Powdery mildew
Today oak powdery mildew is one of the most common diseases in European forests but it is thought to not always have been present in these forests. The first reports of the disease were made in Paris and other regions of France, Spain, Luxembourg and the Netherlands in 1907. By 1908 the disease had spread into many other European countries, becoming an epidemic. By 1909 it had reached Russia and Turkey, followed by Brazil in 1912 and soon after was distributed around the world. The sudden outbreak of the disease puzzled mycologists as they were unsure about the origin of the pathogen. Its anamorph was easily distinguished from "Phyllactinia guttata" which had previously been reported to cause powdery mildew on oaks in Europe at low intensity. It did however share morphological similarities with "Oïdium quercinum", "Calocladia penicillata" and "Microsphaera penicillata" which had previously been reported to cause powdery mildew on oaks in Europe. The sudden appearance and high incidence of the disease made it unlikely that it was caused by any of these species however and instead some authors hypothesised that it was caused by the introduction of a new species from outside Europe. Powdery mildew had already been reported from North America so it was thought that the new disease could have been imported on American oaks into Europe. The absence of the disease on American red oaks growing in France however, made this unlikely. Four years after the initial outbreak, a teleomorph was found in south-east France which Arnaud and Fox identified as an American species, "Microsphaera quercina" (now included in "Microsphaera alni"). A detailed morphological study by Griffon and Maublanc in 1912 suggested that the species was different from all previously described species and proposed the name of "Microsphaera alphitoides", referring to the floury appearance of the abundant white sporulation. This identification remained controversial until the 1940s. The origin of the disease was still uncertain however, in 1927, Raymond suggested that the disease may have been caused by a fungus identified in 1877 in Portugal that had been imported from Portuguese colonies. In 1980, Boesewinkel demonstrated that the same species was responsible for powdery mildew on "Quercus robur" and on a mango species from New Zealand, supporting the hypothesis that the fungus had shifted its host.