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Horse chestnut leaf blotch is an unsightly, but not seriously damaging, disease of white- and red-flowered horse chestnuts. It causes irregular brown blotches on leaves.
Leaf blotch is an infection of the leaves of horse chestnuts by the fungus Phyllosticta sphaeropsoidea, which causes irregular brown blotches, often with yellow margins. You're most likely to see attacks in summer.
It is specific to horse chestnuts and both the white- and red-flowered forms are affected.
You may see the following symptoms:
The symptoms may be confused with those of horse chestnut leaf mining moth, the larvae of which tunnel within the leaves but mainly between two of the main lateral veins, giving a more elongated lesion.
Horse chestnut leaves may also gradually turn brown and shrivel all round the edge, looking like severe water stress. This is not leaf blotch, and the cause of this condition is as yet unknown.
Although horse chestnut leaf blotch can be unsightly, the disease is not seriously damaging. Where control is needed, there are a few things you can try.
The fungus was introduced accidentally into the UK from North America in the last century.
The life cycle has not been studied in detail, but the fungus is known to pass the winter in fallen leaves, releasing airborne spores in the spring. These infect the leaves and produce a second type of spore in the infected tissues. These spores are responsible for most of the infections, which are worse in wet conditions.
Although it can be unsightly, the disease is not seriously damaging. Severe attacks tend to build up in late summer and may be mistaken for early development of autumn colour. The disease is more prevalent in the south and west of the UK.
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