Suppressive effect of aquaponic water on the development of root rot caused by Pythium aphanidermatum in lettuce

Aquaponic systems, define as recirculating soilless systems combining aquaculture and hydroponic, are at the core of innovative researches. However, plant pest and more especially plant pathogens management is still unclear. In fact, in this kind of one loop device, where the nutrient plant solution returns to the fish part, chemical pesticides and disinfecting agents are not allowed due to the presence of fishes. Furthermore, they might be toxic for beneficial bacteria present in the system, such as nitrifying bacteria. Among the large possibility of diseases occurring in soilless systems, oomycetes pseudo-fungi, responsible of root rot diseases like Pythium aphanidermatum (Edson) Fitzp, are problematics due to their capacity to produce a mobile form, making the dispersion of the disease easier. Two recent articles open the hypothesis of a natural protective action of aquaponic water or fish effluents against plant pathogens during in vitro trials (Gravel et al., 2015; Sirakov et al., 2016). This phenomenon could be linked to the presence of antagonistic microorganisms or inhibitory compounds in fish water. Assumptions that don’t seem aberrant in light of suppressive action already observed in hydroponic systems (Postma et al., 2008). To confirm these observations, in vitro experiments with aquaponic water have been carried out and completed for the first time with in vivo trials to assess its capacity to procure a plant protection effect towards P. aphanidermatum. In vitro results show a very highly significant decrease of mycelium production when 25% of aquaponic water is added to a V8 CaCO3 broth. But no difference was made between the control (standard V8 CaCO3 broth) and the broth containing 25% of 0.2 µm filtrated aquaponic water. Based on this test, inhibitory action of aquaponic water seems to be linked to a microbial action. Nevertheless, an indirect action on pathogens by the way of plant stimulation by water compounds cannot be totally excluded. During in vivo tests, aquaponic lettuces inoculated with P. aphanidermatum have significantly less disease symptoms and a better root yields compared with inoculated lettuces grown with hydroponic water. These results highlight that aquaponic water can contribute to find a novel source of BCAs. To complete the study, water samples of both experimentations were conserved for 16S rDNA Illumina sequencing, in order to identify bacteria potentially antagonist against P. aphanidermatum.