Bioactive volatile organic compounds produced by Bacillus spp. against potato storage disease pathogens

More than a billion people worldwide eat potatoes, and the total crop production exceeds 374 millions of tons. Unfortunately, the culture requires high pesticide use due to disease pressure. This use of chemical pesticides is increasingly criticized and it is necessary to develop new alternatives to control plant pathogens. To tackle this issue, biopesticides are being intensively studied, with particular attention to Bacillus bacteria. This genus represents an extraordinary source of bioactive compounds. Among them, volatile organic compounds (VOCs) have attracted significant research interest. As naturally occurring chemicals, VOCs might be a potential alternative to pesticides, both to reduce the amounts of chemical inputs and to limit the risk of pathogen resistance development. This research aimed at selecting strains of Bacillus spp. displaying antagonistic activity through volatile production against two storage pathogens. Fusarium solani is a fungus causing potato dry rot and Streptomyces scabies is a bacterium that triggers potato common scab. The majority of isolates showed an activity against at least one pathogenic strain with various efficiencies. This experiment, using “superposed Petri dishes”, allowed the selection of the most active strains: 6 out of 24. The second objective was to characterize volatiles produced by the selected strains thanks to SPME-GC-MS analyses. Although results revealed different volatile profiles depending on the strain, sulfur-compounds were mostly represented. One strain, Bacillus mycoides 14AB7 produced all the interesting identified compounds. These molecules were individually tested against one strain of F. solani. Preliminary results showed that DMTS (dimethyl trisulphide) was particularly interesting as antifungal volatile. In addition to B. mycoides 14AB7, two other strains produced DMTS: B. pumilus 10BB5 and 30BB2. They were then chosen as treatment to run an in vivo experiment with infected potatoes. Unfortunately, the difficulty to turn the efficacy observed in vitro into in vivo success has been highlighted. Besides these assays, a conidial germination experiment was carried out on F. solani in order to understand the volatile mode of action. The results indicated an important reduction of the mycelial growth but no inhibition of conidial germination. Finally, Bacillus volatiles offer different perspectives for future uses. Yet, it will be necessary to explore VOCs produced by Bacillus spp. and their effects under natural conditions, in multi-species communities and with different pathogens. Although the very VOCs’ mode of action remains to be thoroughly understood, our results open new perspectives and highlight the need to study interactions mediated by VOCs between soil-borne pathogens and bacteria.