In the spring of 1997, commercial plantings of daikon or Chinese radish (Raphanus sativus L. cv. longipinnatus) in the San Joaquin Valley, Fresno County, were damaged by a disease previously undocumented in California. Symptoms consisted of small (2 to 5 mm diameter), circular to oblong, brown to light tan leaf spots that contained concentric rings. Following rains in January, leaf spots proliferated, coalesced, and resulted in significant necrosis of foliage. An Alternaria species with 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 daikon leaves had mean dimensions of 190.1 × 23.1 μm, with a mean beak length of 60.0 μm. The isolates were identified as Alternaria brassicae (2). Pathogenicity was confirmed by culturing six representative isolates on sterilized 20% V8 juice solution for 14 days under a 12 h light/12 h dark cycle, filtering the suspensions through cheesecloth, adjusting spore concentrations to 25,000 conidia per ml, and spraying suspensions onto 4-week-old seedlings of daikon cvs. April and Summer. Plants were incubated in a humid chamber for 48 h and then maintained in a greenhouse. After 5 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. With the method described above, daikon isolates were also inoculated onto the following A. brassicae hosts (1): radish (Raphanus sativus L. cv. EXCO 517), broccoli (Brassica oleracea L. subsp. botrytis cv. Greenbelt), Chinese cabbage (Brassica campestris L. subsp. pekinensis cv. Cha-Cha), and broccoli raab (Brassica rapa L. subsp. rapa cv. Spring Raab). After 5 (radish) to 12 (other hosts) days, similar leaf spots developed on all inoculated plants and A. brassicae was reisolated and identified. Control plants sprayed with distilled water remained symptomless. These inoculations were repeated and results were the same. For California, this is the first report of black spot caused by A. brassicae on commercially grown daikon.
References: (1) S. T. Koike. Plant Dis. 81:552, 1997. (2) E. G. Simmons. Mycotaxon 55:55, 1995.
