ABSTRACT

A field survey to investigate the diseases of cowpea in Abia State, Nigeria was conducted between 1997 and 1998. The following diseases were found to be associated with the crop; anthracnose, ^bacterial blight, bacterial pustule, Ascochyta^blight, Macrophomina blight, brown blotch, Cercospora leaf spot, Pseudocercospora leaf spot, Septoria leaf spot, Choanephora pod rot, powdery mildew, Sphaceloma scab, Sclerotium stem rot and web blight. Of these diseases, Pseudocercospora leaf spot, brown blotch, bacterial blight and bacterial pustule were of major importance and were consequently studied. Field studies were conducted during the early and late cropping seasons of 1999 and 2000 at the Michael Okpara University of Agriculture, Umudike, a location in the humid rain forest zone of southern Nigeria, to investigate the effect of cropping systems on these major cowpea diseases. The treatments comprised four cropping systems; sole cowpea, cowpea/maize, cowpea/cassava and cowpea/maize/cassava intercrop and six varieties of cowpea; IAR48, IAR1696, 1T89KD-288, IT89KD-374, IT86D-719 and IT93K-452-1 in a 4x6 factorial in randomized complete block design with three replications. The incidence and severity of these diseases were significantly reduced when cowpea was intercropped with other crops without any depressive effect on its growth and yield. 'I he least disease incidence and severity were recorded on cowpea in cowpea/maize/cassava intercrop, while the highest incidence and severity were recorded on sole cowpea The cowpea varieties reacted differently to the diseases; IAR48 was the most susceptible to brown blotch and together with IT89KD-288 and IT89KD-374 were the most susceptible to pseudocercospora leaf spot and bacterial pustule respectively. The highest incidence of bacterial blight was recorded on 1T93K-452-1. Cowpea yields varied significantly among varieties, the highest grain yield (838 kg/ha) was obtained from cowpea variety 1T86D719 in cowpea/maize/cassava intercrop during the late planting of 2000. Time of planting had a significant effect on the disease epidemics and crop yield. Disease development and spread were high in early cropping season but low in late cropping season which encouraged better crop yield, with grain yield of 552 kg/ha and 408 kg/ha obtained respectively in the late and early cropping seasons of 2000. The inclusion of cowpea in the prevailing intercropping system especially in the late season planting should be adopted as an important approach to integrated disease management in cowpea production. Consequently, early maturing cowpea varieties, IT86D-719 and IT93K-452- 1 are recommended for use in maize/cassava intercrop systems during the late cropping season ( August - November) in humid groecological zone of South-Eastern Nigeria.