drops its two- or three-year-old needles every fall.

Not to be confused withIt's rather the opposite you should worry about: many hedges that get treated aren't sick at all. A true tip blight is scattered in speckles through green foliage, whereas winter drought or salt damage browns one face of the hedge evenly, often with a sharp horizontal line at the height of the splashing — those two problems have their own entries.

PreventionThere's nothing to prevent, but a cedar that goes into winter well hydrated bronzes less: water generously in the fall until the ground freezes, and mulch 8 to 10 cm deep. Too much nitrogen in late summer heightens the contrast in spring.

TreatmentNothing to do, and above all no panic spraying or fertilizing. If the build-up of brown scales bothers you, gently shake the twigs or run a gloved hand or a hand rake through the inside of the hedge. Just remember that cedar doesn't put out new shoots on bare old wood: as long as the twig tips are green, there's no reason to worry.

HostsOn the conifer side, junipers — eastern red cedar, Rocky Mountain juniper, creeping junipers and their cultivars. On the rose family side, it depends on the fungus species: crabapple and apple, hawthorn, then serviceberry, quince and mountain ash for cedar-quince rust. Eastern white cedar, despite its name, is not a host of these rusts. Three species mainly: G. juniperi-virginianae, G. globosum and G. clavipes.

Not to be confused withOn crabapple, this rust is often confused with scab, whose spots are olive to black and velvety, never bright orange. The gelatinous horns on juniper don't look like anything else — the problem is rather that they cause far more worry than they deserve.

PreventionIt all happens at purchase: choose resistant cultivars on both sides, crabapple as well as juniper, rather than hoping to keep the hosts apart. The spores travel one to three kilometres, so removing your own juniper won't solve the problem if the neighbourhood is full of them. Unlike scab, picking up the crabapple's spotted leaves does nothing here: the fungus doesn't overwinter on that side; it starts again every year from the galls on the juniper.

TreatmentThe only step that really makes a difference is to cut off and discard the galls on the juniper in winter or very early spring, before they turn gelatinous. Sulphur has weak and inconsistent efficacy against these rusts, the truly effective active ingredients being commercial class, and in any case it can only be applied to a young plant under three metres, wearing goggles: on a mature crabapple or hawthorn, nobody can cover the crown at arm's length. If you use sulphur anyway, check that your plant and this rust are named on the label, which alone governs, don't use it on a tree whose fruit you harvest for jelly, and check your municipality's bylaw, as several cities are more restrictive than the province.

HostsAustrian pine above all, by far; then Scots pine, mugo pine, red pine, jack pine and white pine. During heavy attacks, black spruce and tamarack can be affected. The fungus has gone by the names Diplodia pinea and Sphaeropsis sapinea.

Not to be confused withThe white pine weevil, which makes the terminal leader droop and die in a shepherd's crook, but only one shoot, not dozens of twigs. A late frost, which hits all the shoots at the same time, once, and doesn't come back the following year. Scleroderris canker, where the needles redden from the base toward the tip and the wood scraped under the bark looks yellowish green.

PreventionThis disease strikes trees that are already stressed, so deep watering during summer droughts does more than any spray. Always prune in dry weather, never in a wet spring, and pick up the cones and dead twigs that have fallen to the ground, which are the fungus's main reservoir. If you're planting new, be aware that Austrian pine as a hedge or screen is almost a lost cause in Montérégie past twenty or twenty-five years.

TreatmentNo home-class product names pine or this fungus in Quebec, and the fungus is lodged in the wood of the shoot anyway: there's no useful spraying to do. What remains is sanitation pruning, cutting each affected twig well below the dead area, in dry weather and preferably in winter, disinfecting the tool between cuts, followed by a thorough cleanup of the cones and twigs on the ground. A tree whose top has died and that has lost more than a third of its crown won't recover; have it assessed by a certified arborist, who will also take care of cutting it down if needed.

