In the fall of 1996, commercial plantings of broccoli raab or rappini (Brassica rapa subsp. rapa) in the Salinas Valley, Monterey County, were damaged by a disease previously unreported in California. Symptoms consisted of small (7 to 8 mm in diameter), circular to oblong, brown to light tan leaf spots that contained characteristic concentric rings. An Alternaria sp. having conidia with prominent beaks and spores produced singly or in chains of two was consistently observed on and isolated from the host. Conidia from naturally infected broccoli raab measured 144.5 to 194.5 × 22.2 to 30.6 μm (beak length ranged from 33.3 to 75.0 μm). The isolates were identified as Alternaria brassicae (Berk.) Sacc. (2). Pathogenicity was confirmed by culturing five representative isolates on sterilized 20% V8 juice solution for 10 days under a 12-h light/12-h dark cycle, filtering the suspensions through cheesecloth, obtaining spore concentrations of 30,000 conidia per ml, and spraying suspensions onto 6-week-old seedlings of broccoli raab cv. Spring Raab. Plants were incubated in a humid chamber for 48 h and then maintained in a greenhouse. After 5 to 7 days, leaf spots similar to the original symptoms developed on all inoculated plants. The pathogen was reisolated and identified as A. brassicae. Control plants sprayed with distilled water remained symptomless. The inoculation test was repeated and results were the same. Because of the extensive plantings of cole crops in the Salinas Valley, isolates from broccoli raab were inoculated onto two sets each of broccoli (Brassica oleracea subsp. italica cv. Greenbelt), Chinese cabbage (Brassica campestris subsp. pekinensis cv. Cha-Cha), and Japanese mustard (Brassica campestris subsp. nipposinica cv. Mizuna), using the method described above. After 5 to 7 days, similar leaf spots developed on all inoculated plants and A. brassicae was reisolated and identified. Control plants sprayed with distilled water remained symptomless. This fungus does not appear to be a pathogen specific to particular cole crops in this area (1). For California, this is the first report of black spot caused by A. brassicae on commercially grown broccoli raab.
References: (1) S. T. Koike. Plant Dis. 80:822, 1996. (2) E. G. Simmons. Mycotaxon 55:55, 1995.
