Recently, an unknown species of Phytophthora fungus was discovered as an underlying cause of Sudden Oak Death. A tree may be infected with Phytophthora for a number of months or years before exhibiting a sudden change in foliage...
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One possible reason is that a fungus called anthracnose is infecting the leaves.
Various species of anthracnose (Apiognomonia, Discula, Gleosporium, Glomerella, and Gnomonia) cause partial to total defoliation of trees in late spring or early summer. When symptoms are severe, we do not recommend treatment. Resistant plant material, good sanitation practice, fertilization to stimulate vigor and fungicide applications may provide some control but will not eradicate an existing infection. Moist weather in April and May increases susceptibility to this pathogen.