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Step out and have a look at your local conker trees… if you live in England or Wales you may be surprised at what you see. Some of them look like they are shedding their leaves prematurely.
Closer examination suggests otherwise.
This season’s leaves are normal over some of their area, but something is attacking them, causing them to go brown and shrivelled.
This twig is actually regrowing new leaves where the previous ones fell off. The culprit is the Horse Chestnut Leaf Miner moth, Cameraria ohridella. This is the time of year when the damage caused by the leaf miner is becoming apparent. The caterpillars of the moth get in between the front and back of the leaves and nibble away at the soft flesh inside, hollowing it out (that’s the big white spots on the leaf in the photo). The moth can have up to six generations in a growing season, so the caterpillars just keep coming back while the leaves are still on the tree. This can affect the whole leaf too, and towards the end of the season it may die. When the leaf dies and falls off the caterpillar pupates inside the leaf, emerging in the spring as an adult to mate and lay eggs on the new leaves. The Horse Chestnut Leaf Miner was first observed in Macedonia in northern Greece in the late 1970’s, and was described as a new species in 1986. It spread west and was first found in the UK in Wimbledon in 2002. It now occurs commonly in horse chestnuts throughout central southern and eastern England, and is spreading steadily at 40-60km per year. Despite the alarming visual effect of the moth, it apparently has no long-term detrimental effect on the trees. However, as most horse chestnuts are planted for their visual impact, the miner attacks are still a serious problem. The best treatment is to clear up the fallen leaves and burn them, destroying the pupae, although this won’t get rid of the disease, but it might keep it under control.
Perhaps more alarmingly, there is another, unconnected, threat to horse chestnuts, Bleeding Canker. This infection causes sap to ooze from the tree, attracting secondary infections and causing all sorts of problems. It’s becoming a lot more common in the last few years. Luckily this isn’t yet to be seen on the trees in The Ranger’s road, but it is around on the Island and elsewhere in England. Survey results show that in 2007 around half the horse chestnut trees in Britain showed some degree of symptoms. It was originally thought to caused by a fungal disease called Phytophthora, but is now thought to be most often due to a bacterial pathogen, Pseudomonas syringae pv aesculi – although there is still some debate about that. Unlike the leaf miner, bleeding canker does kill trees, and many thousands have already succumbed. Not all die though: many chestnut trees have the infection and survive for years, although there is an increased risk of branches failing and dropping off, which can obviously be a problem in public areas.
Because of these two problems, quite different but both impacting on horse chestnuts, there does these days seem to be an understandable view amongst some tree experts that it’s just not worth planting conker trees any more. Even the Forestry Commission says:
In a few places where mature trees have succumbed to bleeding canker and been replaced with young horse chestnuts, some of the replanted trees have shown signs of infection within a few years. Therefore, replanting with the same species is not recommended.
So, enjoy your conkers while you can. Your children and grandchildren may not be so lucky.