Effect of Sowing Date on the Incidence, Apparent Infection Rate and Severity of Scab on Cowpea

The main aim of this study was to investigate the effect of sowing dates on the incidences, apparent infection rates andseverities of cowpea scab, caused by Sphaceloma sp. on three varieties of cowpea (Vigna unguilata L. Walp). The varieties were: TVx 3236, SAMPEA-6 and IT93K452-1. The investigations were undertaken during the 2004, 2005 and 2006 cropping seasons at Samaru and Shika in Zaria, Nigeria. Four sowings were done at 7-day interval starting from late July and ending in mid August of each year. Scab disease incidence and severity ratings were taken every seven days starting from the first visible symptoms of infection on the plant parts. The design used was a factorial concept in a Randomized Complete Block Design (RCBD) with three replications consisting of single row plots, each 75 cm wide, 6 m long, and 75 cm apart. For all the 3 seasons, the early sown cowpeas had higher scab incidences, apparent infection rates and severities than the late sown crops, even though not significantly higher in all cases or parameters. No scab symptoms were observed on the leaves of the more resistant cultivar, TVx 3236, in all 3 years of the investigation. Under northern Nigerian conditions, early cowpea plantings would result in higher infections from scab, unless more resistant cultivars are used in such early plantings.