HostsMugo pine and Austrian pine are the two main victims in landscaping, to the point that it's hard to recommend them for a damp spot. Scots pine, white pine, red pine and jack pine can also be affected. These are pathogens of the genus Pinus: a spruce that is losing its needles belongs to another entry.

Not to be confused withRed band needle blight (Dothistroma septosporum), another fungus that produces truly red bands: unlike this one, it isn't documented as common in landscapes in Quebec. Lophodermium needle cast, where the needles brown along their whole length and drop massively in spring, with no bands or sharp boundary. The normal fall drop of two- or three-year-old needles, which turn yellow evenly inside the tree in September, without spots.

PreventionIt's a disease of humid, stagnant air: space pines well, especially in hedges or screens, and never water the foliage. Pick up the fallen needles, which carry the fruiting bodies, and put them in the garbage. In a damp or poorly ventilated spot, avoid Austrian pine and mugo pine altogether in favour of a less susceptible species.

TreatmentNo home-use fungicide names this disease in Quebec: the documented programs rely on commercial-class copper products, which require a permit and an applicator certificate. For you, the answer is sanitation — picking up needles, pruning the worst-affected lower branches, opening up the planting — and, eventually, choosing another species. If you're set on having a valuable tree treated, know that the useful window is mid to late May, before the new needles emerge, with a repeat around mid-July, and that it's a job for a licensed company.

HostsMostly young pines under ten years old: Scots pine and mugo pine at the top of the list, then red pine and Austrian pine. Pine hedges, tight groupings and young nursery plants are the classic situations.

Not to be confused withBrown spot needle blight, where the spot circles the needle, the tip dies while the base stays green, and everything shows in August rather than May. Winter drying, which mostly browns the side exposed to the wind and doesn't spare the current year's needles.

PreventionSpace the plants and avoid tight groupings of young pines, where the foliage stays wet. Water at the base only, and above all don't wet the foliage from late July to September, which is precisely the infection period — even the lawn sprinkler counts. Pick up the fallen needles and put them in the garbage; they carry the inoculum.

TreatmentNo home-use fungicide names this disease in Quebec. The really useful thing to know is that the timing is the opposite of other needle diseases: protection applied in spring, when the needles drop, comes a year too late, since infection took place at the end of the previous summer. A well-spaced pine, watered at the base and cleared of its fallen needles, recovers on its own in two or three seasons.

HostsBalsam fir above all, Fraser fir and other ornamental firs. It's an occasional disease on a lone fir in a landscape, but common in Christmas tree plantations and near natural fir stands.

Not to be confused withA late frost at the end of May, which produces exactly the same picture — scorched new shoots, green old needles — but hits all the exposed shoots at once, the day after a cold night, and doesn't come back the following year. If the picture repeats two springs in a row, it's the disease.

PreventionLeave at least 1.5 m between firs so the air circulates and the new shoots dry quickly, and avoid low-lying spots where fog and cold air linger in spring. Pick up and discard dead shoots that have fallen. If affected balsam firs grow along the edge of your property, removing them reduces the source of spores, but have it done professionally: cutting a tree almost always requires a municipal permit, and vegetation in a shoreline buffer or a protected woodlot can't be cut.

TreatmentNo home-use fungicide names this disease in Quebec; what remains is to cut off and discard the dead shoots, which carry the next spring's spores, and to open up the planting so the air circulates. The disease follows the weather: after a dry spring, the tree bounces back and fills in again within two or three seasons. A fir that loses its shoots several years in a row won't regain its shape and deserves to be replaced.

HostsRhododendron and azalea, pieris (also called andromeda), holly, boxwood, yew, viburnum, dogwood, serviceberry, juniper and fir. Among perennials, lavender and coral bells regularly succumb in heavy beds. In Quebec, it's mostly P. cactorum, P. plurivora and P. cryptogea.

