Anthracnose on white oak had been common this spring. Leaf symptoms range from large areas of browning, especially on the leaf margins, to scattered small necrotic spots. The leaves have an overall scorched appearance. The lower branches tend to show the most severe symptoms.
Photos of oak anthracnose can be found at the Plant Disease Clinic website .
The disease is caused by the fungus Apiognomonia quercina and is favored by rainy spring weather. Oak anthracnose can occur over a wide range of temperatures, but lower temperatures promote the most severe symptom development. Midsummer conditions tend to cause outbreaks to subside.
Acervuli, fruiting structures of the pathogen, eventually become visible on the undersides of leaves, especially in the necrotic areas next to leaf veins.
Twig infections may also occur, causing dieback before the buds open in the spring. The fruiting structures on the dead twigs can provide a source of spores to infect emerging leaves.
Although unsightly, anthracnose is a minor problem on established trees. Basic cultural practices such as mulching, proper watering, and removal of fallen leaves will help maintain tree vigor. Remember, oaks should not be pruned during April, May, or June. Pruning wounds can attract the beetles that spread the oak wilt fungus.
Further information on oak anthracnose can be found in Pm-1280 Anthracnose of Shade Trees. This bulletin is available from your local county extension office or from the Extension Distribution Center, (515) 294-5247. You may contact the Distribution Center by E-mail at email@example.com. Please include publication title and ID number in your message.
This article originally appeared in the June 21, 2002 issue, p. 85.
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