Occurrence of Bacterial Brown Spot on Dry Beans in Zimbabwe

A previously unreported leaf spot disease was observed in dry bean variety trials in Harare, Zimbabwe, during March 2000. It was subsequently noticed on the cultivars Ex-Rico and C20 in commercial fields. The irregular, brown, necrotic lesions (0.5 to 5 mm in diameter) resembled typical symptoms of bacterial brown spot. A fluorescent bacterium was isolated from diseased leaf samples on King's B medium. LOPAT tests (2) classified the bacterium as a Group 1a fluorescent plant pathogenic pseudomonad. Agglutination with a specific antibody reagent, from the Adgen Express test kit, positively identified the bacterium as Pseudomonas syringae pv. syringae, the causal organism of bacterial brown spot. The pathogen was distinguished from P. syringae pv. phaseoli, the halo blight organism, by its ability to utilize mannitol, inositol, and sorbitol and its ability to grow on KBC medium (3). The utilization of sucrose as a sole carbon source distinguished the pathogen from P. viridiflava. Pathogenicity of isolates was verified by inoculating young attached pods of the cultivar Teebus, using the method of Cheng et al. (1). Control plants were inoculated with a sterile syringe needle dipped in sterile distilled water. Inoculated plants were maintained in a greenhouse at 28°C day/18°C night for 4 days before being evaluated for symptom development. P. syringae pv. syringae was consistently recovered from typical lesions that developed on inoculated pods. No symptoms developed on control pods. This is the first report of bacterial brown spot caused by P. syringae pv. syringae on dry beans in Zimbabwe. P. syringae pv. syringae was, however, isolated in South Africa from extracts of bean seed received from Zimbabwe in 1996, although it is uncertain whether the disease actually occurred in the field during that specific season. References: (1) G. Y. Cheng et al. Plant Dis. 73:419, 1989. (2) R. A. Lelliott et al. J. Appl. Bacteriol. 29:470, 1966. (3) S. K. Mohan and N. W. Schaad. Phytopathology 77:1390, 1987.