Application of aquaponic microorganisms alone or in consortium as original biocontrol method of lettuce root rots in soilless culture

Root pathogens in hydroponic culture are often difficult to control without the use of synthetic pesticides. Moreover, most of the available biocontrol agents were isolated from soils. They were not developed for soilless application and often resulted in poor efficacy. It is therefore important to find novel sources of beneficial microorganisms that could grow and protect plant root in such aquatic environment. In this context, aquaponic systems that combines hydroponic plant culture and fish farming were described as a promising source of biocontrol agents. From a previous study, three aquaponic microorganisms were isolated and selected to evaluate their capacity to control P. aphanidermatum root rot disease on lettuce. Sphingobium xenophagum SHb30, Mycolicibacterium fortuitum C13 and Aspergillus flavus G2 were the three strains used alone or in consortium to protect soilless lettuce. Treatments were compared with a biocontrol agent registered against Pythium diseases, a propamocarb fungicide and the direct use of aquaponic water. G2 treatment alone protected lettuce as well as the fungicide and the foliar fresh mass of lettuce was similar to healthy lettuce. C13 had no effect on the disease, while SHb30 limited foliar yield loss. Consortium containing G2 gave similar results than G2 alone but the SH30+C13 combination tended to increase the protective effect in comparison with separated application. This study highlighted that aquaponic water or some of its microorganisms applied alone or in consortium could control P. aphanidermatum root rot disease on lettuce in soilless conditions in a similar way than a fungicide and with a better action than a registered biocontrol agent. Further research will aim at developing appropriated formulation to stabilize and improve biocontrol efficacy of these strains.