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Oak Pests - A Guide to Major Insects, Diseases, Air Pollution and Chemical Injury

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Apiognomonia errubunda
Discula umbrinella

Importance. -- Severely affected oaks may be defoliated by midsummer, which reduces growth, predisposes trees to other diseases and makes the trees unsightly. White oaks are most severely affected. Anthracnose does not result in major losses in forests.

Identifying the Fungus. -- Small, black, cushion-like, fruiting bodies form on necrotic tissue where spores are produced. A beaked, flask-shaped, fruiting structure can be found on dead leaves.

Identifying the Injury (figure 67). -- Round to irregular, lightbrown to black areas appear on the leaf, most frequently along veins. Affected leaves often appear scorched, and may curl or twist and drop from the tree. Infrequently, cankers and dieback occur on small twigs.

Biology. -- The fungus overwinters in dead leaves. Spores (ascospores) are wind-blown to the new, expanding leaves and shoots. Another spore type (conidia), which reinfects other leaves or shoots, is then produced.

Control. -- Collect and dispose of fallen leaves and twigs. Reduce the density of branches to increase air movement. Fertilize to increase vigor and use fungicide sprays as needed.

Figure 67
Figure 67 (close-up)
Figure 67. -- Oak anthracnose; full leaf and closeup views.
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