Using isothiocyanate plant allelochemicals as biocontrol against plant pathogenic Ralstonia solanacearum bacterium

Ralstonia solanacearum is a bacterial plant pathogen with a global status, able to infect many economically important crops including tomato and potato. Traditionally R. solanacearum has been controlled using agrochemicals. However, stricter legislations and environmental damage caused by agrochemicals have made these options unfeasible. Biofumigation is a biocontrol method that exploits Brassica plant allelochemicals called isothiocyanates (ITCs) and could be used to protect against crop disease. While recent studies have shown promising results, the potential for pathogen ITC tolerance evolution and the effects of ITCs on non-target microbes and crop hosts remain understudied. This thesis identifies that allyl-ITC is very effective at suppressing R. solanacearum pathogenic bacterium in vitro and in vivo. However, ITC exposure can also select for ITC tolerance evolution. Mechanistically, tolerance was associated with insertion sequence movement particularly in the megaplasmid. In multi-species communities, it was found that plant non-pathogenic Pseudomonas bacteria were less susceptible to ITC than R. solanacearum in liquid and soil microcosms and in vivo tomato mesocosms. Further, synergistic suppressive effects of ITC and Pseudomonas on R. solanacearum densities were observed. However, ITC effects were less specific in more complex rhizosphere communities, where ITC application reduced the diversity and affected the composition and potential connectivity of soil microbiota. Crucially, ITC application had negative effects on the plant flowering and biomass where Pseudomonas were present. The work presented here lay the groundwork for systematic understanding of ecological and evolutionary effects of biofumigation on R. solanacearum pathogen, non-target microbiota and crop host.