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Bulletin P

Abstract

In the past 5 years wilt has come to be recognized as the most important oak disease in the upper Mississippi valley. The disease has been reported from Iowa, Wisconsin, Minnesota, Missouri and Illinois. The fungus Chalara quercina Henry has been shown definitely to be the cause of the disease.

Trees of the red oak group are killed quickly, usually within 30 to 60 days after wilt symptoms are apparent. Those of the white oak group may survive several years after infection with only a few branches being killed each year.

All types of oak apparently are susceptible since all of the 28 species inoculated in the greenhouse were infected. The disease has been found on 9 of the 11 native species in Iowa.

Infected trees have not been observed to recover from oak wilt. Prevention of spread of the disease has been attempted by various methods of sanitation. In general, removing diseased trees as soon as symptoms appear has been effective in stopping or retarding spread in state park and state forest test areas. Pruning experiments on trees that had only a few infected branches indicate that white oaks may often be saved by removal of such infected branches at a point well back of the latest symptoms.

Frost injury, leaf blight and insect and rodent injury often result in leaf discoloration which from a distance may be mistaken for oak wilt.

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