Aquaponic water, a novel source of biocontrol agents against pythium aphanidermatum root rot in lettuce

In aquaponics, phytosanitary treatments for plant diseases management are a delicate matter because of the presence of fish. However, it appears that aquaponic systems could be naturally armed against plant pathogens. This natural protective action is called suppressiveness. Aquaponic water has shown a direct inhibitory effect on in vitro Pythium spp. growth (Gravel et al. 2015; Sirakov et al. 2016; Stouvenakers et al. 2018). To confirm this discovery, in vivo experiments using Pythium aphanidermatum have been carried out on lettuces growing in hydroponic (HP) water, aquaponic (AP) water or aquaponic water complemented (COMP) with mineral nutrients to reach HP levels. Suppressiveness property of AP, COMP and HP waters was evaluated by comparing root symptoms and lettuces yields. For each treatments, lettuces root microbiota were analysed after Illumina high throughput sequencing of ITS and 16S rDNA genes. Results showed that yields and roots health of AP lettuces were significantly better compared to COMP and HP lettuces. The bioinformatics analysis of sequenced microbiota showed differences in terms of microbiota diversity and features composition when comparing AP with COMP and HP water. These results highlight that the physicochemical modifications of the AP water to make the COMP water disturb the AP microbiota and lead to the loss of its suppressive capacity. Moreover, AP could be an interesting source of novel biocontrol agents for plant pathogens control in aquaponics.