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Identification of Sources and Inheritance of Resistance to Sphaceloma Scab in Cowpea

Resistance in cowpea to Sphaceloma scab was characterized based on foliar and pod infection. Disease severity on cowpea plants was rated on a scale of 1 to 5, as follows: 1 = no symptoms, 2 = less than 10% infection, 3 = 10 to 20% infection, 4 = 20 to 50% infection, and 5 = more than 50% infection. Mean severity scores were used to calculate areas under the disease progress curve. Of the 75 cowpea lines evaluated, 10 were resistant, 30 were moderately resistant, and 35 were susceptible based on foliar infection. For pod infection, 24 lines were resistant, 40 were either moderately resistant or susceptible, and 11 were very susceptible. Local lines were less infected than plant introductions. In a separate experiment, 25 lines previously considered resistant were evaluated in the field with infested cowpea crop debris and susceptible rows as sources of inocula. Only 3 lines (39, KVu/175, and 46) maintained their high levels of resistance to Sphaceloma scab. Five cowpea lines were subsequently diallel-crossed, and parents and F2 were evaluated in the field to determine the nature of inheritance of resistance to Sphaceloma scab. The majority of the crosses between the resistant and susceptible lines showed intermediate reaction to scab, and high variation was obtained due to genotype and combining abilities. Partitioning of the variance into components due to general and specific combining ability revealed that additive genetic variation constituted the major portion of the total genetic variance for resistance to scab in cowpea.