Horse chestnut leaf blotch

Horse chestnut leaf blotch is a common fungal disease caused by Phyllosticta paviae. This pathogen primarily affects the foliage of horse chestnut trees, leading to premature leaf shedding and a reduction in the overall vigor and aesthetic value of the affected specimens.

The primary host for this fungus is the common horse chestnut (Aesculus hippocastanum). While the disease rarely kills mature, healthy trees, it can significantly weaken young or stressed trees, making them more susceptible to secondary pests and environmental stressors like drought or poor soil conditions.

Symptoms typically manifest in early to mid-summer as irregular reddish-brown spots on the leaf surface, often surrounded by a yellow halo. As the infection progresses, these spots can enlarge and coalesce, causing the leaf tissue to shrivel, curl, and turn brown, which gives the tree a scorched appearance.

The development of the fungus is heavily dependent on moisture and humidity. The spores (pycnidiospores) are produced in the small black fruiting bodies located within the infected leaf spots. Rain splash and wind disseminate these spores to healthy leaves, where they initiate new infections under favorable weather conditions.

Management strategies focus on sanitation and chemical intervention when necessary. Because the fungus survives the winter in fallen leaves, raking and destroying all leaf litter is critical to breaking the disease cycle. In severe cases or high-value landscapes, applying fungicides during the leaf-expansion phase can protect the tree from initial infection.