SCREENING FRENCH BEAN GENOTYPES (Phaseolus vulgaris L.) USING PHENOTYPIC AND SEQUENCE TAGGED SITE (STS) MOLECULAR MARKER FOR ANGULAR LEAF SPOT (Pseudocercospora griseola) RESISTANCE

Abstract

French bean, a sub-group of the common bean (Phaseolus vulgaris L.), is grown for its tender green pods and consumed as a green vegetable. It is a major export crop in Kenya but production levels are constrained by both biotic and abiotic stresses. Angular leaf spot (ALS) disease, caused by Pseudocercospora griseola, can cause major reductions in yield. Genetic resistance against ALS is a high breeding priority, as effective cultural control is difficult. The aim of this study was to evaluate French bean germplasm for ALS resistance in the field, in controlled conditions with ALS races and using a sequence tagged site (STS) marker. The French bean germplasm panel tested included 26 commercial varieties, three breeding lines and three landraces. The 32 test genotypes were planted in a randomized complete block design with three replicates, in three sites in western Kenya (Eldoret, Kakamega and Kisii) in 2014. Two dry bean varieties—Mexico 54 and GLP X92—were included as resistant and susceptible checks, respectively. Further, the genotypes were screened under greenhouse conditions using 8 races of ALS. The ALS severity on the French bean genotypes was scored based on the CIAT standard scale of 1-9 of disease assessment. Populations derived from a cross between the Mexico 54 (resistant dry bean variety) and Amy (Susceptible French bean variety) were phenotyped and genotyped to study the inheritance of ALS resistance in Mexico 54 and also determine the potential application of STS G796 marker in screening for resistance to ALS. The field results showed significant (P ≤ 0.05) differences in disease severity between the three sites: the Kakamega site experienced the greatest disease pressure (mean severity of 6.2), followed by the Kisii site (5.7) and the Eldoret site (5.3). Only Mexico 54 out of the 34 entries showed field resistance to ALS. Most exhibited either an intermediate or a susceptible reaction at all three sites. For greenhouse conditions, genotype Manakelly showed resistance to 6 of the 8 races used. Race 63-23 and 63-39 were the most virulent races (6.8) affecting most French bean genotypes. The phenotypic data confirmed that Mexico 54 has a single dominant ALS resistance gene and the co-dominant STS marker was polymorphic between Amy and Mexico 54 and was able to distinguish Mexico 54 from the other 11 ALS differential cultivars and 31 French bean genotypes. The present study has demonstrated that commercial French bean germplasm lacks a source of robust resistance against ALS. However, the exotic variety Mexico 54 could be used to improve ALS susceptible elite varieties. Mexico 54 is an indeterminate dry bean variety, so an intensive backcross program would be needed to restore the necessary pod quality and determinacy. In the meantime, there is a need to identify other sources of resistance, preferably within the French bean genepool, both to increase the diversity of resistance and to simplify the improvement of current elite varieties. The study also showed that STS G796 is a potential molecular marker that can be used for indirect selection during the development of ALS resistant lines using Mexico 54 as one of the progenitors.