Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2009
Publication Date: 1/1/2010
Citation: Wechter, W.P., Keinath, A.P., Farnham, M.W., Smith, J.P. 2010. First Report of Bacterial Leaf Blight on Broccoli and Cabbage Caused by Pseudomonas syringae pv. alisalensis in South Carolina. Plant Disease. 94:132. Interpretive Summary: In May 2009, leaf spot and leaf blight symptoms were observed on broccoli and cabbage on several farms in Lexington County, the major vegetable-growing region of South Carolina. Affected areas ranged from scattered disease within fields to losses of entire fields. Diseased leaves were collected from fields in different parts of Lexington County. Using biochemical, molecular and host range testing, the pathogen was determined to be the bacterium Pseudomonas syringae pv. alisalensis. This represents the first report of this pathogen in South Carolina on broccoli and cabbage. Although cabbage has been grown in South Carolina for many years, broccoli is a relatively new and rapidly expanding production vegetable. This disease may represent an important factor in future production of both broccoli and cabbage in the State. This rapid detection and identification of this pathogen, especially while the broccoli market in South Carolina is in the early years of development, will allow plant researchers to get an early start on studying possible control strategies for this disease.
Technical Abstract: In May 2009, leaf spot and leaf blight symptoms were observed on broccoli (B. oleracea var. italica) and cabbage (Brassica oleracea var. capitata) on several farms in Lexington County, the major brassica-growing region of South Carolina. Affected areas ranged from scattered disease foci within fields to entire fields. Symptoms on all crops included circular and irregular shaped necrotic lesions, often with yellow halos. Water soaking was evident in early infection sites. Necrotic spots, which ranged from 3 to 10 mm in early infections, tended to coalesce into a general blight of the entire leaf in more advanced infections. Diseased leaves, from both broccoli and cabbage, were collected from four fields in different parts of the county. All samples produced large numbers of bacterial colonies that fluoresced blue under UV light after 24 h growth. In total, 23 isolates were collected. These isolates were Gram negative and fluoresced blue-green or yellow under UV light after 48-h growth on KB agar. Two isolates from broccoli and two isolates from cabbage were selected at random and tested for pathogenicity to cabbage cv. Early Jersey Wakefield, broccoli cv. Decicco and Waltham 29, turnip cv. Topper, broccoli raab cv. Spring, collard cv. Hi-Crop, and oat cv. Montezuma in greenhouse tests. A strain of Pseudomonas syringae pv. maculicola and P. syringae pv. alisalensis were included as controls, along with a water-inoculated negative control. The four unknown bacterial isolates and P. syringae pv. alisalensis were pathogenic on all of the brassica plants tested, as well as on oat. In contrast, the P. syringae pv. maculicola strain was not pathogenic on broccoli raab or oat. The symptoms produced by all isolates and strains tested were similar to those observed in the field. No symptoms were observed on water-inoculated plants. Bacteria isolated from these test plants were identical to the isolates used in the inoculation tests. LOPAT tests determined the isolates to be P. syringae. Comparative rep-PCR DNA analysis using the BOXA1R primer resulted in a DNA banding pattern of each of the strains that was identical to the P. syringae pv. alisalensis strain. Based on the LOPAT and rep-PCR assays, as well as the differential host range, the current disease outbreak on broccoli and cabbage in South Carolina is caused by the bacterium P. syringae pv. alisalensis. Broccoli is a relatively new, albeit rapidly expanding, production vegetable in South Carolina, this disease may represent an important factor in future production.