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Disease Overview

July 10, 2002

Frequently, callers to the Plant Clinic request information about currently active diseases. Sometimes con-sultants want this information before making client field calls. Other times, growers are trying to diagnose their own problems. Many times, master gardeners are just trying to get a handle on diseases before staffing the county phone lines. Here are some diseases we have seen frequently this summer.

We have seen many cases of root rots on herbaceous hosts. Earlier in the season, Pythium was the most common because of wet weather. In recent drought stress, Rhizoctonia root and crown rot has appeared. Both diseases cause root loss and stress in hot, dry weather.

Dutch elm disease and oak wilt have been confirmed in a few cases. Calls about elms and oaks have been numerous. We have not seen too many cases of pine wilt but expect that soon.

The early season leaf diseases were abundant this year because of the cool, wet spring. This includes anthracnose of many hosts, including ash, oak, and maple. We saw Septoria on dogwoods and leaf blisters on oak as well. New leaves have looked better, without infection, but old leaves remain for the season. These should be raked when possible.

Fire blight has been very common on crabapple, apple, pear, and a few other rosaceous plants. This damage followed by heat and drought may contribute to the decline of many woody plants.

We have seen many juniper samples this season, but none of those have been infected with Phomopsis blight. Kabatina blight has been common; this fungal disease invades wounded or weakened tissue. It is speculated that many junipers suffered winter injury followed by Kabatina infection.

Cedar-quince rust has been very common on haw-thorn fruit. This is a good year to get a picture of that rust. You won’t have to look hard to find it. The same goes for Sphaeropsis blight of pines, black spot of rose, Cytospora of spruce, and slime molds in general. Chlorosis on river birch started particularly early this year and may be stressing these plants enough to cause branch dieback.

Author: Nancy Pataky


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