Tree health bulletin Posted on August 27, 2008

How bad is this horse chestnut problem? Everyone wants to know. They are looking grim all over the south of England, at least; the leaves on their lower branches, and sometimes higher, motheaten and brown. They are worse this year than last. And some trees are dying of a distressing plague that makes their trunks bleed sap.

They are show trees, horse chestnuts, prominent in parks, solo performers in essential avenues. Paris depends on them, perhaps, more than London does, but from Hyde Park to the village smithy they are crucial to our landscape. And they are in danger.
It is not one disease that is threatening them – unlike the elms that 30 years ago were destroyed by a fungus carried by a parasite (and are still dying from it). The fatal chestnut disease is a canker. But the disfigurement you see everywhere is not connected. A little leaf-miner moth (or rather its larvae) is the culprit – first seen in this country in Wimbledon in 2002, and since moving on at about 40 miles a year. It feeds on the lower branches, eating the leaves from the inside, starting in late spring and moving higher up the tree – or on to another. It rarely reaches the top of a tall tree (so far) before leaf-fall. Then it winters on the ground until next spring. The only practicable measure you can take is to burn, bury or compost the leaves to prevent the critter (Cameraria is its name) from re-offending next spring.

Aesthetic disaster though it is, the leaf-miner will not kill (or even substantially weaken) a tree. Hopefully biological control measures will soon be available (says the Forestry Commission).

The canker is more serious, and more mysterious. Many tree species have cankers and many agents are at work. The chestnut one apparently started with a Phytophthora (related to the scary Sudden Oak Death) but is now blamed principally on something called Pseudomonas syringae var. aesculi. It causes weeping lesions in the bark, which become dangerous if they girdle the trunk – or indeed a major branch. Once girdled it dries out and is liable to break. Big specimens often survive, only partially girdled, and form strong scars. Small trees are more apt to die.

No doubt it is serious threat, with nearly half a million horse chestnuts in the country, but neither of these problems should be used as an excuse for felling trees. We have already seen it happen in this village: a farmer sees an unhappy-looking tree, unilaterally declares it dangerous and cuts it down. But then he’d like to do that to all his trees.

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