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Outbreak of White Rust, Caused by Albugo Candida, on Japanese Mustard and Tah Tsai in California. S. T. Koike, University of California Cooperative Extension, Salinas 93901. Plant Dis. 80:1302. Accepted for publication 5 September 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1302D.

During the spring months in 1995 and 1996, damaging outbreaks of white rust occurred in coastal California counties on the leafy crucifers Japanese mustard (Brassica campestris L. subsp, nipposinica) and tah tsai (Brassica campestris L. subsp, narinosa), which are grown in adjacent rows and harvested for use in packaged salad mixes. The typical white pustules of the disease developed profusely on the abaxial sides of leaves of both crops, causing growers to harvest only portions of their fields. Leaf hypertrophy was observed only on infected Japanese mustard. Morphological characters were similar for isolates from both hosts, and the pathogen was confirmed to be Albugo Candida (Pers.) Kunze. Sporangia diameters ranged from 18.8 to 19.0 µm. Inocula for pathogenicity tests were prepared by scraping sporangia from infected leaves and suspending them in sterile, distilled water for 8 h at 5C to induce zoospore formation (1,2). Inocula from each host were sprayed onto corresponding test plants of 4-week-old Japanese mustard (cv. Mizuna) or tah tsai (cv. Tokita). Plants were incubated in a moist chamber for 48 h and then maintained in a greenhouse. After 10 to 15 days, white rust pustules similar to the original symptoms developed on all plants. Because red mustard (Brassica juncea (L.) Czernj. & Coss. subsp, rugosa) is always planted alongside Japanese mustard and tah tsai, red mustard (cv. Red Giant) was inoculated with the Japanese mustard isolate. However, after four attempts, white rust failed to develop on the red mustard. Pathogenicity tests with inoculum from diseased Chinese cabbage (Brassica campestris L. subsp, pekinensis) resulted in white rust on Japanese mustard and tah tsai, but not on red mustard. For California, this is the first report of while rust caused by Albugo Candida on commercially grown Japanese mustard and tah tsai.

References: (I) G. A. Petrie. Can. Plant Dis. Surv. 66:43, 1986. (2) R. S. Pidskalny and S. R. Rimmer. Can. J. Plant Pathol. 7:283, 1985.