Not to be confused withRoot suffocation from too much water, a root flare buried too deep and bark gnawed by a vole give exactly the same picture seen from above. So does Armillaria root rot, but it leaves a white felt under the bark at the crown. Before digging out more than one or two plants, have the diagnosis confirmed by a plant diagnostic laboratory.

PreventionIt all comes down to drainage and above all to planting: set the root ball 5 to 10 cm HIGHER than the soil level, never at the bottom of a hollow or the foot of a slope. In clay soil, don't dig a narrow, deep hole that will act like a bathtub; amend wide and shallow, and keep the mulch a few centimetres away from the trunk. Turn off daily automatic watering, which is a common cause, and look at where the water from a downspout goes.

TreatmentThere's nothing to spray: Phytophthora isn't a fungus but an oomycete, an organism related to algae, and that's precisely why the sulphur and copper on the garden centre shelf have no hold on it. Dig out the plant with its root ball, put it in the garbage, and consider the spot contaminated for years: never replant a susceptible species there; switch to another genus. Wash and disinfect your shovel, boots and wheelbarrow wheels before moving elsewhere on the property. Only dig out yourself a shrub or young plant that you can remove with both feet on the ground: an established tree with rotten roots can topple without warning, and removing it is a job for a certified arborist.

HostsPractically all hardwoods and conifers, with a marked preference for trees that are already stressed. Very common on lots carved out of a former woodlot or planted around old stumps, a commonplace situation in Quebec residential developments. Several species share the territory in Quebec, including A. gallica, A. sinapina, A. calvescens and A. ostoyae.

Not to be confused withPhytophthora and Verticillium wilt produce the same slow decline, but neither leaves a white fan under the bark or black rhizomorphs. Be careful with the mushrooms themselves, too: honey mushrooms grow on the same wood as the funeral bell (Galerina marginata), which is deadly. Never eat these mushrooms on the strength of an information sheet alone — here, they are only used for diagnosis.

PreventionDuring construction or landscaping, have old stumps and large root debris dug out and removed rather than buried. After that, it's all about vigour: water deeply during droughts, avoid wounding the crown or piling fill at the foot of trees, and don't compact the root zone with machinery. On a lot where Armillaria is established, replant species known to be more tolerant rather than putting the same thing back in the same spot.

TreatmentNo product available to gardeners acts on Armillaria, and there is no curative treatment. An affected tree is a dangerous tree: the fungus has destroyed the anchoring roots and the tree can topple entirely, including during felling — have it assessed and cut down by a certified, insured arborist, never yourself, as soon as it is taller than your ladder or could reach a building, a wire or the street. Have the stump ground out by a contractor and check with your municipality: several require a tree-cutting permit even for a dead tree.

HostsCedar, yew, boxwood, rhododendron and azalea, holly, hemlock, blue spruce and mugo pine in exposed spots. On roses, it's the canes that are found blackened when the winter hilling is removed.

Not to be confused withDe-icing salt causes the same browning, but on the street side rather than the wind side. The spruce spider mite discolours the foliage a bronze-grey: shake a twig over a sheet of white paper, and if tiny moving specks fall, that's it. On roses, winter burn and grey mould under the winter cover account for the vast majority of dead canes, much more often than a true canker.

PreventionWater deeply in October until the ground freezes — it's the measure that pays off most — and mulch 7 to 10 cm deep to delay deep frost. On an exposed plant, put up a burlap screen on stakes, on the wind and sun side, without the fabric touching the foliage. Choose plants that are hardy for YOUR area, and one zone hardier for an evergreen in an open spot, and stop all nitrogen fertilizing from the end of July.

TreatmentDon't prune anything before bud break: wait to see where the green comes back, in May, then cut back to live wood. Be aware that a cedar, a tightly sheared yew or a spruce won't grow new foliage on completely bare old wood — if the gap goes down to the wood, it will stay, and on a young plant under three years old, replacement is often the only way out. Be wary of aerosol anti-transpirants sold for this problem: university trials show no consistent benefit, and they replace neither October watering, nor mulch, nor a burlap screen.

