Dual protection of hydroponic tomatoes from rhizosphere pathogens Ralstonia solanacearum and Fusarium oxysporum f.sp. radicis‐lycopersici and airborne conidia of Oidium neolycopersici with an ozone‐generative electrostatic spore precipitator

Abstract

An ozone-generative electrostatic spore precipitator was developed to protect nursery-stage seedlings of tomato from both airborne conidia of powdery mildew ( Oidium neolycopersici ) and root-infecting pathogen propagules of bacterial wilt ( Ralstonia solanacearum ) and fusarium crown and root rot ( Fusarium oxysporum f.sp. radicis-lycopersici ). The device was a cylindrical electrostatic spore precipitator (S2 cylinder) in which a positively charged straight conductor wire insulated with a transparent acrylic cylinder originated from a spore-precipitation cylinder (S1 cylinder) designed to physically control airborne conidia of tomato powdery mildew in greenhouses. The S2 cylinder consisted of two sites for conidial attraction and ozone production. The site for ozone production was located at the end of the cylinder, where an earthed copper conductor ring (as a cathode) was attached to the edge of the cylinder, responding to the anodal tip of a positively charged central conductor wire. Distinct types of discharge (corona, corona-streamer, streamer and arc discharge) occurred between the two electrodes and were dependant on the voltages applied to the wire and the distances between the electrodes. The highest ozone production was observed through streamer discharge. The remaining portion of the S2 cylinder, which was dielectrically polarized by a positively charged wire, created a non-uniform electric field outside the cylinder to attract conidia that came into the generated field. Hydroponic culture troughs to raise tomato seedlings in a nursery greenhouse were paralleled with S2 cylinders. The aim was to control rhizosphere pathogens R. solanacearum and F. oxysporum f.sp. radicis-lycopersici and to prevent them entering the hydroponic system during cultivation, while at the same time trapping O. neolycopersici conidia in the spaces between the cylinders. The results indicated that susceptible tomato plants in culture troughs attached to the S2 cylinders remained uninfected by both rhizosphere and aerial pathogens throughout the experimental period (2 and 3 weeks, respectively). This suggests that the present system will enable the dual control of both these pathogens in hydroponic systems in greenhouses.Keywords : fusarium crown and root rot of tomato, ozone production, physical control of pathogens, polarized dielectric insulator, tomato bacterial wilt, tomato powdery mildew IntroductionWhile a greenhouse environment is excellent for growing tomatoes, it is even better for propagating disease-causing organisms. Because of the higher temperature, higher relative humidity and lush green foliage, diseases are constant threats once they enter a greenhouse. Management of disease at the seedling nursery stage is important to prevent the establishment of diseases that may affect cropping. In greenhouses, powdery mildew of tomato ( Oidium neolycopersici ) has frequently been detected on hypocotyls, cotyledons and first leaves of nursery-stage seedlings, as well as on well-developed plants in the propagation stage (M...

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