Control of Pythium wilt and root rot of hydroponically grown lettuce by means of chemical treatment of the nutrient solution

Abstract

Hydroponic production was initially explored as an alternative to field production due to the ease of plant growth control and the hopes of preventing the majority of disease causing agents known to be present in general soil environments. Of primary concern in terms of pathogens are the water-borne and water-motile zoosporic fungi (especially Pythium spp.) which are able to spread easily throughout the system and cause root-rot and wilting. Few pesticides are currently registered for use in hydroponic systems due to the high costs of registration, while registered pesticides carry a high cost to the grower. Recent legislative moves by numerous countries are also resulting in a trend towards the re-use of hydroponic nutrient solution. As a result such hydroponic solutions require a greater level of disinfection to prevent disease outbreaks but without resulting in chemical buildup of phytotoxic and environmental concern. Sanitiser formulation has seen significant changes over the last few years resulting in sanitisers being used in many new areas and in a more environmentally friendly nature. Although sanitisers are not designed to have specific action against micro-organisms (as is the case with fungicides and anti-microbial agents such as antibiotics), most sanitisers are able to act on cell membranes due to the inherent surfactant properties. This study attempted to determine the suitability of various sanitisers and chemicals as alternate means of control of Pythium in recirculating gravel hydroponic systems by: 1). Exposing Pythium zoospores in a water suspension to the sanitisers Actsol®, Agral 90®, Fitosan®, Prasin®, Purogene®, TecsaClor®, Sporekill® and copper (as copper (I) sulphate) which all managed to eliminate 80% or more of the viable inoculum within a 10 minute exposure time at relatively low concentrations. 2). Testing the above sanitisers for phytotoxicity effects on cucumber plants in a static hydroculture system under laboratory conditions and lettuce plants in a gravel bed hydroponic system under greenhouse conditions. Purogene® and TecsaClor® exhibited a slight growth promotion effect at low concentrations, yet still caused negative phytotoxic effects when dosed at high concentrations. All other sanitisers exhibited some measure of phytotoxicity, observed as growth retardation and leaf discolouration, with phytotoxic effects increasing with increasing concentrations. Copper sulphate was found to be the most phytotoxic chemical tested. 3). Addition of the sanitisers to a small scale hydroponic system (greenhouse), as well as to a semi-commercial scale (field) gravel bed hydroponic system artificially infested with Pythium and cultivated with lettuce. The sanitisers were also compared to a commercially available fungicide, Phytex®. Only Phytex® and Purogene® managed to effectively reduce disease incidence and promote growth over an untreated, Pythium infested control. The results indicated that Purogene® was the most effective for application into a gravel bed hydroponic system cultivated with lettuce, while no sanitiser treatment was able to equal the improved growth and disease control recorded with treatment of the commercial fungicide Phytex®. Although all the sanitisers were able to reduce levels of Pythium inoculum in the hydroponic nutrient solution, this beneficial effect did not translate into increased yields, due to the growth retardation due to phytotoxic effects.