Authors
W. P. Wechter, USDA-ARS, U.S. Vegetable Laboratory, Charleston, SC 29414;
A. P. Keinath, Clemson University, Charleston, SC 29414;
J. P. Smith, Clemson University, Lexington, SC 29072; and
M. W. Farnham, USDA-ARS, U.S. Vegetable Laboratory, Charleston, SC 29414
Severe outbreaks of leaf spot disease of leafy vegetable brassica crops have occurred from early spring to late fall for at least the past 7 years in Lexington County, South Carolina, the major growing region for leafy greens in the state. Significant economic losses to this disease totaling $1.7 million have been incurred by large and small growers. In 2005, Pseudomonas syringae pv. maculicola was reported as one of the causal organisms of leaf spot disease in South Carolina (2). Investigations during 2006 and 2007 have led to the isolation of another bacterium causing leaf spotting of brassica crops. Symptoms in the field were nearly identical to symptoms caused by P syringae pv. maculicola, i.e., small, brown necrotic spots, often with chlorotic halos that expand and coalesce to cover the leaves. Colonies recovered from diseased tissues were xanthomonad like, nonfluorescent on Pseudomonas Agar F, mucoid on yeast extract dextrose chalk medium, grew at 35°C, hydrolyzed starch, positive for protein digestion, alkaline in litmus milk, and produce acid from arabinose. Sequence data from the 16S rDNA and fatty acid methyl ester analysis gave the best homology to Xanthomonas campestris pv. campestris with a similarity score index of >0.98 and >0.70, respectively, confirming genus and species. Excised-cotyledon assays, used to differentiate between pathovars campestris and armoraciae, confirmed the pathovar as campestris (1). Pathogenicity assays with spray inoculations (1 × 107 CFU/ml) (3) on eight plants each of ‘Topper’ and ‘Alamo’ turnip, ‘Early Jersey Wakefield’ cabbage, and ‘Money maker’ tomato produced leaf-spot symptoms within 10 days in the greenhouse and growth chamber on the turnip and cabbage plants, but not the tomato. X. campestris pv. campestris, which is common throughout the world, also is the causal agent of black rot in brassica. Typical black rot symptoms are seen often in Lexington County fields in summer and are quite different from the leaf spot symptoms observed. Leaf-spotting X. campestris pv. campestris (LS) strains and black rot (BR) strains, recovered from black rot-symptomatic plants lacking leaf spots, from the same fields were compared in greenhouse pathogenicity assays on six plants each of ‘Topper’ turnip and ‘Early Jersey Wakefield’ cabbage. Spray inoculations with 20 individual LS strains and 10 individual BR strains, collected from 2005 to 2007, produced symptoms unique to each group. These symptoms included chlorotic ‘V’-shaped lesions initiating from the leaf margins with black veining when plants were inoculated with BR strains, versus rapid and severe leaf spotting followed by chlorotic ‘V’-shaped lesions typically lacking black-veining 10 to 16 days postinoculation associated with LS strains. Additional inoculation tests gave similar results. To our knowledge, this is the first report of a severe leaf spotting disease of field-grown brassica leafy greens caused by X. campestris pv. campestris in South Carolina. These findings may have importance in differentiation of bacterial leaf spot pathogens in brassica crops.
References: (1) A. M. Alvarez et al. Phytopathology 84:1449, 1994. (2) A. P. Keinath et al. Plant Dis. 90:683, 2006. (3) W. P. Wechter et al. Hortic Sci. 42:1140, 2007.
