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Powdery mildew of oak is caused by the fungus Erysiphe alphitoides (also known as Microsphaera alphitoides) and it is a common foliar pathogen of oak throughout Europe. First found in England in 1908, it was considered a contributory factor in the oak dieback episode the 1920s.
Erysiphe attacks young leaves and soft shoots, covering them with a felty-white mycelium, causing them to shrivel and blacken. Mild overcast conditions are optimal for development of the disease, which usually appears in summer when warmer conditions prevail.
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