Bacterial brown spot; disease incidence and primary inoculum sources

The first major outbreak of bacterial brown spot of dry bean, caused by Pseudomonas syringae pv. syringae, in South Africa, occurred in 1992 on the Eastern Transvaal Highveld. The incidence of the disease, and damage caused by it, remained significant ever since. Primarily inoculum sources of bacterial brown spot were determined. Infected or contaminated seed was found to be a major source of P. syringae pv. syringae. The pathogen was found to survive between seasons in infected debris left on the field. Volunteer bean plants, resulting from spilled seed left in bean fields after harvest, was also found to be a potential source of bacterial brown spot. Diseased plantings, and some plantings without the disease, was found to host high epiphytic populations of P. syringae pv. syringae that can serve as primary inoculum for other plantings in the season. The pathogen could not be isolated from plants or plant material other than bean. Elimination of primary inoculum sources will help to control bacterial brown spot.