A strain of Bacillus subtilis subsp. subtilis shows a specific antagonistic activity against the soil-borne pathogen of onion Setophoma terrestris
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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.
KeywordsBacillus subtilis Setophoma terrestris Biocontrol Onion
This work was principally supported by a grant from the National Institute of Agricultural Technology (INTA). Grants from the National Council of Scientific and Technological Research (CONICET) and Secretary of Research and Technological Linkage of the Catholic University of Cordoba are also acknowledged. Andrea G. Albarracín Orio is member of the Research Career of CONICET, Elsa Brücher had a PhD fellowship from CONICET and Daniel D. Ducasse is researcher of INTA.
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