Genotypic Response of Dry Bean (Phaseolus vulgaris L.) to Natural Field Infection of Ascochyta Blight (Phoma exigua var. diversispora (Bubak) Boerema) under Diverse Environmental Conditions in Rwanda

Ascochyta blight, caused by Phoma exigua var. diversispora (Bubak) Boerema, is a serious constraint in the cultivation of the common bean (Phaseolus vulgaris L.) in Rwanda, particularly in the cool and wet highland production areas. In order to identify resistant genotypes, a germplasm evaluation study was conducted to quantify the impact of the disease on phenotypic and agronomic traits under natural conditions. Field screening trials of 39 bush (Types I, II and III) and 36 climbing (Type IV) genotypes from different accessions within and outside the country were conducted at three sites, namely, Rwerere, Nyamagabe and Musanze Research Stations, for two seasons. The relative area under the disease progress curve (RAUDPC) based on evaluations of the disease severity (percentage leaf area infected), was used to evaluate the genotypes. Thirteen genotypes were identified with some level of ascochyta resistance. The study revealed Rwandan genotypes G 2333 and SMC 18 as new sources of resistance to Ascochyta blight. Additional results showed a negative relationship (r = -0.42 and -0.51 for Seasons A and B, respectively) between ascochyta infection and yield. Further relationships were identified between the plant flower colour and seed size to ascochyta resistance. Some of the identified resistant genotypes can be used to introgress ascochyta resistance into susceptible Rwandan market classes of common bean genotypes.