Efficacy of biosurfactants in the management of Phytophthora capsici on pepper in recirculating hydroponic systems

Zoosporic pathogens are among the most destructive root pathogens in recirculating cultural systems, and zoospores are the primary infectious propagule responsible for their spread via the recirculating nutrient solution. More stringent regulation of pesticide usage in greenhouses has resulted in the need for alternative management strategies. This investigation was designed to evaluate the efficacy of rhamnolipid and saponin biosurfactants in managing zoosporic plant pathogens in recirculating systems where plants are cultivated hydroponically in either an inorganic or an organic substrate. The efficacy of biosurfactants in the control of root rot was demonstrated in vivo using a pepper – Phytophthora capsici pathosystem. Amending the recirculating nutrient solution with either a rhamnolipid or a saponin biosurfactant (150 and 200 µg a.i./mL, respectively), which selectively kill zoospores, resulted in 100% control of the spread of the pathogen. Disease control was achieved in both ebb and flow and top-irrigated cultural systems, with either an organic potting mix or rockwool as the planting medium. In the absence of either biosurfactant, all plants within a cultural system were killed within 6–7 weeks following hypocotyl inoculation of a single plant in the system, which served as the source of secondary inoculum. Injecting the rhamnolipid biosurfactant into the irrigation line during every irrigation also resulted in 100% control of the disease. These results provide evidence that biosurfactants may be suitable alternatives to synthetic surfactants and microbial agents for the management of diseases caused by zoosporic pathogens in recirculating systems.