Anthracnose is a very common springtime disease affecting many different shade trees. Ash, oak, and maple are among the most severely affected. The name - Anthracnose - is given to a variety of shade tree fungi. These fungi cause similar looking leaf spots, and can also cause leaf distortion and defoliation.

Anthracnose typically appears on the leaves as dead, brown spots. The shape and size of these spots varies greatly. Leaves infected in early spring often become heavily spotted, curled, and twisted. If infection is severe enough, anthracnose also causes the leaves to drop. In contrast, late season infections usually appear on the leaves as small brown spots, with little to no distortion.

Anthracnose is most active in early spring when conditions are cool and wet. Frequent rainfall stimulates spore production and spreads them from infected leaves on the ground to healthy leaves in the tree. As this disease progresses it moves upward and outward from the inner, lower branches where moisture levels are the highest. Since anthracnose is highly regulated by rainfall, disease severity is greatest during wet years and lowest during dry years.

Anthracnose does not permanently damage trees, therefore, fungicide applications are not usually recommended. The best management strategy is to maintain healthy trees. Water trees during dry periods, fertilize based on a soil test and mulch. To help reduce the severity of disease next spring, rake and dispose of leaves in the fall.

Credit: University of Minnesota Extension, 1999

more info: http://www.na.fs.fed.us/spfo/pubs/fidls/anthracnose_east/fidl-ae.htm