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.
What is horse chestnut leaf blotch?
Leaf blotch is an infection of the leaves of horse chestnuts by the fungus Guignardia aesculi, 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 are affected.
You may see the following symptoms:
- On leaves: Irregular brown blotches of dead tissue, sometimes with yellow edges. Severe attacks can cause the leaves to shrivel completely
The symptoms may be confused with the recently-arrived horse chestnut leaf mining moth, whose larvae 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 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.
- Raking up and burning or composting the fallen leaves in autumn will help reduce the amount of fungus available to initiate infections the following spring
- Consider growing less-affected horse chestnuts. The pathogen also infects Aesculus indica, but infections are less severe and lack the yellow margins. In the USA, A. turbinata is said to be rarely affected
- Chemical control is usually not required. In any case, it is impractical for most gardeners to spray large trees
The fungus Guignardia aesculi 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.