Gadenin日本語Get the appChestnut blight: what a canker looks like
Chestnut blight is a bark fungus that enters through wounds and then girdles the branch it is on. The first sign is usually not on the bark at all but in the canopy: a single limb whose leaves go yellow, then brown, and hang on dead through the summer while everything around it is green. Follow that limb back toward the trunk and somewhere along it you will find the canker.
On young smooth bark the canker is a sunken orange-brown patch, sometimes swollen and split at the edges, often speckled with pinhead-sized orange pustules. On older bark it is harder to read: a rough, cracked area with the bark sitting slightly proud. Cut a sliver of bark at the edge of the suspect patch. Healthy chestnut is green and moist underneath; blighted tissue is dry, brown and dead right down to the wood. Below the canker the tree often throws a burst of vigorous shoots off the trunk, which is a distress signal, not a recovery.
There is no spray that cures it. What works is speed and hygiene. Cut the affected limb out well below the canker, at least eight inches (20 cm) into clean green tissue, in dry weather. Burn or remove the wood; do not chip it under the tree. Disinfect the saw before and after, and again between every cut. Wounds are the entry point, so keep mowers and string trimmers away from the trunk, avoid unnecessary pruning, and never leave a torn stub after a limb breaks in wind. If a canker sits on the main trunk of a young tree there is nothing to cut back to, and removing the tree protects the rest of the planting.
Weevil grubs inside the nut
You crack an outwardly perfect nut and find a fat, legless, cream-colored grub with a brown head and a tunnel of frass where the kernel should be. That is a chestnut weevil larva, and the reason it is a surprise is that it was laid inside the nut months earlier. The adult is a small brown weevil with a remarkably long curved snout, and it uses that snout to drill through the burr and the developing shell in summer to lay a few eggs directly into the kernel. Nothing visible happens on the outside.
The grubs eat through the autumn, then chew a clean round exit hole about a sixteenth of an inch (1.5 mm) across, drop out of the fallen nut, and burrow into the soil under the tree, where they wait to emerge as adults the following season or the one after. That life cycle is what makes the control obvious: everything happens under your own tree.
The float test at harvest catches most infested nuts before they reach your store, because the grub has already removed enough kernel to change the density. A hot water dip is the traditional next step: hold the sorted nuts in water at 120F (49C) for twenty minutes, then cool and dry them well before curing. That kills eggs and small larvae inside the shell without cooking the kernel, and it also knocks back surface molds.
The long-term fix is sanitation. Pick up every nut, every day, including the split, the rotten and the obviously grubby ones, and destroy the culls rather than throwing them into the hedge. Nuts left lying are grub factories that reinfest you next year. Keeping poultry or pigs under the trees after harvest works well for the same reason, because they clear the fallen culls and disturb the overwintering larvae in the soil.
Do not compost culled nuts in a cool heap. The grubs survive it comfortably and you will be spreading them back under the tree with the compost.
Root and collar rot on wet ground
Ink disease is a soil-borne water mold that attacks chestnut roots and the base of the trunk on ground that stays wet. Above ground it looks like slow decline: small pale leaves, sparse growth, dieback starting at the top of the crown, and a tree that goes backwards over two or three years rather than dying suddenly.
The confirming sign is at the collar. Look for dark inky-black staining seeping from cracks at the base of the trunk, and cut into the bark just above soil level: rotted tissue is dark and often has a sharp line between dead and healthy wood, sometimes with a violet tinge.
There is no treatment once a tree is affected. Everything you can do is done at planting: acid but well-drained soil, a raised mound on heavy ground, no irrigation ponding at the collar, and mulch kept clear of the bark. If one tree goes down to it, do not replant a chestnut in the same spot.
The rest, and what to do about each
Most other chestnut troubles are cosmetic or self-limiting, and knowing which is which saves you a lot of unnecessary spraying. The table below covers what you are most likely to meet on a garden tree, in the order you are likely to meet it through the year.
| Symptom | Likely cause | What to do |
|---|---|---|
| Whole limb browns in summer, leaves hang on dead | Blight canker on that limb | Trace back to the canker, cut 8 in (20 cm) into clean wood, burn the prunings |
| Sunken orange bark patch with pinhead pustules | Blight canker | Remove the limb; if on the trunk of a young tree, remove the tree |
| Round 1.5 mm hole in a stored nut | Weevil grub has exited | Discard, float-test the whole batch, tighten daily pickup next season |
| Nut floats but shell is intact | Grub damage or an unpollinated blank | Discard; if most of the crop floats, check the pollination pairing |
| Green swollen galls on shoots and leaf stalks in spring | Gall wasp | Cut out and burn galled shoots in late winter; a light attack is tolerable |
| Burrs shed in quantity in midsummer, contents undersized | Drought stress after flowering | Water deeply in dry spells; mulch out to the drip line |
| Inky black seepage at the trunk base, crown thinning | Root and collar rot on wet soil | No cure; improve drainage, do not replant chestnut on the spot |
| Leaves yellow between green veins, poor growth | Soil pH too high for chestnut | Test pH, use acidifying mulch; never lime the root area |
A written record beats memory
Chestnut problems build over years, which is exactly the timescale memory is worst at. Each season note the date you first saw a dead limb and where on the tree it was, the position of any canker you cut out, and the percentage of the crop that floated at harvest.
Those three numbers, kept for four or five years, answer the questions that actually matter: is the blight spreading through the planting or holding still, and is the weevil pressure rising because your pickup has slipped. A photograph of each canker with its date, taken from the same spot, is worth more than a page of description when you are trying to judge whether a tree is losing ground.