A common type of fungal leaf spot on ash trees is anthracnose. This disease, caused by the fungus Discula sp., survives winter on infected plant tissue. During the spring and summer when the weather is cool and wet, spores are produced on infected tissue. These spores are spread by wind and splashing rain to buds, shoots, and expanding leaves where new infections occur.

Leaf spots are irregular in shape and usually appear brownish in color. Infections early in the season may result in large necrotic (dead) areas, blotching, and/or distortion of leaves. Leaf infections later in the season result in discrete necrotic spots. Severe leaf infection can cause extensive defoliation. Anthracnose is most severe on the lower and inner portions of the tree where the humidity and moisture levels are higher. This helps distinguish it from wilt diseases and root problems where symptoms develop first in the top of the tree.

Anthracnose infection on ash can cause large necrotic areas and distortion of leaves. Photo credit: Chad Behrendt

In most cases, anthracnose does not cause permanent damage to established trees. However, trees stressed by root restrictions, drought, heavy insect infestation, etc., are much less tolerant of anthracnose and may show decreased vigor after only a single season of severe anthracnose defoliation. Cultural control measures should include proper watering, mulching, and sanitation procedures. Raking leaves in the fall and pruning out dead or dying branches help reduce the number of new infections the following year. Fertilizing stressed trees in the spring may also help boost tree vigor.