Potential of Bacillus subtilis as a biological control agent against three rot-causing pathogens in seed yams

Ghana is the largest exporter of yams in West Africa. However, yam production, particularly seed yam production, is constrained by storage rot during the off-season. Farmers seldom use synthetic pesticides to control seed yam rot. However, these are costly and pose adverse health risks to farmers. Biological antagonists offer a sustainable, relatively cost-effective, and safe alternative to synthetic pesticides. Therefore, this study aimed to test the efficacy of Bacillus subtilis as an alternative to synthetic pesticides. Bacillus subtilis supplied through the biofungicide Serenade ASO (Bayer) was assayed against three storage-rot pathogens: Lasiodiplodia theobromae, Aspergillus niger, and Rhizopus sp. These pathogens were previously isolated from the tissues of rotten seed yams. The efficacy of the bacterium was tested at concentrations of 17 %, 33 %, and 50 % in both in vitro and in vivo bioassays. B. subtilis (50 %) completely inhibited the growth (100 %) of L. theobromae in the in vitro studies. In contrast, there was little to no growth inhibition of the other two test fungi. In the in vivo assay, B. subtilis (50%) significantly (P < 0.01) inhibited L. theobromae, resulting in minimal rot lesions. However, B. subtilis (50 %) was ineffective against the other two test pathogens, resulting in large rot lesions on the seed yams. This suggests that B. subtilis could be an ideal alternative to synthetic pesticides for controlling L. theobromae on seed yams.