HostsCedar is the most sensitive of all, along with white pine, white spruce, sugar maple, red maple and linden. Norway maple, juniper, rugosa rose, potentilla and common lilac cope much better, without being immune.

Not to be confused withWinter burn causes the same browning, but on the wind side, not the street side. On a cedar hedge, browning scattered here and there is more often the normal fall drop of inner needles, a lack of water, or spider mites. The white paper test, shaken under a twig, settles the spider mite question in fifteen seconds.

PreventionShovel early and often; that's what reduces the amount of salt spread the most. On risky surfaces, use an abrasive on its own — sand, fine gravel, stone dust — rather than a salt-sand mix, which just spreads the same amount of chloride around. Set your plantings back at least 3 m from a street cleared with salt, put up a burlap screen on the street side for the winter, and accept that on the first two metres along a municipal street, losing the plants completely is the normal scenario.

TreatmentFor salt sprayed onto the foliage, rinse with clean water as soon as the snow melts; gypsum is no use in that case. For salt absorbed by the roots, leach with 50 to 75 mm of water over two or three waterings — but only if the soil drains: dig a 20 cm hole, fill it with water, and if it doesn't empty in under four hours, don't leach, you would drown the roots. In clay soil, gypsum helps displace the sodium, but it has to be followed by leaching to do any good.

HostsBigleaf hydrangea, magnolia, crabapple in bloom, Japanese maple, hosta, peony, fern, and the soft new growth of spruce, fir and larch.

Not to be confused withFire blight also blackens shoots, but they curl over like a shepherd's crook and stay attached, an amber ooze seeps out and the damage progresses day by day in June — frost, on the other hand, doesn't progress any further. Anthracnose of hardwoods causes brown spots along the veins, not a general collapse overnight. On conifers, a fungal shoot blight spreads from one shoot to the next over several weeks.

PreventionDon't uncover your plants too early and don't push them with nitrogen in April. When frost is forecast, water the soil in late afternoon and cover at sunset with a cotton sheet or a horticultural fabric reaching down to the ground, removed in the morning — never plastic touching the foliage: it conducts the cold and burns exactly where it touches. The same freeze-thaw cycle heaves perennials planted in the fall out of the ground, which is why you should mulch AFTER the ground freezes rather than before.

TreatmentThere's nothing to apply and nothing urgent to do. Let the plant regrow and wait until the new growth is well under way, in June, before cutting off the blackened parts: you'll then know exactly where to cut. On a bigleaf hydrangea or a magnolia, that year's flowering is lost, but the shrub itself isn't.

HostsPin oak above all, and to a lesser degree red oak, birch, red maple, hydrangea, rhododendron and azalea, pieris or andromeda, holly and white pine. The problem is almost always seen at the foot of a foundation, along a driveway or in crushed-stone fill.

Not to be confused withA nitrogen deficiency yellows the OLD leaves first, at the bottom, evenly and without the green net. Root suffocation and salt also cause yellowing and burnt margins, but without that sharp contrast between green veins and yellow leaf blade.

PreventionHave your soil pH tested before planting, especially along a foundation or in fill. Above pH 7, don't plant acid-lovers: choose species that are comfortable in alkaline soil instead, such as bur oak, lilac, viburnum, dogwood or juniper. A mulch of dead leaves or pine needles renewed every year slowly acidifies the surface and never does any harm.

TreatmentStart with the vinegar test: pour some on a handful of soil, and if it fizzes vigorously, your soil is calcareous and any attempt to acidify it is doomed to fail, because the limestone neutralizes acidity as fast as it forms. On soil that is merely alkaline, with no fizzing, GRANULAR horticultural sulphur sold as a soil amendment — never the sulphur from the pesticide shelf, whose label only allows foliar spraying — spread over two or three years lowers the pH slowly. On calcareous soil, all that's left is an iron chelate listing EDDHA in its guaranteed analysis, renewed every spring, or moving the plant; an EDTA or DTPA chelate, the most common kind at garden centres, breaks down above pH 7 and won't correct anything.

