Japanese Mustard, Tah Tsai, and Red Mustard as Hosts of Alternaria brassicae in California. S. T. Koike, University of California Cooperative Extension, Salinas 93901. Plant Dis. 80:822. Accepted for publication 6 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0822B.
In California, the leafy crucifers Japanese mustard (Brassica campestris L. subsp, nipposinica), red mustard (Brassica juncea (L.) Czernj. & Coss. subsp, rugosa), and tah tsai (Brassica campestris L. subsp, nari-nosa) are grown in adjacent rows and harvested for use in packaged salad mixes. In the spring seasons of 1995 and 1996, a foliar disease caused significant damage to Japanese mustard and tah tsai. On Japanese mustard, symptoms consisted of small (2 to 4 mm), circular to oblong, brown leaf spots that contained concentric rings. On tah tsai, leaf spots were slightly larger (3 to 6 mm), circular, tan to brown, contained concentric rings, and were surrounded with yellow borders. Elongated, brown spots also developed on petioles of both crops. An Alternaria species having conidia with prominent beaks and spores produced singly or in chains of two was consistently observed on and isolated from both hosts. Conidia produced by isolates from Japanese mustard and tah tsai measured 181.1 to 198.9 x 21.7 to 24.4 µm (beak length was 63.3 to 75.0 µm) and 164.4 to 183.3 x 22.8-26.1 µm (beak length was 57.2 to 69.5 µm), respectively. Both isolates were identified as Alternaria brassicae (Berk.) Sacc. (2). Pathogenicity was confirmed by culturing both isolates on sterilized 20% V8 juice solution (I) for 10 days under a 12-h light/12-h dark cycle, filtering the suspensions through cheesecloth, adjusting spore concentra tions to 100,000 conidia/ml, and spraying each suspension onto 4-week-old seedlings of Japanese mustard (cv. Mizuna), tah tsai (cv. Tokita), red mustard (cv. Red Giant), and Chinese cabbage (Brassica campestris L. subsp, pekinensis cv. Cha-Cha). Plants were incubated in a moist chamber for 48 h and then maintained in a greenhouse. After 7 days, leaf spots similar to the original symptoms developed on all hosts inoculated with both isolates, and the pathogen was reisolated. Control plants sprayed with distilled water remained symptomless. For California, this is the first report of black spot caused by A. brassicae on commercially grown Japanese mustard and tah tsai and also the first report of red mustard as a host of the same pathogenReferences: (1) D. P. Senior et al. Trans. Hr. Mycol. Soc. 89:244, 1987. (2) E. G. Simmons. Mycotaxon 55:55, 1995.