Occurrence of bacterial leaf spot disease on lettuce caused by Pseudomonas viridiflava in the Kingdom of Saudi Arabia

During the spring of 2007, Romaine type lettuce plants (Lactuca sativa cv. Darkland) grown in four commercial fields in the province of EL-Kharj region of Saudi Arabia were observed with numerous lesions typical of bacterial leaf spot. Lesions were irregular, small, pale green to black. Disease incidence approached 30%. Bacteria were isolated by cutting lesions into small (1 mm) pieces and soaking them in 2 ml sterile distilled water. The resulting suspensions were streaked onto sucrose peptone agar and King's medium B. Three isolates from different fields, selected for further study were aerobic, Gram-negative, non-spore-forming rods. All isolates induced a hypersensitive response in tobacco plants, caused soft rot of potato tubers, and were negative for levan, oxidase, and arginine dihydrolase (Lelliot & Stead, 1987). Acid was produced aerobically from D-glucose, mannitol, and sorbitol, but not from D-arabinose, L-rhamnose, or sucrose; starch was not hydrolysed, glucose metabolism was oxidative, nitrates were not reduced to nitrites, gelatin was hydrolysed. Isolates were catalase positive and unable to grow in the presence of 5% NaCl. On this basis isolates were identified as Pseudomonas viridiflava. The identity of bacterial strains was confirmed by BIOLOG™ analysis (carbon source utilisation at 37°C), with a similarity index of 0.75. Pathogenicity tests were performed by spraying leaves of five-week-old lettuce plants (cv. Darkland) with bacterial suspensions (c. 10⁸ cfu/ml) prepared from 48 h cultures on yeast dextrose chalk agar. Control plants were sprayed with sterile water. Following inoculation plants were covered with polyethylene bags for 48 h at 25° C, after which bags were removed and plants were transferred to a greenhouse at 25-28° C (Aysan et al., 2003). All isolates were pathogenic, causing leaf spot symptoms, similar to those observed on the samples collected, within two weeks of inoculation. No symptoms developed on control plants. The bacterium was re-isolated from inoculated plants and identified as P. viridiflava. To our knowledge, this is the first report of bacterial leaf spot of lettuce in Saudi Arabia caused by Pseudomonas viridiflava.