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Title: First Report of Severe Outbreaks of Bacterial Leaf Spot of Leafy Brassica Greens Caused by Xanthomonas Campestris pv. Campestris in South Carolina

Author
item Wechter, William - Pat
item KEINATH, A - CLEMSON UNIVERSITY
item SMITH, J - CLEMSON UNIVERSITY
item Farnham, Mark

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2008
Publication Date: 7/1/2008
Citation: Wechter, W.P., Keinath, A.P., Smith, J.P., Farnham, M.W. 2008. First Report of Severe Outbreaks of Bacterial Leaf Spot of Leafy Brassica Greens Caused by Xanthomonas Campestris pv. Campestris in South Carolina. Plant Disease. 92:1134.

Interpretive Summary: Severe outbreaks of leaf spot disease of leafy vegetable Brassica have occurred from early spring to late fall for at least the past five years in Lexington County, the major growing region for leafy greens in South Carolina. In 2005, we reported Pseudomonas syringae pv. maculicola (Psm) as one of the causal organisms of this disease in South Carolina. Investigations during the past two years have led to the isolation of another bacterium which also causes leaf spotting of these field-grown Brassica. Through standard microbiological as well as advanced biochemical and genetic techniques, the causal agent, a bacterium Xanthomonas campestris pv. campestris (Xcc), has been isolated and identified. This bacterium, which is found world wide causing a disease called black rot of Brassica, has not previously been reported to cause severe leaf spotting of leafy vegetable greens. This is the first report of a severe leaf spotting disease of field-grown Brassica leafy greens caused by Xcc in South Carolina. These findings may have considerable importance in differentiation of bacterial leaf spot pathogens in Brassica crops.

Technical Abstract: Severe outbreaks of leaf spot disease of leafy vegetable Brassica have occurred from early spring to late fall for at least the past five years in Lexington County, the major growing region for leafy greens in South Carolina. Significant economic losses to this disease have been reported by both large and small field operations in Lexington County. In 2005, we reported Pseudomonas syringae pv. maculicola (Psm) as one of the causal organisms of this disease in South Carolina. Investigations during the past two years have led to the isolation of another bacterium which also causes leaf spotting of these field-grown Brassica. Symptoms in the field were nearly identical to the leaf spot symptoms caused by Psm, i.e. begin initially as small, brown necrotic spots, often with chlorotic halos. These necrotic lesions expand and coalesce to cover large portions of the leaf surface. Leaves become ragged as the disease progresses. Colonies recovered from diseased tissues are Xanthomonas-like, non-fluorescent on Pseudomonas Agar F, mucoid on yeast extract-dextrose-calcium carbonate medium, grow at 35C, hydrolyzed starch, are positive for protein digestion, alkaline in litmus milk, and produce acid from arabinose. Sequence data from the 16S rDNA gave the best homology to X. campestris pv. campestris (Xcc) with a similarity score of >0.980, and fatty acid methyl ester (FAME) analysis had the highest correlation values (>0.700) to Xcc. Excised-cotyledon assays confirmed the pathovar as campestris. Pathogenicity studies with spray inoculations on eight plants each of ‘Topper’ and ‘Alamo’ turnip and ‘Early Jersey Wakefield’ cabbage produced similar symptoms within 10 days in the greenhouse and growth chamber. An equal number of plants sprayed with sterile water displayed no disease symptoms. Bacteria isolated from the diseased, inoculated plants were confirmed to be Xcc by FAME analysis and excised-cotyledon assay. Xcc, which is common throughout the world, is the causal agent of black rot in Brassica. Typical black rot symptoms, which are often seen in the Lexington County fields during mid to late summer, are quite different from the leaf spot symptoms being observed in these same fields. Side by side comparisons of the leaf-spotting Xcc (Xcc-ls) to Xcc isolates recovered from the same fields from black rot-symptomatic plants lacking leaf spots (Xcc-br isolates) were performed in the greenhouse on six plants each of ‘Topper’ turnip and ‘Early Jersey Wakefield’ cabbage. Spray-inoculations with 20 isolates of Xcc-ls and 10 isolates of Xcc-br collected during 2005 to 2007 produced symptoms unique to each group, e.g. chlorotic ‘V’-shaped lesions initiating from the leaf margins with black veining with Xcc-br, versus rapid, severe leaf spotting followed by chlorotic ‘V’-shaped lesions typically lacking black-veining 10 to 16 days post-inoculation, associated with Xcc-ls. Representative isolates of Xcc-ls and Xcc-br were used in additional inoculation tests with similar results. This is the first report of a severe leaf spotting disease of field-grown Brassica leafy greens caused by Xcc in South Carolina. These findings may have considerable importance in differentiation of bacterial leaf spot pathogens in Brassica crops.