Disease Note.

First Report of Powdery Mildew of Tomato in California Caused by an Oidium sp. C. R. Arredondo, Department of Plant Pathology, University of California, Davis 95616 . R. M. Davis and D. M. Rizzo, Department of Plant Pathology, University of California, Davis 95616; and R. Stahmer, Heinz Co., Stockton, CA 95201. Plant Dis. 80:1303. Accepted for publication 5 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1303D.

A powdery mildew of tomato (Lycopersicum esculentum Mill.) was observed in greenhouses in several counties in California in 1995 tol996. Circular, white, powdery colonies occurred on leaves, stems, and petioles and sometimes covered the entire surface of infected leaves. Severely infected plants yellowed and defoliated. The mean dimensions of the ovate to cylindrical conidia were 16.0 µm (11.0 to 16.3 µm) x 34.7 µm (20 0 to 38.8 µm). Conidia were produced in short chains of 2 to 6 conidia. Conidial germination was generally polar; equatorial germination occurred infrequently. Conidiophores were unbranched and 3 or 4 celled. The sexual stage was not found. Conidia from infected tomato leaves were shaken onto leaves of pepper (Capsicum annuum L), lima bean (Phaseolus lunatus L. cv. Henderson Bush), cowpea (Vigna unguiculata (L.) Walp), pumpkin (Cucurbita pepo L. cv. Small Sugar), sweet com (Zea mays L. cv. Jubilee), wheat (Triticum aestivum L. cv. Anza), broccoli (Brassica oleracea var. botrytis cv. Green Sprouting), cauliflower (B. oleracea L. var. botrytis cv. Snowflower), cabbage (B. oleracea var. capitata cv. Steins Flat Dutch), tobacco (Nicotiana tabacum L), amaranth (Amaranthus sp), and several tomato cultivars, including H1916, H2710, H9280, Ontario 7710, Peto 31, Peto 111, Spectrum 579, LA 3118, and LA 3119. Noninoculated plants served as controls. Symptoms developed on all inoculated tomato plants after 7 days and on tobacco after 15 days. Symptoms on tobacco were small and localized. A hypersensitive reaction developed on cowpea 7 days after inoculation. While this powdery mildew fits the generic characteristics of Oidium, the taxo-nomic identity remains uncertain. Oidium lycopersicum Cooke and Mas-see was described on tomato in Europe, but our specimens do not match the description of that fungus. Another powdery mildew of tomato caused by an Oidium sp. was reported in several countries, including Greece, Canada, Bulgaria, Australia, and England (1,2), but no formal taxonomic description of this fungus is available. This is the first report of powdery mildew of tomato caused by an Oidium sp. in California.