Guignardia blotch is a foliage disease of many Aesculus species, including California, Ohio, red, and yellow buckeye and common, red, and Japanese horse chestnuts. Bottlebrush buckeye (A. parviflora) and some varieties of Ohio buckeye (A. glabra varieties arguta, monticola, and sargentii) may be resistant.

The leaves of affected plants develop large reddish-brown blotches surrounded by yellow tissue, often causing the foliage to curl and brown. In severe cases, leaves may fall prematurely in late summer. Since the foliage is usually not badly damaged until after the tree has completed much of its annual growth, the disease is primarily aesthetic.

Wet weather promotes the growth of Guignardia blotch. In the United States, this fungal disease only occurs east of the Great Plains, perhaps because of the drier conditions of the West. This disease may become severe in nurseries or in plantings where tree crowns are close together.

Symptoms and Diagnosis

Guignardia blotch first appears as water-soaked irregular areas. These enlarge quickly and in a few days are reddish-brown to brown leaf spots with clear bright yellow margins. The blotches vary in size and may frequently grow together, covering large areas of leaf tissue. This causes the leaf to curl and brown, becoming dry and brittle, and may cause early leaf drop. Fruiting bodies of the fungus, seen as black pinhead-sized specks, may be visible in the lesions. Occasionally petioles and immature fruit may also have small reddish brown lesions. Symptoms of this disease are similar to those of environmental leaf scorch. Leaves affected by scorch will be on the sunny or windy side of the tree while Guignardia blotch may affect most leaves. Scorched leaves will not have the black fruiting bodies. See also “Scorch of Trees and Shrubs”.

Life Cycle

The Guignardia fungus overwinters on fallen Aesculus leaves. In early spring, its fruiting bodies mature and during wet weather, they release spores into the air. If the spores land on newly developing susceptible leaves that remain wet for several hours, the leaves become infected, resulting in blotches within 10–20 days. New fruiting bodies develop in early June and in wet weather, more infections may continue throughout the summer.

Integrated Pest Management Strategies

1. Sanitation. Rake and dispose of infected leaves.

2. Pruning. Thin the tree canopy to improve air circulation and to speed drying of leaves.

3. Fungicides. Chemical treatment is usually not necessary or recommended in the home landscape. Serious damage may be controlled by applying a fungicide containing chlorothalonil or mancozeb at bud break and then repeating the application at 10 to 14 day intervals as long as wet conditions continue.

Organic Strategies

Strategies 1 and 2 are strictly organic approaches.