COWPEA (Vigna unguiculata (L.) Walp) SEED QUALITY AND YIELD AS INFLUENCED BY MANIPULATING SOWING DATES FOR THE MANAGEMENT OF SCAB INDUCED BY Elsinoe phaseoli Jenkins, AT SAMARU, NORTH-WEST NIGERIA

ABSTRACT
Field experiment was conducted at the Institute for Agricultural Research (IAR) farm at Samaru, North-western Nigeria to investigate effects of sowing dates on incidence and severity of scab on yield and seed quality of cowpea. The varieties, SAMPEA 6, SAMPEA 8, SAMPEA 11, TVx 3236 and Ife brown were planted at three sowing dates, August 9, 19 and 29 in 2016. The design used was Randomized Complete Block Design (RCBD) with three replications. Parameters measured were disease incidence, disease severity and yield. Disease incidence and severity scores were taken at weekly intervals for ten weeks, and data was analysed using SAS statistical software. Cowpea sown on the first sowing date August 9, had the highest scab incidence, 48.89 %, 54.44 %, 36.67 %, 20 % and 13.33 % and severity, 25.11 %, 31.44 %, 18.33 %, 19.56 %, 18.11 % on leaves, stems, peduncles, flower cushions and pod, respectively while plants sown on the third sowing date, Aug 29, had no scab infection. The varieties, SAMPEA 11 and TVx 3236 had low incidence and severity and can be considered to be moderately resistant to scab. The yield of cowpea sown on August 9 was the highest (1,792.60 kg/ha), followed by that sown on Aug 29 (1,422.20 kg/ha) while cowpea sown on Aug 19, had the lowest (1,192.60 kg/ha). However, the yield of Aug 19 and 29, sowing dates were not significantly different from each other (P>0.05). The variety, SAMPEA 11 had highest yield (1,691.40 kg/ha) while in this study, TVx 3236 had the lowest (1,296.30 kg/ha); average potential yield of TVx 3236 had been put at 1,200 kg/ha. Path coefficient analysis showed that severity of scab on pod contributed highly to reduction in yield per hectare (18.94 %), followed by scab on flower cushion (16.42 %). Seed quality tests, germination and accelerated ageing were conducted at Legumes Pathology Laboratory, Department of Crop Protection, Ahmadu Bello University, Zaria while electrical conductivity test was at National Agricultural Seeds Council Laboratory, Sheda, Abuja. Seeds harvested from cowpea sown on August 29 sown cowpea had highest germination of 90 % and vigour, 82.07 % which was significantly different (P<0.05) from those sown on August 9 with lowest germination of 67.20 % and vigour of 64.47 %. Correlation coefficient analysis showed that scab severity on the different plant parts had negative and highly significant effects on viability and vigour of the seeds. Scab severity had a positive correlation with electrolyte leakage. Negative and significant correlation was observed between electrical conductivity and germination (r-0.330*). This was similarly observed with accelerated ageing tests (-0.224), though not significant. Path coefficient analyses showed that scab severity had effects on seed quality, causing a reduction of 73 % and 92.5 % on seed viability and vigour respectively. SAMPEA 11 and TVx 3236, despite its low yield which could be attributed to the small seed size, sown by second week of August should form an integral part for the management of scab in cowpea.