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What is the disease that is attacking horse chestnut trees? Can I do anything to prevent it?

Horse Chestnut Tree leaf damage

In recent years, horse chestnut trees Aesculus hippocastanum have been displaying signs of ill-health. Symptoms include bark splitting, 'bleeding' from trunk and branches, branch die-back and leaves browning, shrivelling and falling in summer. These symptoms have been observed in trees throughout England and Wales, although the majority of cases have occurred in south and south-east England.

At first these symptoms of ill-health were all thought to relate to one problem, however, we now know they are caused by two entirely different factors. The first is predation by the Horse Chestnut Leaf Miner which is larvae of the moth Cameraria ohridella, and the second is a disease called Bleeding Canker Pseudomonas syringae pv. aesculi.

The Horse Chestnut Leaf Miner was first recorded in Greece in the late 1970s and since then has spread right through Europe. It was first observed in the UK on Wimbledon Common in July 2002 and is now well established in an area which extends westwards to Cardiff, northwards to Derbyshire, eastwards to Norfolk and is increasingly found outside this area too. In early summer the larvae begin to eat their way through the leaves, causing them to initially turn mottled brown, then shrivel and then fall early, sometimes months before autumn. Although very unsightly, this on its own is not causing a significant decline in the trees health, although continuous predation, year after year, can eventually weaken the tree and make it more likely to succumb to diseases such as Bleeding Canker.

Although chemical insecticides have shown some success in reducing the population of this predatory insect, it is not a recommended solution, being difficult to apply safely and efficiently, especially on larger trees. Even when applied, the chemical applications are seldom successful for more than one or possibly two years, so there would be the need to reapply the chemical on a regular basis.

In a garden situation, the best control is to remove and burn all fallen leaves during the autumn. The pupae of the moth overwinter on the fallen leaves and so by destroying the leaves you are destroying the pupae. Composting the leaves is not so effective, as the heat generated in compost heaps does not always reach the required temperature to kill the pupae.

Bleeding Canker Disease of horse chestnut is a more serious problem. It causes tree bark to become infected, which then splits and start to 'bleed' a sticky red-brown liquid. As the liquid ages it will dry out to form a black soot-like deposit. Although some trees have been known to recover from the infection, or at least manage to keep the disease under control, in the majority of cases branch die-back will occur followed by death of part, or in severe cases, all of the tree. Affected trees have been found as far north as Glasgow in Scotland and westwards into Wales. There is currently no chemical control available to combat this disease. In the early stages of the disease, removal and burning of infected branches can slow down the spread of the disease, but if the disease continues to spread then consider complete removal of the whole tree because it may become a source of infection for other horse chestnut trees in your area.

If in any doubt as to the reason for ill-health of trees in your garden, it is always advisable to contact the Arboricultural Association they can provide you with excellent advice and hold a directory of recommended professional arborists and tree surgeons in your area.

Tony Russell
Tony Russell is widely regarded as one of Britain's leading authorities on trees and shrubs. Over the past 11 years he has presented many gardening, horticultural and countryside programmes for both TV and radio. Tony regularly writes for several magazines and newspapers including 'The Daily Telegraph', 'The Garden', 'Country Life', The English Garden' and 'BBC Gardeners' World Magazine', and he is the author of 13 books and is the editor of the annual publication 'Great Gardens to Visit' which was nominated for the 'Best Printed Information Product of the Year' in 2008. More information at www.gardenstovisit.net