The oak mildew invaded Western Europa in the years 1908 and 1909. Since then this parasite, Microsphaera alphitoides Griff. & Maubl. (syn. M. quercina (Schw.) Burr.) has occurred regularly in the Netherlands on oak seedlings and oak coppice, mainly Quercus pedunculata Ehr. (syn. Q. robur L. ). After the appearance of the fungus its identity and also its mode of hibernation has been amply discussed. Perithecia with ascospores were discovered by Arnaud and Foëx (1912) in France. In the Netherlands perithecia were only occasionally observed, as their occurrence seems to depend on weather conditions during the summer months (Hartsuyker, 1939). Besides ascospores, if present, chlamydospores on old infected leaves described by Ferraris (1909) might be a source of infection of young leaves in spring (Petri, 1923). However, these observations were not confirmed by other investigators. Neger (1911), Peglion (1911), and somewhat later also Van Poeteren (1912) observed hibernation of the mildew in infested buds from which malformed shoots developed in spring, stem and leaves being covered with mycelium and conidia. Woodward et al. (1929) described the way the mildew spreads: conidia from these shoots infect young healthy leaves in the neighbourhood, on which circular colonies develop. The growth of these white patches stops rather soon, probably owing to thickening of the cuticle of the ageing leaves. As has been noted by Ditu et al. (1964) leaves are severely attacked up to the age of 20 days. A second outbreak may occur on the newly developed Lammas shoots. Van Poeteren (1918) gave a clear description of the two periods of spread.