Efficacy of electrolytically-derived disinfectant against dispersal of Fusarium oxysporum and Rhizoctonia solani in hydroponic tomatoes

Demand for conservation and recycling of water has increased significantly. Therefore irrigation water used for horticultural or agricultural purposes needs to be treated before being reused to eradicate plant pathogens and thereby reducing the risk of pathogen dispersal and losses due to disease. The economically important fungal plant pathogens Fusarlum oxysporum (Synder and Hans) and Rhizoctonia solani (Kuhn) were selected to examine the efficacy of nutrient solution treatment by electrolytic disinfection to prevent the dispersal of these pathogens in the hydroponic production of tomatoes (Solanum lycopersicum Mill.). First, we determined the efficacy of the disinfectant to inactivate F. oxysporum and R. solani in vitro. The electrolytically generated potassium hypochlorite (KClO) was tested at five concentrations of free chlorine (0.2, 0.5, 0.8, 1.0, 2.0 mg/L) in nutrient solutions of pH 5.5, 6.0 and 6.5 with four contact times (5, 30, 60, 120 min). Best sanitation was achieved in nutrient solution at pH 6.0. In vitro, F. oxysporum required 2 mg/L at 30 min for complete inactivation whereas chlorination had only a minimal effect on viability of R. solani. Subsequent trials under practical conditions applied the disinfectant via a new sensor-based disinfection procedure. Potassium hypochlorite solution produced on site and injected into a recirculating nutrient solution once a week for 60 min at a free chlorine concentration of 0.5 mg/L (ORP 780 my) inhibited the dispersal of F. oxysporum and R. solani during the entire test period of 16 weeks. In contrast all tomato test plants irrigated with untreated nutrient solution became infected with F. oxysporum and a third of them additionally with R. solani. At the applied dose no plant damage occurred. Thus, the treatment proved to be effective and applicable to prevent dispersal of fungal pathogens by nutrient solution under simulated field conditions.