Genetics of resistance to Sphaceloma scab of Cowpea

Although scab caused by Sphaceloma sp. is a major disease of cowpea in tropical and sub-tropical areas, little is known about the genetics of resistance to the disease. This study investigated i) relative importance of general (G.C.A) and specific (S.C.A) combining ability effects in the inheritance of resistance to scab on foliage and pods; ii) heritability of the resistance to scab infections on leaves and pods; iii) the genetic relationship between resistance to foliar and pod infections; and iv) the direct and indirect effects of scab foliar and pod infections on seed yield and yield components of cowpea. Ten cowpea lines [39, 46, Kvu 454, Kvu 175 -resistant; Iv 1658, Iv1075, 82, Kvu 530, SLA 59 -moderate resistance; and Era7 -susceptible] were half-diallel crossed during the first season of 1996 at Kabanyolo, Uganda. The F1 and F2 seeds were field grown together with their parents during the second season of 1996 and during the first season of 1997, respectively. A randomised complete block design (RCBD) with three replications consisting of single row plots, 3 m long, spaced at 75 cm apart, were used for all trials. Scab infected cowpea residues were introduced between rows three weeks after emergence (WAE) to ensure uniform scab infection. Foliar and pod scab severities were scored independently basing on a scale of 1 (resistant) to 5 (susceptible). Diallel analysis was done according to Griffing's (1956) method 2 model 1. Both G.C.A and S.C.A effects were important for resistance to scab infection whether on the foliage or pods suggesting the importance of both additive and non-additive gene actions for scab resistance. However, high G.C.A:S.C.A ratios (37.2 and 35.8 for foliar and pod scab severities, respectively) indicated preponderance of additive genetic variance for each trait. The resistant parents were found good general combiners for resistance to scab infections. Broad-sense heritability estimates were high for both resistance to foliar (93.8%) and to pod scab infections (97.0%). Narrow-sense heritability estimates were also high for both traits (79.8% and 84.5%, respectively). Correlation analysis revealed high phenotypic (0.983) and genotypic (0.949) correlation coefficients indicating a strong relationship between the two traits. Foliar scab severity exhibited a negative direct effect on yield, but also indirectly affected it through reduced pods per plant and pod length. Similarly, pod scab severity indirectly reduced yield through po