Abstract
In a survey from 2001 through 2006 of host range, distribution and other factors affecting powdery mildew diseases on various plant species in seven mountainous regions of Akita Prefecture (including the environs of Akita and Daisen cities), 126 plant species representing 48 families were recorded as host plants. Most host species in the northern region of Akita Prefecture were in the Caprifoliaceae, whereas in the central and the southern regions they were mostly in the Compositae. Nine species were newly recorded as host plants in Japan, with two species each in the Rosaceae, Caprifoliaceae and Compositae, and one species each in the Guttiferae, Araliaceae, and Ericaceae. Powdery mildew fungi belonging to 10 genera (7 genera in a new taxonomic system) were isolated from host plants, predominantly from genera Uncinula (Erysiphe section Uncinula and Parauncinula novo systema) and Microsphaera (Erysiphe section Microsphaera). The numbers of diseased host plants varied considerably by region. Akita Prefecture, where more than 30% of the potentially susceptible host species were infected, had three characteristic features: average rainfall from June through September was less than 166 mm/month; average wind speed from June through September was greater than 1.2 m/s; and average soil calcium exceeded 1900 mg/kg. Disease incidence was low in the Oga region, where soil calcium was as low as 1400 mg/kg, despite having the other criteria for high incidence. Therefore, the main factor affecting disease incidence may be soil calcium.
