Erysiphe alphitoides is a species of fungus which causes powdery mildew on oak trees. The fungus can be easily recognized trough the distinctive disease symptoms. It produces white powdery patches on shoots and buds of oaks. The disease progresses to form a felt-like, white mildew growth. Large portions of the plant, especially leaves, may be infected (Figure 1A-B). Late in the season, small spherical, dark brown-to-black fungal structures (cleistothecia) may be seen mixed within the felt-like mildew (Figure 1C).
The white mycelium growing in the leaves is in charge of extracting nourishment from the host plant through specialized hyphae that penetrate the epidermal cells known as haustoria. The teleomorphs are usually more distinctive and diverse than the anamorphs. In Erysiphe alphitoides, the cleistothecia have the asci arranged in a hymenial layer, resembling perithecia. Cleistothecia are normally black and usually, do not have a diameter much more than 0.1 mm (Figure 1C). A key characteristic of Erysiphe phytoid are branched flattened dichotomously appendages in the cleistothecia (Figure 1D). Oval shaped conidia can also be produced by the anamorph.