Anthracnose is the most common malady of green ash in North Dakota. Ash anthracnose is caused by the fungus Gnomoniella fraxini. It infects buds, leaves and sometimes twigs. Spring infection of buds or expanding leaves causes necrosis and distortion of leaves, the most typical symptom of ash anthracnose. Often such leaves have irregular brown blotches associated with leaf veins (Figure 6). Whole shoots may be stunted and the leaves deformed or killed. Infections occurring later, after leaves have expanded, cause small brown circular lesions which may later expand or coalesce.
In some years anthracnose is associated with extensive defoliation of ash trees in May or June. The exact conditions under which defoliation occurs are not known, nor are the reasons why it occurs in some years and not others.
The ash anthracnose fungus develops best at temperatures of 60 to 70 F. Wet weather in May favors anthracnose development.