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Horse chestnut problems
Horse chestnut leaf-mining moth (Cameraria ohridella)
New to Britain in 2002, this pest is now widespread in much of England and is spreading into Wales.
The main host is common horse chestnut (Aesculus hippocastanum) and some other Aesculus species. Symptoms include white or brown elongate blotches on the foliage from mid-June onwards. Caterpillars or circular pupal cocoons can be seen within the mined areas if the leaf is held up to the light.
The tiny adult moths lay eggs on the foliage. After hatching, the caterpillars enter the leaves and eat the internal tissues. There are three generations during the summer; by August the foliage may be extensively damaged, leading to early leaf fall. This pest overwinters as pupae in the leaf mines.
Spraying is not feasible on large trees. Collecting and burning fallen leaves in the autumn will reduce the overwintering pupae. Alternatively, the leaves can be composted in sealed bags that are kept closed until the following July, by which time the adult moths will have emerged and died. This can delay the build up of damage during the summer but is only worthwhile for isolated trees where most of the fallen leaves can be gathered up. Some chestnuts, such as Aesculus indica, A. x neglecta and A. chinensis are not attacked or suffer only slight damage.
Leaf blotch (Guignardia aesculi)
This is caused by the fungus Guignardia aesculi, which was accidentally introduced from North America early last century. Both common horse chestnut and the red flowered Aesculus x carnea are affected. Irregular brown blotches develop on leaves from midsummer onwards, often most frequent at tips and margins. The initial appearance is distinct from the leaf miner, but the end result is similar and damage by both may be mistaken for early autumn colour. The life cycle of the fungus has not been confirmed in the UK, but it probably overwinters in fallen leaves, liberates airborne spores in the spring and then water-dispersed spores from the infected areas in the summer. Destroying fallen leaves is the only practical control measure, but is unlikely to have much useful effect. Although a serious attack is unsightly, the overall health of the tree is little affected.
Bleeding canker
Cankers are infections of the bark of woody plants by fungi or bacteria. Some of the causal organisms stimulate the bark to 'bleed', releasing dark liquid which can stain the outer bark black.
Several species of Phytophthora cause this problem in horse chestnuts, but recently there has been a large upsurge of new cases in UK and in northern Europe which have been shown to be due to infection by a bacterium. This has not been recorded previously on chestnuts and it has not yet been conclusively identified. Cankers cause problems firstly because they kill the bark, allowing decay organisms to enter, and secondly because they can girdle the stem, killing all parts above the infection.
It may take several years for these problems to become serious and detection of bleeding canker is not automatically a reason to fell the tree. It should however be kept under careful observation. There are no chemical control measures available.
See www.forestresearch.gov.uk/fr/INFD-6KYBGV for further information.
Horse chestnut scale