First report of horse chestnut powdery mildew caused by Erysiphe flexuosa in Castilla y León, Spain.

Horse chestnut (Aesculus hippocastanum L.) is an important ornamental tree in many gardens in Castilla y Leon (northern Spain). During the summer and autumn of 2005 and 2006, disease signs resembling powdery mildew were observed in Condesa de Sagasta Boulevard and Quevedo Botanical Garden in Leon. Whitish fungal growth was visible macroscopically on the upper and lower leaf surfaces of both young and old leaves and petioles. The disease reduced tree attractiveness and could cause important aesthetic damage in the parks. Five symptomatic trees older than 30 years were observed in each park, and 10 leaves per tree were collected. Microscopic examinations of the leaf surfaces revealed ectophytic, hyaline hyphae with lobed appresoria, solitary or in pairs. Conidia were barrel to broadly subglobose and ranged from 23 to 37 μm long and 8 to 16 μm wide. They were produced singly on two- or three-celled conidiophores. Chasmothecia were produced in abundance, mostly on the lower leaf surface. They were globose, 12...