Effect of intercropping cowpea with maize or cassava on cowpea bacterial blight and yield

Abstract

The effect of intercropping cowpea (Vigna unguiculata (L.) Walp.) with cassava (Manihot esculenta Crantz) or maize (Zea mays L.) on bacterial blight of cowpea caused by Xanthomonas axonopodis pv. vignicola was investigated in five cropping systems: cowpea monoculture at high density and at low density, cowpea-maize ‘within row’, cowpea-maize ‘in alternate rows’, and cowpea-cassava ‘in alternate rows’, in the forest savanna transition zone of Benin over two years. After inoculation of field border plants, disease severity index and incidence were measured as areas under the disease progress curve (AUDPC). The AUDPCs for disease incidence and severity were reduced in one of four treatments in each of two subsequent years, and one of four treatments in one year, respectively, in the ‘in alternate row’ patterns compared to monoculture at high density. Disease incidence was higher in cowpea monoculture at high density than in cowpea-cassava ‘in alternate rows’ in year 1996, and in cowpea-maize ‘in alternate rows’ in year 1997. The disease severity index was reduced by 50% in the cowpea-cassava ‘in alternate rows’ pattern in 1997, but not in 1996. Highest percentages of leaves with spots or blight occurred only in the cowpea monoculture at high density. Compared with monoculture, cowpea yield was about 60% in intercropping with cassava and about 40% in intercropping with maize. Cassava yielded 45–57% and maize 53–60% compared to monocropped cassava and maize, respectively. Comparing the land efficiency use of cowpea monoculture at high density to intercropping, the land equivalent ratio was similar in the intercropping systems with ‘alternate row’ patterns, and a yield loss of cowpea in intercropping was compensated by the additional yield of the intercrop. Although no stable effect of intercropping on disease reduction was observed, intercropping cowpea with maize or cassava ‘in alternate rows’ reduced bacterial blight in some cases and generally gives the farmer more yield security in case of failure of one crop. Therefore, intercropping as one element of an integrated control strategy for cowpea bacterial blight should not be abolished by farmers in favour of monocropping cowpea.