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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 León (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 León. 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, 120 to 160 μm in diameter, dark brown, and bore two types of appendages; long and flexuous with simple hook-like apices or short, simple, tapered, and rough walled. The long appendages measured 87 to 182 μm and the shorter ones measured 12 to 32 μm. Chasmothecia contained three to five asci, each developing six to eight hyaline, round, and single-celled ascospores when matured. Pathogenicity was confirmed on Aesculus sp. branches by healthy leaves being inoculated by touching them with powdery mildew-infected leaves. Noninoculated leaves served as controls. The appearance of colonies in the infected leaves was observed 15 days after the inoculation, but not in the controls. The pathogen was identified as Erysiphe flexuosa (Peck) U. Braun & S. Takamatsu, a North American powdery mildew. It has been recently introduced in Europe (1) and is now present in a number of countries (2). To our knowledge, this is the first report of E. flexuosa in this region of Spain.

References: (1) N. Ale-Agha et al. Cryptogam. Mycol. 21:89, 2000. (2) K. Zimmermannová-Pastirčáková et al. Schlechtendalia 8:39, 2002.