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May 7, 2003

Anthracnose will be visible on several tree species soon. The fungus infects as buds open and requires warm, wet conditions for infection. Succulent new plant growth is most susceptible. Older leaves and drier conditions later in the season usually retard repeating cycles of infection. Anthracnose causes water-soaked spots; young lesions from dark green to brown; and possibly some stem cankers on ash, oak, maple, and sycamore. Other trees are susceptible to varying degrees, but the trees listed are those on which we traditionally see anthracnose in Illinois. Dogwood anthracnose is much more severe but occurs later in the season.

Rust galls of cedar-apple rust are now fully swollen and full of spores in central Illinois. Spores are most likely moving from cedars to their alternate hosts as this article goes to press. Keep this in mind if you are fighting cedar-apple, cedar-hawthorn, or cedar-quince rusts.

It is very tempting to prune out any dead wood in trees and shrubs as it appears. In most cases, I would agree with that practice as long as conditions are dry. Pruning in wet weather allows pathogens to survive and move on equipment. In addition, wetness allows the pathogen to remain alive or possibly germinate on the cut surface to which it has been inoculated. There is one other exception to the rule. In areas where oak wilt is present, do not prune oaks in the spring or early summer. Sap on fresh cuts attracts beetles that may be carrying the oak wilt fungus to your tree. Oaks should be pruned in late summer or the dormant season.

Author: Nancy Pataky


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