Distribution and severity of bacterial brown spot on dry beans in South Africa: An update

Bacterial brown spot (BBS) of common bean (Phaseolus vulgaris) is a seed-borne bacterial disease caused by Pseudomonas syringae pv. syringae (Pss). Dry bean is an important field crop in South Africa and all commercially available South African dry bean cultivars are susceptible to BBS. The aim of this study was to determine the current distribution and severity of BBS in the dry bean production areas of South Africa. We surveyed 31 locations in five provinces. Disease severity was evaluated on selected plots on a 0-9 scale in four different farming systems: commercial and subsistence farms and strip and national cultivar trials. Leaves with typical BBS symptoms were harvested and transferred to the laboratory. Bacteria were isolated following standard procedure using King's B medium. Identification of pure isolates was done using physiological and biochemical techniques. Incidence and severity values were used to calculate a disease index. BBS was observed in 88% of locations. BBS incidence was recorded in 54% to 100% of all the farming systems surveyed during the 2008/2009 and 2010/2011 seasons combined. BBS was more severe in the strip and national cultivar trials than on subsistence and commercial farms during the 2008/2009 season and more severe on the commercial farm and in national cultivar trials than on subsistence farms and strip trials during the 2010/2011 season. Findings of this study signal the importance of developing BBS-resistant dry bean cultivars for South Africa.