Abstract

Web blight of common beans caused by Rhizoctonia solani is a very important disease in bean growing regions of Latin America and the Caribbean. There are no cultivars with high levels of resistance available mostly because of a lack of information on the pathogen genetic diversity and virulence patterns and the need to find sources of resistance that can be introgressed into common bean.

To study virulence patterns, the development of a set of differential bean lines is essential. Two groups of candidate bean lines were screened with isolates from AG 1-1F, AG 1-1E and AG 1-1A using the detached leaf technique. No significant differences were found among lines at any of the different growing conditions tested. All lines were rated as susceptible to all isolates even though they were selected based on the moderate resistance they showed in field trials. Thus new screening methods and more cultivars/lines need to be studied in order to develop a set of differentials.

Genetic diversity of isolates collected in Puerto Rico, Honduras and the Dominican Republic over ten years from the two newly described subgroups AG 1-1E and AG 1-1F was assayed with four molecular markers. While some isolates were found to be clonal, Universal Rice Primers (URP) markers were the most informative showing differences between countries and years of collection for most of the isolates. This would support the hypothesis that population structure has changed over time and that there could be sexual recombination among a few genotypes.

A group of 71 accessions from several species of Phaseolus including wild and landrace collections was tested for resistance to isolates from AG 1-1F, AG 1-1E and AG 1-1F. Fifteen lines were classified as moderately resistant to AG 1-1A but susceptible to the other two subgroups. Line 01-006-3B from P. coccineus was found to be resistant to AG 1-1F, intermediate to AG 1-1E and susceptible to AG 1-1A making it the best line to be considered for common bean breeding programs wanting to introgress web blight resistance into commercial varieties.