DISINFESTATION OF POLYSTYRENE SEEDLING TRAYS FOR SUBSTRATE-BASED FLOAT SYSTEMS IN AUSTRALIA

Abstract

Internationally, a move by the tobacco industry to substrate-based ‘float systems’ for transplant production has dramatically reduced their use of methyl bromide (MB) for soil disinfestation. The float system utilises expanded polystyrene trays, filled with soil-less substrate, seeded with tobacco, and floated on a common pool of water (containing fertilisers) within polyhouses. Prior to 2005, tobacco growers in Australia (and other countries) disinfested their used trays with MB to reduce the risk of carrying pathogens into the float system. Our research investigated four approaches to provide growers with alternatives to MB for tray disinfestation: (a) improved cultural practices, (b) heat disinfestation, (c) biocidal dips, and (d) alternative fumigants. Results showed there was a strong association between improved hygiene practices by growers (particularly tray handling and storage, p=0.01) and reduced pathogen populations on their used trays. Heat disinfestation with aerated steam (1 h, maximum temperature 80°C) or solarisation (6 wks, maximum temperature 75°C) provided practical and consistent alternatives to MB, reducing fungal populations on used trays to undetectable levels and the viability of artificial inoculum of R. solani embedded in trays to nil. Although biocides (e.g. NaOCl) and fumigants (e.g. methyl iodide) also reduced pathogen populations on trays, they sometimes caused phytotoxicity in transplants. Overall, our research showed that improved hygiene practices and tray disinfestation with steam or solarisation provide viable alternatives to MB for minimising the risk of used trays carrying pathogens into the float system.