Powdery mildew, Microsphaera alphitoides (syn. Erisyphe alphitoides), on a young leaf of pedunculate oak, Quercus robur, in September in the UK. The leaf is from a shoot 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 leaf surface 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