HostsAnything that doesn't like wet feet: yew, boxwood, rhododendron, lavender, thyme, pine, birch, Japanese maple, panicle hydrangea, and most bed perennials. Willows, dogwoods and viburnums tolerate it better.

Not to be confused withPhytophthora almost always moves in under those conditions and gives the same picture; the difference shows at the crown, chocolate brown with a sharp boundary in the case of the pathogen. A root flare buried too deep and a root ball left in its wire basket also cause this slow decline. You often have to scrape away the soil to tell.

PreventionFix the drainage before planting: a 15 to 20 cm mound in heavy soil, a wide, shallow hole rather than a narrow, deep one, and never plant at the low point of a lot. Direct downspouts away from the beds, and don't let machinery or occasional parking compact a tree's root zone. Water rarely and deeply rather than a little every day.

TreatmentStop watering, pull back the mulch and soil piled on the crown, and let the soil drain. NEVER aerate waterlogged soil: working saturated clay smears it and makes the compaction permanently worse — wait until a handful of soil crumbles instead of rolling into a sausage, which often takes several weeks. In the meantime, 3 to 5 cm of compost spread on the surface, around the outer edge of the crown and away from the large roots, lets the earthworms do the work of loosening the soil.

HostsAll trees and shrubs planted from containers or balled, with a special mention for maple, linden, crabapple, standard lilac, cedar hedges and spruce.

Not to be confused withThe picture from above is identical to that of Armillaria, Phytophthora and Verticillium wilt. The difference shows with a shovel: here, no white felt, no chocolate-brown crown, only a buried trunk or a strangling root.

PreventionAt planting, look for the trunk flare before digging and remove the soil covering it in the pot, even if it means sacrificing 5 cm of the root ball; the flare must stay visible above the finished grade. Tease apart or cut the roots circling the bottom and sides of the pot, completely remove the wire, string and burlap from the top of the root ball, and never pile mulch in a volcano against the trunk. A wide, shallow hole is always better than a narrow, deep one.

TreatmentOn a young tree, clear the root flare with a jet of water and cut the girdling root with pruners disinfected with 70% alcohol, before and between each tree. On a tree more than 15 cm in diameter, have the work done with an air spade by a certified arborist: a shovel destroys precisely the fine roots you're trying to save, and a large girdling root often also serves as an anchoring root — cutting it on one side only can make the tree unstable in the wind. On a young tree badly planted a year or two ago, the simplest thing is still to take it out and replant it properly.

HostsYoung trees with thin, smooth bark: maple, linden, crabapple, ornamental cherry and plum, oak, beech, birch and pear. An exposed trunk, planted facing due south in a parking lot or beside a driveway, is the typical case.

Not to be confused withA mower or string trimmer wound is found at ground level and on several sides at once. A fungal canker makes a wound that progresses year after year in all directions, often with small black dots. Rodent damage leaves parallel tooth marks and wood stripped bare all the way around.

PreventionPut a white reflective sleeve or a light-coloured spiral guard on the trunk of young trees from November 1 to May 1, and remove it in spring so as not to trap moisture. Be careful: a spiral guard does NOT stop voles, which travel underneath it; in rodent areas, add a cylinder of metal mesh. Keep the lower branches on a young tree as long as possible, as they shade the trunk, and stop nitrogen from the end of July.

TreatmentDon't paint anything and don't put any sealant or tar in the wound: the tree closes it better in the open air. With a disinfected knife, remove the strips of bark that have already come loose, following the outline of the living wood, then water deeply during droughts in the following years — closing takes several seasons. A deep frost crack on a large tree is a structural defect: have it assessed by a certified arborist rather than judging the risk of breakage yourself.

