AQUAPONIC WATER SUPPRESSIVENESS ON PYHTIUM APHANIDERMATUM ROOT ROT IN LETTUCE AND ITS ORIGIN

Gilles Stouvenakers*; Sébastien Massart; M. Haïssam Jijakli 
Laboratory of Integrated and Urban Plant Pathology, University of Liège, Gembloux Agro-Bio Tech, Passage des déportés 2, 5030 Gembloux, Belgium 
E-mail: g.stouvenakers@uliege.be

Introduction
In couple aquaponic system, hydroponic plants production, fish rearing and nitrifying bacteria are constituents of the same system and consequently share the same water. Because of this coexistence, phytosanitary treatments for plant diseases management are a delicate matter. Among plant pathogens, fungus-like Oomycetes responsible of plant diseases are among the most problematic because of their possible fast spread linked to zoospores (mobile spores) release in the irrigation water. However, it appears that aquaponic systems could be naturally armed against plant pathogens. This protective action is called suppressiveness (i.e. suppressive action) and refers to the cases where (i) the pathogen does not establish or persist; or (ii) establishes but causes little or no damage (Postma et al. 2008). In in vitro laboratory experiments, aquaponic water has shown a direct inhibitory effect on 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 plant pathogen have been carried out on lettuces growing in aquaponic (AP) water, hydroponic (HP) water or complemented aquaponic (CAP) water. CAP water results from the addition of mineral salts in AP water to obtain identic concentrations of mineral nutrients as the ones contained in HP water.
Materials and methods
Suppressiveness of AP, CAP and HP water were evaluated by comparing root symptoms, leaf yields and root yields of P. aphanidermatum inoculated lettuces with the corresponding negative controls. For each treatments, samples of lettuces rhizosphere, rhizoplan and endosphere have been conserved for microbiota analysis by Illumina high throughput sequencing of ITS and 16S rDNA genes.
Results and conclusions
Results show that root symptoms of inoculated lettuces are significantly lower when they grow in AP water compared to CAP and HP waters. Foliar yield, root yield and leaf turgidity also decrease significantly less when using AP water. These results also highlight that the physicochemical modifications of AP water to make the CAP water probably disturb the natural microbiota and lead to the loss of its suppressive capacity. To elucidate the origin of AP water suppressiveness, a comparative bioinformatics analysis of microbiota diversity and OTU's composition of the different samples is in progress. Moreover, this study could contribute to find novel biocontrol agents for plant pathogens control in aquaponics.
References
Gravel, V. , Dorais, M., and Dey, D. 2015. Fish effluents promote root growth and suppress fungal diseases in tomato transplants. Canadian Journal of Plant Science 95: 427–436.
Postma, J., van Os, E. & Bonants, P.J.M. 2008. Ch 10 - Pathogen detection and management strategies in soilless plant growing system p.425-457. In Soilless Culture: Theory and Practice. Elsevier B.V..608p.
Sirakov, I. et al., 2016. Potential for combined biocontrol activity against fungal fish and plant pathogens by bacterial isolates from a model aquaponic system. Water, 8: 1–7.
Stouvenakers, G., Massart, S. and Jijakli, M.H., 2018. Suppressive effect of aquaponic water on the development of root rot caused by Pythium aphanidermatum in lettuce. In Biological and integrated control of plant pathogens. Lleida: IOBC-WPRS Bulletin Vol. 133: 11–12.