ASSESSMENT OF CROP LOSS IN COWPEA [VIGNA-UNGUICULATA (L) WALP] CAUSEDBY SPHACELOMA SP, CAUSAL AGENT OF SCAB DISEASE

IT 84S-2140 and IT 85F-1517 (cowpea varieties) susceptible to scab dis ease caused by Sphaceloma sp. were treated with severely infected cowp ea plants in 1989 and 1990 field trials. Different levels of disease i ntensity were generated by treatment with benomyl using two different methods of application: varying the frequency of application from 4- t o 35-day intervals resulted in grain losses ranging between 25-70% in 1989 and 35-50% in 1990. When the concentration of benomyl was varied at a 10-day interval, crop losses ranged between 10 and 71% in 1989 an d 9-70% in 1990. Grain yield was significantly negatively correlated w ith leaf, stem, peduncle, flowering cushion and pod scab severities. T he linear regression of grain yield on disease was computed using the critical point model. The results indicate that a weekly spray of beno myl at 0.5 kg/400 1(-1) will prevent abscission of flowers, reduce hea vy scabbing of young pods which in all interact to play an important r ole in crop loss due to cowpea scab. Cowpea scab can cause up to 71% c rop loss in susceptible varieties if uncontrolled.