Pathogenic Variability of Angular Leaf Spot Disease of Common Bean in Western Kenya

Abstract

Common bean (Phaseolus vulgaris L.) is the most important legume and is second only to maize as a food crop in Kenya. Despite its importance, bean productivity is declining in western Kenya due to several biotic and abiotic constraints including several fungal diseases. Among these diseases, angular leaf spot (Phaeoisariopsis griseola (Sacc.) is one of the most damaging and widely distributed diseases of common bean, causing yield losses as high as 80%. Furthermore, the problem is compounded by limited information on pathogen distribution and variability in western Kenya hindering breeding for angular leaf spot (ALS) resistance. Therefore, this study was carried out to characterise the ALS pathogen (Phaeoisariopsis griseola) (Sacc.) into different pathotypes. Forty-two isolates of P. griseola were collected from different bean growing areas of western Kenya and characterized into six pathotypes (63:11, 30:26, 33:23, 63:7, 31:10 and 63:63) by use of 12 differential cultivars. Advanced lines and commercial varieties obtained from KALRO-Kakamega were separately inoculated with six pathotypes of P. griseola and evaluated for disease reaction in the screenhouse. A screening trial of Mesoamerican and Andean bean genotypes showed that two varieties were tolerant (disease scores 1 to 3), fourteen varieties were moderately resistant (scores 4 to 6) and four varieties were susceptible (7 to 9). The tolerant varieties were small-seeded, while the susceptible varieties were mostly large-seeded. Keywords: Beans, Pathotypes, Phaeoisariopsis griseola, Severity