A strain of Bacillus subtilis subsp subtilis shows a specific antagonistic activity against the soil-borne pathogen of onion Setophoma terrestris

Pink Root is among the major diseases of onion and its causal agent, Setophoma terrestris, is one of the most severe pathogens in soils of tropical and subtropical climates. The management of Pink Root is difficult and the strategies currently used are ineffective. Although, there are some varieties resistant to S. terrestris, they are not resistant to every isolate of the pathogen and it is not unusual to find isolates that break that resistance. Moreover, chemical control is not technically or economically feasible. So, we decided to address biocontrol as a mean to manage the main onion disease in Argentina. We tested the efficiency of a strain of Bacillus subtilis subsp. subtilis isolated from the rhizosphere of onion plants to inhibit the growth of S. terrestris in vitro. Our strain of Bacillus subtilis showed a strong capacity of growth inhibition of S. terrestris. No antagonistic activity against two other onion pathogens, Fusarium oxysporum f. sp. cepae and F. proliferatum, was observed. Interestingly, we found a high growth inhibition of S. terrestris on plates containing cell-free supernatant of B. subtilis previously grown in the presence of the fungus. No significant differences in the fungal growth were obtained between control plates and plates containing cell-free supernatant from B. subtilis grown without previous contact with S. terrestris. Electron microscopy of S. terrestris from co-cultures plates revealed thickened, tortuous or coiled fungal hyphae, with granules and globular like terminations. These results suggested that the strain of B. subtilis under study has a strong biocontrol activity against S. terrestris and that it would be acting diffusible bacterial inhibitory compounds specifically induced by this pathogen.