Reaction of selected common bean germplasm accessions to bacterial brown spot in South Africa

• Interactions on ratings, yield and AUDPC were highly significant.
• Correlations between ratings and yield, AUDPC and yield were highly significant.
• A trend that high yielding accessions have lower disease severity.
• VAX 4, VAX 1 and VAX 2 are potential resistance sources for bacterial brown spot.

Bacterial brown spot (BBS), caused by Pseudomonas syringae pv. syringae (Pss), is a disease of common beans (Phaseolus vulgaris L.) in South Africa. Planting of pathogen-free seed and application of copper-based chemicals do not guarantee disease control. Genetic resistance is considered the most effective control measure. BBS occurs regularly in major dry bean production areas. The aim of this study was to screen selected accessions for their reaction to BBS. Twenty-seven common bean accessions were assessed over three seasons and locations for resistance to BBS. Trials were arranged in a randomized complete block design with three replications and artificially inoculated with Pss (1 × 108 CFU/mL) using a Stihl mistblower at 21, 28 and 36 days after planting. Plots were rated for disease reaction on a modified 1–9 CIAT scale with 1 being resistant and 9 being susceptible. Ratings were done at 7, 14, 21 and 28 days after the first inoculation. Trials were harvested manually and yield data were recorded. Disease ratings were used to construct the area under the disease progress curve (AUDPC). Highly significant genotype × environment interactions were recorded and additive main effects and multiplicative interaction analysis was used to identify lines in which disease response and yield were significantly affected by environment. VAX 4 and VAX 2 had lowest disease ratings of 1.8 and 2.0, and AUDPC values of 38.6 and 41.6, respectively, and are therefore potential resistance sources.