Gadenin日本語Get the appDogwood anthracnose, the one that kills
Dogwood anthracnose is a fungal disease that has destroyed flowering dogwoods across large parts of their range, and it is genuinely fatal rather than merely disfiguring. Unlike most leaf diseases, it does not stop at the leaves: it moves from leaf into twig, from twig into limb, and from limb into the trunk, and a tree can go from mildly spotted to dead in three to five seasons.
Learn the sequence, because early recognition is the only real leverage you have. It begins with tan or purple-rimmed blotches on the leaves, usually starting in the lower, shadiest part of the canopy in a wet spring. Infected leaves then die but hang on the twig instead of dropping, and dead leaves clinging to bare branches through winter are the diagnostic sign for this disease. The twigs behind them die back. The tree responds by throwing out shoots from the trunk and main limbs, and those shoots get infected in turn, carrying the fungus into cankers on the trunk itself. Once the trunk is cankered all the way round, the tree is finished.
It thrives on cool, wet, shaded, still conditions and on leaves that stay wet for long periods. Trees in deep shade at the bottom of a damp slope, crowded by other planting, are the ones that die. Trees in morning sun with air moving through them, on a site that drains, very often live alongside the disease without ever being seriously damaged.
| Symptom | Likely cause | What to do |
|---|---|---|
| Dead brown leaves clinging to bare twigs in winter | Dogwood anthracnose | Cut back to healthy wood in dry weather, improve light and air flow |
| Tan blotches with purple rims, lower canopy first | Dogwood anthracnose, early stage | Rake and remove fallen leaves, stop all overhead watering |
| Clusters of soft shoots on the trunk and main limbs | Stress response that anthracnose then colonises | Rub or cut them off flush while they are still small |
| Sunken cracked patch on trunk or main limb | Anthracnose canker or an old mower wound | Assess how far round it runs, a fully girdled trunk is not recoverable |
| White dusty film on upper leaves in midsummer | Powdery mildew | Improve air movement, water properly, do not feed |
| Small purple-rimmed spots on the bracts in spring | Spot anthracnose, a cosmetic disease | Ignore it, it does not threaten the tree |
| Sawdust-like frass and loose bark near ground level | Dogwood borer in an old bark wound | Keep machinery away permanently, widen the mulch ring |
| Dry brown leaf margins with green centers | Drought scorch, not disease | Deep watering and a wider mulch ring |
How to make the site hostile to the disease
There is no spray that a home grower can rely on to save a badly sited flowering dogwood, so the whole strategy is to change the conditions the fungus needs. Every action below removes one of them, and together they are far more effective than anything you can apply.
Give the tree light and moving air. Thin or remove whatever is crowding it, particularly on the side that stays damp longest in the morning. A dogwood whose leaves are dry by mid-morning is in a different disease category from one that stays wet until noon.
Keep water off the foliage entirely. Water at the base, in the morning, never with an overhead sprinkler. At the same time do not let the tree go into drought, because stressed trees are colonised faster. And keep nitrogen low, because soft fast growth is the easiest tissue for the fungus to enter.
- Rake and remove every fallen leaf Infected leaves on the ground and clinging to the twigs are where the fungus overwinters. Clear the whole area under the canopy after leaf fall and take the leaves away rather than composting them in place. This one job, done every autumn, measurably lowers next spring's infection pressure.
- Cut out dead twigs in dry weather Prune dead and dying twigs back into clean healthy wood during a dry spell in the dormant season. Wet weather spreads spores on every cut. Disinfect the blades between cuts with alcohol wipes when you are working on a tree that is already infected.
- Remove trunk shoots while they are small Soft shoots on the trunk and main limbs are the bridge the disease uses to reach the trunk. Rub them off with a thumb while they are green, or cut them flush before they harden. Check for them two or three times through the growing season.
- Open up the site Thin overhanging branches from neighboring trees and pull back shrubs crowding the lower canopy. The aim is direct morning sun on the leaves and a steady flow of air through the branches by mid-morning, every day, not just on breezy ones.
- Re-check the base of the trunk each winter Pull the mulch back and look at the bark right round the trunk for cankers, wounds and frass. Finding a canker early means you can decide about the tree with time in hand rather than after the canopy has thinned.
If the tree you are dealing with sits in permanent shade on damp ground and already shows twig dieback and trunk cankers, be honest about the odds. Removing it and replanting on a brighter, drier, better-ventilated spot is a better use of the next five years than treating it in place.
Powdery mildew and the diseases that only look serious
Powdery mildew is the disease you will actually see most years. It shows as a dull white or grayish film on the upper leaf surface from midsummer, thickening through late summer, with affected leaves curling, browning at the edges and dropping early. Repeated heavy defoliation weakens a tree and reduces the following spring's bloom, so it is not entirely harmless, but it does not kill.
It behaves differently from anthracnose in one important way: it does not need wet leaves. Warm days, cool nights and stagnant humid air are what it wants, which is why boxed-in corners are worse than open positions. The response is air movement and steady watering rather than spraying. Clear crowding growth, keep the tree from going dry, and rake up infected leaves in autumn.
Spot anthracnose is a separate and far milder disease that speckles the bracts with small purple-rimmed spots in a wet spring, sometimes distorting them. It spoils the look of one display and does nothing else. Do not confuse it with the leaf and twig blight described above, and do not start cutting into the tree because of it.
Borers, and the wound that lets them in
Dogwood borer larvae tunnel under the bark and can girdle a trunk, but they almost always need an existing wound to get started. The wound is nearly always ours: a mower deck bumped into the trunk, a string trimmer run round the base, a stake tie left on until it cut in, or a pruning cut left on the lower trunk.
Look for coarse sawdust-like frass in bark crevices near ground level, bark that sounds hollow or lifts away, and a sudden crop of sprouts from the base of a tree that never produced them before. Thinning in the upper canopy with no other explanation is a late sign that the trunk is being girdled.
The only prevention that works is not to wound the tree. A mulch ring three to four feet (1 to 1.2 m) across removes the reason for machinery to approach at all, and it costs an afternoon. Keep the mulch off the bark itself, remove stakes and ties by the end of the second season, and avoid pruning cuts low on the trunk.
Keep a symptom log
Record the date you first see leaf blotches each spring and where in the canopy they appeared, plus what the weather did in the ten days before. Anthracnose runs on wet springs, and a log like this quickly shows whether your tree gets hit every year or only in the wet ones, which is the difference between a manageable site and a doomed one.
Note the same for the first white mildew film each summer, and count the trunk shoots you remove each season. A rising count of trunk shoots year on year is an early warning that the tree is under stress and worth investigating at the base before the canopy shows it.