Occurrence of powdery mildew caused by Cystotheca kusanoi on Oak trees in China

Quercus spp. (Fagaceae) are evergreen and deciduous trees or shrubs, of which some are keystone species in forest ecosystems. Oak trees are common host plants of the powdery mildews within the genus Cystotheca. Cystotheca kusanoi, previously treated as heterotypic synonym of C. lanestris, was recently reinstated as a species of its own based on morphological re‐examinations and phylogenetic analyses of collections on Quercus spp. in Korea and Japan. The hyphal appressoria and presence or absence of fibrosin bodies in conidia of Cystotheca spp. was so far unclear. The present morphological examinations and phylogenetical analyses revealed that a fresh collection of powdery mildew on Q. spinosa from northwest China (Tibet Autonomous Region), and two herbarium specimens on Q. serrata from southwest China (Sichuan Province) and Q. variabilis from southeast China (Jiangsu Province) within re‐evaluated 12 herbarium specimens of C. lanestris s. lat. on Quercus spp. belong to C. kusanoi. To the best of our knowledge, this is the first report of C. kusanoi from China. Additionally, this is the first report of Q. spinosa as a host for C. kusanoi. Fibrosin bodies were not found in the fresh specimens of C. kusanoi. Nipple‐shaped to lobed hyphal appressoria have been observed and are described in the genus Cystotheca for the first time.