Plant Protection Institute, Hungarian Academy of Sciences, P.O. Box 102, Budapest, H-1525
Department of Plant Pathology, Cornell University, Riverhead, NY 11901
Since 1997, powdery mildew infections have been repeatedly observed on Sedum spectabile plants, cv. Autumn Joy, grown as ornamentals in commercial greenhouses in New York. Circular patches of gray mycelia appeared and spread on upper and occasionally on lower leaf surfaces followed by necrosis of the leaf tissues and defoliation. The new disease reduced the market value of the infected ornamentals and required chemical control. The pathogen produced conidia singly on 2- to 3-celled conidiophores occurring on the ectophytic hyphae. Conidia were subcylindrical, measured 27 to 36 μm × 13 to 17 μm, and contained no fibrosin bodies. Germinating conidia produced a short germ tube, 5 to 30 μm, terminating in a lobed appressorium. Hyphal appressoria were lobed to multi-lobed, opposite or spread along the hyphae. Cleistothecia were not found. Based on conidial characteristics, the pathogen was identified as Erysiphe sedi Braun. To confirm pathogenicity, potted healthy S. spectabile plants were placed near infected plants in the greenhouse. In addition, detached S. spectabile leaves were inoculated with the pathogen by touching them to powdery mildew colonies and then placed in plates filled with one layer of polystyrene balls floated in water. Plates were covered and kept in the laboratory. Uninfected potted plants kept in another greenhouse and noninoculated detached leaves served as controls. After 1 week, powdery mildew appeared on all infected plants and leaves exposed to or inoculated with the pathogen. The pathogen was morphologically identical to the original fungus. No symptoms were observed on the controls. E. sedi is a common Asiatic powdery mildew species infecting many crassulaceous plants (1,2) and was introduced to Eastern Europe from Asia (2). To our knowledge, this is the first report of E. sedi in North America.
References: (1) U. Braun. Beih. Nova Hedwigia 89:1, 1987. (2) U. Braun. The Powdery Mildews (Erysiphales) of Europe. Gustav Fisher Verlag, Jena, 1995.