|Wechter, William - Pat|
Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/1/2006
Publication Date: 3/1/2006
Citation: Keinath, A., Wechter, W.P., Smith, J. 2006. First report of bacterial leaf spot on leafy Brassica greens caused by Pseudomonas syringae pv. maculicola in South Carolina. Plant Disease. 90:683. Interpretive Summary: South Carolina ranks 2nd in the United States in total acreage of turnip greens and collards, and third in mustard greens. Over the past 4 years, a serious leaf-spotting disease of leafy greens has caused huge economic losses for growers in the midlands of South Carolina. In order to address this problem, isolation and accurate identification of the pathogen is critical. Through standard microbiological as well as advanced biochemical and genetic techniques, the causal agent, a bacterium Pseudomonas syringae pv. maculicola, has been isolated and identified. This bacterium is responsible for a disease called “Peppery Leaf Spot” of a number of leafy greens, and has been reported to cause serious problems in both California and Oklahoma greens-growing areas. This is the first reported occurrence of this disease in South Carolina. With the identification of the causal agent, investigation into the epidemiology of this disease and subsequently the development of control strategies are now possible.
Technical Abstract: As of 2001, South Carolina ranked second in the United States in acreage of turnip greens (Brassica rapa) and collard (B. oleracea) and third in acreage of mustard (B. juncea). In June 2001, a leaf disease was found on turnip greens (cv. Alamo), mustard (cv. Southern Giant Curled and Florida Broadleaf), and rape salad greens (B. napus var. napus cv. Essex) on a commercial farm in Lexington County, South Carolina. The disease was found in May and June 2002 on three additional farms in the same county on turnip greens cv. Topper and Royal Crown and collard cv. Top Bunch. Symptoms included small tan spots, water-soaking, yellowing, and brown necrosis of leaves after spots coalesced on the lower halves of plants. Yellowing was more prevalent on older than on younger leaves. Leaf samples were collected in 2001 and 2002 from the affected hosts on the four farms. Bacterial streaming was evident from these samples and 27 strains were isolated on nutrient agar or King’s Medium B (KMB). All strains were Gram-negative and fluoresced blue-green or yellow under UV light after 48-h growth at 28ºC on Pseudomonas Agar F (PAF). Based on LOPAT tests, the strains were identified as P. syringae. All 27 strains were tested for pathogenicity to rape salad greens cv. Essex and then to turnip greens cv. Topper. P. syringae pv. maculicola F41 and P. syringae pv. tomato F33 were included as positive and negative controls, along with a noninoculated control. Bacteria were grown on KMB for 48 h at 24ºC and a bacterial suspension was prepared and adjusted to 0.1 optical density at 600 nm. Three-week-old plants were held at 95 to 100% relative humidity (RH) for 48 h before they were sprayed with inoculum and then held at 95 to 100% RH for 48 h after inoculation. After 5 to 8 additional days in a greenhouse, nine strains and F41 caused symptoms on both Topper and Essex similar to symptoms observed in the field. No symptoms were observed on noninoculated plants or plants inoculated with F33. Based on comparative rep-PCR fingerprint analysis using the BOXA1R primer, the profile of each of the nine pathogenic strains from South Carolina was nearly identical to that of F41. Bacteria isolated from inoculated, symptomatic turnip leaves had identical LOPAT and BOXA1R profiles to the corresponding original strains. Pathogenic strains had blue-green fluorescence on PAF, whereas nonpathogenic strains fluoresced yellow. Five pathogenic strains, as well as F41, were further identified to species and pathovar with fatty acid methyl ester profiles as P. syringae pv. maculicola. To the best of our knowledge, this is the first report of P. syringae pv. maculicola from South Carolina.