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Disease Note

Powdery Mildew (Oidiopsis taurica) on Onion in California. F. F. Laemmlen, Cooperative Extension, Courthouse, El Centro, CA 92243. R. M. Endo, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 69:451. Accepted for publication 20 February 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-451d.

Circular to oblong, 5–15 mm lesions were found on leaves of the onion (Allium cepa L.) cultivar Henry’s Special at three locations in the Imperial Valley of California during June 1983. The yellowish lesions were covered with a hyaline, powdery growth of conidia and conidiophores. A single, apically pointed condidium was produced at the apices of branched conidiophores emerging from stomata. Older leaves were affected first. The pathogen was identified as Oidiopsis taurica Tepper by comparing infected onion specimens with herbarium specimens of tomato leaves infected with the fungus. The Leveillula perfect stage was not found. A survey confirmed that Henry’s Special was the only onion cultivar affected and that foliage damage was minimal. The disease was not found during 1984. This is the first report of powdery mildew on onion in the United States.