Host Specificity and Resistance of Cowpea and Common Bean to Elsinoë spp. isolated from cowpea

Abstract

Cowpea (Vigna unguiculata L. Walp.) is an important leguminous crop in Kenya, but its production is constrained by diseases that reduce yield and market quality. Scab disease, caused by fungi in the genus Elsinoe, is particularly damaging due to its effects on foliage, flowers, and pods. This study investigated cowpea scab under controlled conditions, focusing on pathogen identification, host specificity, and varietal response. The pathogen was isolated from infected cowpea plants, and its identity as an Elsinoe sp. distinct from Elsinoe phaseoli was confirmed based on previously established multi-locus DNA barcoding (ITS and TEF1-) and phylogenetic data. Pathogenicity and cross-infectivity tests were conducted on three cowpea and three common bean (Phaseolus vulgaris L.) varieties. Kunde Tamu and Kunde Soko were highly susceptible, showing increasing disease severity across growth stages. By flowering, lesions had spread extensively on leaves and stems, and during pod filling, more than 90% of plant tissue was affected. Pods developed severe scab lesions, often becoming mummified and failing to fill grain (severity score 5). Kat Kunde exhibited partial tolerance, with low disease severity ([≤]2) during vegetative and flowering stages and minimal symptom progression. However, severity increased during pod filling, with pod lesions and occasional mummification, resulting in a final severity comparable to susceptible varieties, indicating stage-specific tolerance. All common bean varieties (CAL 194, CAL 143, and GLP 2) showed no scab symptoms, with severity scores remaining at 1, confirming absence of cross-infectivity. Typical scab symptoms in cowpea included cork-like lesions on leaves, stems, and pods, leading to flower abortion and pod mummification in susceptible varieties. Cross-infectivity tests confirmed that the isolate possesses strict host specificity to cowpea, remaining entirely distinct in host range from Elsinoe phaseoli which infects common bean. These findings confirm Elsinoe sp. as the causal agent of cowpea scab in Kenya, demonstrate strict host specificity, and identify tolerant germplasm for resistance breeding and improved disease management.