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Disease Briefs, 7/22/98

July 22, 1998

The primary infection cycle for crabapple scab is the initial wave of spores that overwinter and infect new, succulent growth in the spring. This primary infection time is usually targeted for disease control with applications of commercial fungicides. This year the continual spread of the fungus from leaf to leaf on the same tree (secondary cycle) lasted longer than usual because of cool, wet conditions. Now we are seeing extensive defoliation from scab. This is nothing new, but it certainly is a reason to replace old, susceptible varieties with newer resistant varieties. Seek out varieties with resistance to scab, rust, and mildew.

Hawthorns have been heavily infected with cedar-quince rust. This rust causes fruit infection, but not leaf spots, on hawthorn. Hawthorns with leaf rust are most likely infected with cedar-hawthorn rust.

Sycamores were hit very hard by sycamore anthracnose fungi this year. Although it appeared for a while that the trees would not recover, most of those with severe cases that I have been watching seem to have recovered--they have many large, healthy leaves. A few, however, still look quite thin and it appears that they will lose some branches this year.

Sphaeropsis blight of pine can be seen readily in Illinois. The extensive wet weather has created an ideal environment for the fungus. Pines that receive plenty of moisture are less likely to be infected than drought-stressed pines. Despite the more-than-adequate rains, at least in the midsection of the state, 1998 has produced a bumper crop of Sphaeropsis blight.

Author: Nancy Pataky


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