Powdery mildew, Microsphaera alphitoides (syn. Erisyphe alphitoides), on young leaves of pedunculate oak, Quercus robur. The picture shows the shoot tip of an oak tree in September in the UK. The shoot is young; part of the so-called lammas growth that occurs in summer (Lammas day is 1st August, the Celtic harvest festival). The mildew forms a white covering on the upper surface of young leaves, and is conspicuous from a distance. The covering consists of a mat of fungal hyphae and conidia, attached to the leaf by haustoria. The presence of the mat leads to a lowering of photosynthesis in the infected leaf. This does not cause much harm to mature trees because only a small proportion of their leaves are affected. It may however contribute to the failure of seedling oaks growing in low light conditions, such as in natural woodland