ISSUE 4 May 25, 2000
The moderately cool, wet weather seen across much of North Dakota over the last few weeks is causing significant anthracnose infections and subsequent defoliation of ash trees. Currently, we are seeing small purple spots with tan centers on leaves and some defoliation. Over the last two years, ash anthracnose caused significant defoliation in parts of eastern, central, and southwestern North Dakota. Last year, some trees lost nearly 100% of their leaves in eastern North Dakota; however, they did refoliate by midsummer.
The ash anthracnose fungus (Gnomoniella fraxini) overwinters in fallen leaves and twigs and in rachises (long, central part of compound leaves), fruit, and twigs which may be retained in trees throughout winter. Under wet conditions in the spring, spores are produced that infect succulent new growth. Cooler weather favors disease development. Dry weather reduces spore production and disease development. Leaves, shoots, twigs, branches, and seed coverings are infected.
Small purple spots with tan centers may develop on infected leaves (as we are seeing this year); however, the classic symptom is brown, dead, distorted leaf tissue. These leaves may have brown blotches associated with leaf veins and they often fall prematurely. Multiple years of heavy defoliation can stress trees which may cause stunted growth and dieback.
Destroying fallen leaves and dead twigs may reduce infections in
subsequent years. In areas where trees are repeatedly infected by anthracnose, protectant
sprays may be necessary. Three applications work best, and timing of fungicide is critical
for controlling disease development. The first application should be made as buds are
beginning to swell, but before buds break. The second application should be made when the
buds show green tips, and the third fungicide application should be made when the leaves
are half grown. The recommended spray treatment should be lime sulfur first, followed by
treatments with a chlorothalonil product. Treatments associated with the second flush of leaves after defoliation may reduce late season infections. Always follow pesticide labels.