Biocontrol effects of bacteria isolated from compost tea against postharvest disease of tomato fruit (Solanum lycopersicum L.).

Prominent postharvest pathogens of tomato fruit include Alternaria spp. and Botrytis cinerea. While the control of such pathogens typically relies on synthetic chemical fungicides preharvest, microbial biocontrol agents are a sustainable and promising alternative postharvest. Bacillus velezensis SH1 and SH2 and Pseudomonas azotoformans SH3 were previously isolated from disease-suppressive compost tea. To evaluate their effectiveness against tomato pathogens, the bacteria, their cell-free culture filtrates and precipitated subfractions were tested against mycelial growth in vitro. Suppressive effects were also examined against disease development on tomato fruit. From bacterial precipitates, lipopeptides were further identified and quantified. Results showed that all bacteria controlled growth of the three fungi, with P. azotoformans SH3 generally being the most effective. However, in vivo disease lesion suppression varied according to pathogen and tomato cultivar. In cell-free culture filtrate assays, mycelial inhibition was observed from all bacteria against B. cinerea, whereas mainly B. velezensis SH1 inhibited A. solani and A. alternata growth. For bacterial precipitates, B. velezensis SH1 was the best treatment overall, while P. azotoformans SH3 precipitates showed no inhibition. Both B. velezensis strains were confirmed to produce fengycins, iturins and surfactins. Fengycins seemed to play the most important role in the observed activity of the Bacillus lipopeptides. Results suggest that antifungal activity by the bacterial strains is imparted through the release of antifungal compounds, although the role of the produced compounds was variable.