Q What is guinardia leaf blotch?

A It is a fungal infection that produces large brown leaf spots.

Caption: The damage looks horrible but is mostly cosmetic

Q How do I recognise guinardia leaf blotch?

A The leaf blotch develops as large, red-brown patches, irregular in shape, and concentrated at the tips and edges of the leaves. Badly affected leaves roll inwards along the leaflets and may fall prematurely.

Q Where does guinardia leaf blotch occur?

A Guinardia leaf blotch is widespread and common in the southern half of England, but is less often seen further north.

Q What causes guinardia leaf blotch?

A The fungus Guinardia aesculi is the fungus responsible for this leaf blotch.

Q What plants does guinardia leaf blotch attack?

A It only attacks horse chestnuts.

Q What damage does guinardia leaf blotch do?

A The damage is mostly cosmetic, and trees do not appear to suffer any long term harm.

Q Could I mistake guinardia leaf blotch for anything else?

A Mines produced by horse chestnut leaf miner can look like fungal leaf blotches, but are usually a different shape, and are translucent when held up to the light. Guinardia blotches frequently have a yellow halo around them, typical of fungal infection, which is absent from the leaf mines.

Horse chestnut leaves sometimes suffer from browning along the edges for reasons that are not entirely clear but are likely to be related to water stress.

Q When do guinardia leaf blotch attacks occur?  

A Leaf blotches are most obvious in late summer, though the infections begin in spring.

Q How do I treat trees affected by guinardia leaf blotch?

A There is no effective treatment for the problem. The disease probably overwinters on fallen leaves, so clearing these away may help reduce future attacks, though spores can easily blow in from other areas.

Q Are there any horse chestnut varieties resistant to guinardia leaf blotch?

A The Japanese horse chestnut A. turbinata is rarely affected. The common horse chestnut, A. hippocastanum, red horse chestnut (A. x carnea) and Indian horse chestnut (A. indica) are all susceptible.