Gadenin日本語Get the appSilver leaf: the reason you do not prune in winter
Silver leaf is a wood-invading fungus and it is the most serious disease of ornamental cherries. The name comes from the symptom: leaves on an affected branch take on a distinct silvery or leaden sheen, because the fungus releases a toxin that separates the upper leaf surface from the layer beneath and the trapped air reflects light. The silvering usually shows on one branch or one side of the tree first, not evenly all over.
The confirming test is in the wood, not the leaves. Cut through an affected branch and look at the cut face. Healthy cherry wood is uniformly pale. Infected wood shows a dark brown or purplish stain, often as an irregular core or a broken ring. Silvering without that stain is not silver leaf; it can be caused by drought or root damage and it recovers on its own.
There is no chemical cure. What works is removing the affected branch, cutting back at least six inches (15 cm) beyond the last visible staining into clean pale wood, in dry summer weather, disinfecting the saw between every cut. Take the prunings off site and burn or bin them; do not stack them nearby, because the flat purple bracket fungi that eventually form on dead cherry wood release the spores that start the next infection.
Prevention is entirely about wounds and timing. The spores need a fresh wound to enter and they are most abundant in cool damp conditions. Every rule in the pruning guide, summer only, dry weather only, small cuts, clean tools, exists because of this one fungus.
| What you see | Likely cause | What to do |
|---|---|---|
| Silvery sheen on the leaves of one branch, dark stain in the cut wood | Silver leaf fungus | Cut out well into clean wood in dry summer weather, disinfect tools, remove debris |
| Amber gum oozing from the trunk or a limb, sunken dead bark | Bacterial canker | Cut out affected wood in summer, improve drainage and shelter, stop nitrogen feeding |
| Small round holes scattered through the leaves, edges brown | Shot hole, usually following bacterial infection | Largely cosmetic, rake up fallen leaves, no spray needed |
| Flowers and the shoot behind them collapse and turn brown, hanging on | Blossom wilt, worst in a wet flowering season | Prune out the dead spurs in summer, remove any mummified fruit |
| Curled, blackened shoot tips with dense colonies underneath | Cherry blackfly | Tolerate on a mature tree, hose off on a small one, stop feeding nitrogen |
| Whole branch bare in spring while the rest leafs out | Dead branch from canker, silver leaf or old root damage | Cut out in summer, then look for the cause at the roots |
Bacterial canker: gum, sunken bark and dying branches
Bacterial canker is the other killer, and it announces itself with gum. Amber, sticky resin oozes from the trunk or a limb, often in spring, and the bark around it becomes sunken, darkened and slightly flattened compared with the healthy bark either side. Small amounts of gum on a cherry after a physical injury are normal; gum coming out of an area of sunken bark is not.
Above the canker, the branch suffers. Leaves come out small and pale, or the shoot fails to leaf out at all in spring while the rest of the tree is in growth. A canker that reaches all the way around a limb kills everything beyond it. On the trunk, the same process kills the tree.
The bacteria enter through wounds and through leaf scars in autumn, and they are spread by rain splash in cool wet weather. This is why winter pruning is so costly, why mower damage at the base is worth real trouble to prevent, and why a badly drained or stressed tree is far more likely to get it than a well sited one.
Treatment is surgical and partial. In dry summer weather, cut the affected branch back at least six inches (15 cm) beyond the last discolored tissue, disinfecting between cuts. There is no useful spray for a garden tree. Then work on the conditions: relieve compaction, improve drainage, provide shelter, stop feeding nitrogen, and keep machinery away from the bark. Trees under less stress wall off cankers and live with them for years.
Do not try to save a cherry with a canker that has girdled the main trunk. Once the sunken dead bark runs all the way around, everything above it is finished, and the sensible decision is to remove the tree rather than spend three years watching it die branch by branch.
Blossom wilt, shot hole and the problems that look worse than they are
In a wet flowering season, flowers on some spurs turn brown, collapse, and then hang on the tree instead of dropping, and the short shoot behind them dies back too. That is blossom wilt, a fungus that comes in through the open flower in damp conditions. It looks dramatic on a tree in full bloom but rarely threatens the tree itself. Cut the dead spurs out in summer, and remove any shriveled mummified fruits, because those are where the fungus overwinters.
Shot hole is the other cosmetic complaint. Small brown spots appear in the leaves and then the dead tissue falls out, leaving the leaf peppered with clean round holes as though something had shot at it. It follows bacterial infection or wet spring weather and it does not need treatment. Rake up and remove fallen leaves in autumn on a tree that had it badly.
Cherry blackfly is a dense black aphid that colonises the shoot tips in late spring, curling and blackening the new leaves. On a mature tree it is unsightly and irrelevant; the tree grows through it and predators arrive within a few weeks. On a young tree, hose the colonies off repeatedly or rub them out by hand. Do not reach for a broad spectrum insecticide, which kills the ladybirds and hoverfly larvae that would otherwise finish the job. The underlying reason an outbreak takes hold is soft growth, which usually means nitrogen.
Slow decline, and knowing when it is not a disease
Flowering cherries are not long-lived trees. Depending on the form and the site, a garden cherry has a useful life of roughly twenty to fifty years, and some of the most heavily flowering forms are at the short end of that. A tree of that age with sparse growth, a thinning crown, some dead wood in the top, and a shorter flowering season each year is doing what cherries do. It is not a failure of care and there is no treatment that reverses it.
The way to distinguish decline from an active disease is the pattern. Disease is asymmetric and fast: one branch silvers, one limb gums and dies, one side leafs out and the other does not. Decline is even and slow: annual shoot growth drops year by year across the whole tree, the flowering gets thinner everywhere at once, and small dead twigs accumulate throughout the crown rather than in one place.
The third pattern is root damage, which arrives one to three seasons after the event and shows as dieback starting in the very top of the crown and working down. Ask what has been done over those roots: paving, trenching, a border dug, materials stacked or vehicles parked will explain more crown dieback than any pathogen.
A declining mature cherry is still worth having while it flowers. Take out dead wood in summer, keep the root area mulched and undisturbed, stop feeding, and plan its replacement rather than trying to rescue it. Plant that replacement somewhere else if the old tree died of silver leaf or canker.
Keeping a problem log that is actually useful
For anything you notice, record the date, which branch, which way it faces, and what the weather had been doing for the previous fortnight. Silver leaf, canker and simple drought stress can all produce a poorly branch, and in the moment they are easy to confuse, but their records look nothing alike: silvering follows a wound, canker follows wet cool weather and shows gum, drought follows three dry weeks and affects the whole tree.
Log every cut you make to remove diseased wood, with the date and diameter, and note whether the wood was stained inside. If the same limb needs cutting again the following year, you did not get beyond the infection the first time, and that only becomes visible in writing.
Finally, measure the length of the past year's shoots on two or three branches each winter and write the number down. A steady figure means a healthy tree whatever else is going on; a figure that halves over three years is the clearest early warning a cherry gives you, and it is invisible without the record.