Biocontrol potential of some selected native bacterial antagonists against tomato (<em>Solanum lycopersicum</em> L.) early blight causal agent <em>Alternaria alternata</em>

Abstract

Seven selected bacterial antagonists isolated from healthy tomato rhizosphere and phyllosphere were evaluated for their biocontrol efficacy against the tomato early blight causal agent Alternaria alternata under in vitro conditions. The causal organism of early blight of tomato was isolated from a tomato leaf sample which showed early blight symptoms and verified it as Alternaria alternata. Healthy tomato rhizosphere soil, fructosphere and phyllosphere extracts were used as the potential sources of antagonists. Thirty bacterial isolates from tomato rhizosphere soil and 22 bacterial isolates from tomato phyllosphere were isolated using serial dilution technique. Out of those isolates, 24 were subjected to antagonists screening procedure. It was observed that, all the bacterial isolates except RA 5, RA 21, PA 1, PA 2, inhibited the growth of A. alternata significantly (p<0.05) in co-cultivation and dual culture showing an average radial growth inhibition of 69 %. Seven different bacterial isolates coded as RA 8, RA 12, RA 17, RA 18, RA 29, PA 4 and PA 12 were selected for further studies. Based on 16 S rRNA analysis, four of these antagonists were identified as Bacillus thuringiensis, Bacillus cereus and Pseudomonas sp. Microscopic observations of A. alternata hyphae subjected to antagonism showed deviations from the normal hyphae including thickenings, swellings and vacuolation. Inhibition of A. alternata spore germination by selected antagonists was significant at p<0.05 level with an average inhibition of 83.7 %. All the selected antagonists produced diffusible antifungal substances and showed hyperparasitism and competition as their mechanisms in antagonizing the fungal pathogen A. alternata. These results suggested that these seven bacterial isolates can be further explored as potential biocontrol agents in controlling early blight disease in tomato.