Biological control of Macrophomina phaseolina on cowpea ( Vigna unguiculata) under dry conditions by bacterial antagonists

Abstract

Macrophomina phaseolina, the causative agent of charcoal rot of cowpea and many other crops, is a devastating pathogen in many regions worldwide. Single control measures are ineffective or not feasible under farmers' conditions. In order to promote biological control as a component of an integrated management approach under arid ecological conditions, 20 bacterial antagonists were isolated from soil samples collected from the rhizosphere of healthy as well as M. Phaseolina-infected cowpea from fields located in the dry savannah zone of West Africa. In dual culture tests with four media, antagonistic activity was variable and depended on the medium used. Growth inhibition was generally good on tryptic soy agar on which two antagonists also inhibited microsclerotia production by M. phaseolina. Effective antagonists were identified as Bacillus subtilis, B. sphaericus and Paenibacillus polymixa. In greenhouse experiments, seed treatment with B. subtilis strain A11 reduced the incidence of M. phaseolina by 89.29% over the untreated control plants, and lower pathogen quantities in plants were confirmed by DAS-ELISA. Bacillus subtilis A11 was rhizosphere competent and maintained high population densities of up to 6 Log10 CFU/g fresh weight on the roots and 5.73 Log10 CFU/g fresh weight on the hypocotyls of cowpea plants over three weeks after inoculation. This antagonist is recommended for seed treatment in combination with other cultural practices for the management of M. phaseolina under arid conditions.