Geographic and temporal distributions of four genotypes found in Erysiphe gracilis var. gracilis, a powdery mildew of evergreen oaks (Erysiphales)

A survey of 402 samples of Erysiphe gracilis var. gracilis on evergreen oaks collected from a wide area of western Japan showed that they were divided into four distinct genotypes each forming a separate clade with high bootstrap support, which were referred to as E. hiratae (genotype I), E. uncinuloides (genotype II), E. gracilis s. str. (genotype III), and E. pseudogracilis (genotype IV) in a separate taxonomic treatment. However, there are no clear differences in geographic distributions among these four genotypes. Quercus myrsinifolia was only infected by genotype II and Q. salicina only by genotype IV, whereas Q. glauca was infected by all four. These results strongly suggest an association between host species and speciation of these genotypes. A further 312 samples of Q. glauca with E. gracilis s. lat. colonies were collected from four locations in the Mie University campus to investigate frequency of genotypes I and II every month from May 2015 to January 2016. No temporal isolation was found in genotype frequencies. These genotypes frequently co-existed on a single leaf surface, especially at the locations disturbed by human activities. Two oak powdery mildews, E. gracilis s. lat. and Cystotheca wrightii, produced conidia only one month a year and their life cycle differed from most other powdery mildew species. This suggests that these oak mildews developed their unique life cycles to synchronize with the life cycle of evergreen oaks.