HostsHosta, coral bells, astilbe, fern, hydrangea, Japanese maple, dogwood, rhododendron, birch, linden and beech. In general, anything sold for part shade and planted in full sun.

Not to be confused withDe-icing salt also causes brown margins, but in spring and on the street side. Fertilizer burn follows a recent application and affects the whole plant at once. A fungal leaf spot makes round, scattered spots on the leaf blade, not a continuous border; and if the whole plant looks thirsty while the soil is soaked, look to the roots instead.

PreventionThe right plant in the right place takes care of 90% of cases: read the exposure on the label and be wary of white walls and paved surfaces, which reflect heat. Mulch 5 to 7 cm deep, keeping the mulch away from the crown, and water deeply once a week rather than a little every day, especially for the first two years after planting. A temporary shade cloth during a heat wave saves a young plant transplanted in spring.

TreatmentThere's no product to apply and nothing to fix right away: the scorched leaves won't turn green again, but the plant isn't in danger. Cut off the most damaged leaves for appearance's sake, water deeply, and if the problem comes back every summer, move the plant in the fall to a more sheltered spot. Marginal scorch that comes back three years in a row on a tree is a warning sign: check the drainage, the planting depth and the compaction.

HostsYoung trees planted in the middle of a lawn, especially those with thin bark: crabapple, ornamental cherry and plum, maple, linden, mountain ash, serviceberry. Hares also go after roses, hydrangeas, crabapples and young cedars.

Not to be confused withSunscald makes a wound higher up the trunk and on one side only, the southwest, with no tooth marks. A fungal canker progresses year after year and often comes with small black dots. A plant that wilts in July always suggests a root disease: look at the base of the trunk first; it takes ten seconds.

PreventionRemove the lawn in a 60 cm circle around the trunk and replace it with mulch, never piling it against the bark: no more grass to cut, no more mower near the trunk. Against rodents, the only tool that works is a cylinder of metal mesh with 0.6 cm openings, pushed 5 to 10 cm into the ground and taller than the expected snow, put in place BEFORE the first snowfall. A plastic spiral guard isn't enough: voles travel underneath it.

TreatmentDon't use rodenticide baits: they don't work on hares, they come too late anyway since the damage is already done, and secondary poisoning kills birds of prey, foxes and pets. Neatly trim the loose strips of bark with a disinfected knife, don't apply paint or sealant, and water deeply. A trunk gnawed or cut all the way around is lost and has to be replaced; below half the circumference, the tree closes the wound over several years but stays vulnerable to wind at that spot, and on a young tree it's often better to start over.

HostsCedar hedges, columnar junipers, yews sheared into columns, mugo pines, hydrangeas and multi-stemmed shrubs. Among trees, those whose branches grow in a tight V, such as silver maple, birch and Japanese tree lilac.

Not to be confused withNothing else produces this picture; it isn't a disease, in fact. Don't confuse torn bark at the fork with a canker: the tear is fresh, clean, mechanical, and dates from that same spring.

PreventionShape the top of hedges and conifers into a rounded dome or a pyramid, with the base wider than the top: a flat top collects snow and gives way. In the fall, tie up multi-stemmed cedars and junipers in a spiral with a wide, soft strap, without cutting into the bark, and remove it in spring. Don't plant a columnar conifer or a fragile shrub under the edge of a roof or where the snowblower throws snow.

TreatmentNever hit a frozen conifer with a broom or a shovel: twigs caught in ice snap clean off — on fresh, light snow, brush gently from the bottom up; otherwise, let it melt. Tie the spread stems back together with a soft strap for a season or two, until they regain their shape, and cleanly prune broken or bent branches back to healthy wood rather than leaving them. A large branch broken high up, a split fork or a tree leaning after an ice storm is a job for a certified arborist, not for the ladder from the garage.