Sources of Resistance to Ashy Stem Blight Caused By Macrophomina phaseolina in the Cowpea Major Gene Pool Two

Cowpea (Vigna unguiculata L. Walp) is an important source of inexpensive protein and income in sub-Saharan Africa and elsewhere. However, in agro-ecological zones with arid climate where Macrophomina phaseolina is prevalent, the economic and nutritional benefits of cowpea are reduced by yield loss from ashy-stem-blight. Host-plant resistance is the most effective strategy to mitigate M. phaseolina damage to cowpea; however, only limited sources of resistance to this pathogen have been identified. Forty-eight cowpea genotypes, from the southern East Africa germplasm which comprises cowpea gene pool 2, were evaluated for M. phaseolina resistance in a 3-year (2012-2014) series of field experiments conducted under natural infestation at UCRiverside. Based on ANOVA for cumulative plant mortality (CM) and correlations of percent plant mortality (%M) with environmental stress index (ESI, r = 0.53, P < 0.0001) and plant senescence score (SSc) 6 genotypes were consistently highly resistant to ashy-stem-blight disease with %M < 5, while 20 resistant genotypes had % M in a range of 6-12 %. In addition, 17 (%M 12.1-18) and 5 (%M 20-26) genotypes were classified as susceptible and highly susceptible, respectively. Plant senescence was significantly correlated with susceptibility to the disease (r = 0.56, P < 0.0001); however, one early maturing resistant genotype showed delayed senescence. These new sources of M. phaseolina resistance provide an important resource to cowpea breeders for variety improvement in marginal production areas.