Identification of new sources of resistance to charcoal rot caused by Macrophomina phaseolina (Tassi) Goid in cowpea

Charcoal rot caused by Macrophomina phaseolina results in substantial yield losses in cowpea. Host plant resistance is the most reliable and cost-effective method of controlling this disease. Newly developed varieties need to be screened for their reaction to M. phaseolina as new strains of the pathogen keep evolving due to pathogen reproduction. This study sought to identify cowpea genotypes with stable resistance to M. phaseolina. Twenty-three cowpea genotypes were screened on the field and in pots under screenhouse conditions using a virulent M. phaseolina isolate. The pathogen effect on the genotypes and their agronomic performance and yield were determined. Disease parameters such as percentage Disease Incidence (DI) at the seedling stage, Seedling Infection Score (SIS), Root and Stem Severity (RSS), and Colony Forming Unit Index (CFUI) measured on the field were highly correlated with screen house experiments. These parameters were also significantly and positively correlated with each other suggesting that any of these parameters could be used in ranking the resistance or susceptibility of the genotypes. The disease parameters also had a significant negative correlation with percent germination/plant stand and pod yield and grain yield. It was observed that the charcoal rot disease was more severe at the seedling stage than at the reproductive stage. This is evidenced by the significant strong positive correlation between percent germination/plant stand and, pod yield and grain yield as the disease at the seedling stage causes seed rot and damping off. Based on the most Frequent Genotype Rank (FGR) generated by recording the highest occurring genotype rank of SIS, RSS, and CFUI of each genotype and comparison with grain yield data, IT11K-61-82, IT10K-837-1, SARI-3-11-100 and IT84S-2049 were ranked to be resistant to moderately resistant while IT07K-303-1, IT14K-1682-3 and IT13K-1070-2 were moderately susceptible. Additionally, IT14K-2030-2 and IT14K-2113-4 were susceptible to Charcoal rot disease while the remaining were moderately susceptible. Genotypes that combine resistance with farmer-preferred agronomic traits should be released as varieties while those that lack the farmer-preferred traits could be improved through hybridizations.