An increasing number of greenhouses are reusing nutrient solutions in their operations to protect the environment and save water and fertilizer (Richard et al., 2006). One major concern of this practice is the risk of dispersal of soil- and water-borne plant pathogens within the recirculation system (Richard et al., 2006). Various water disinfection technologies have been used in controlled environment plant production systems including greenhouse and nursery operations. However, these technologies are usually ineffective in controlling pathogens in potting substrates, especially when the substrates contain organic materials. For example, the frequently used oxidants (e.g., ozone, chlorine, and chlorine dioxides) may react with organic potting substrates before they reach residual levels that are lethal to plant pathogens. Similarly, control of plant pathogens with copper ion is also challenging because organic potting substrates can bind copper ions, preventing them from reaching critical levels for pathogen control (Zheng et al., 2004).
An alternative to nutrient solution disinfestation technologies in controlled environment plant production systems to reduce pathogen pressure in reused nutrient solution is the use of pathogen-suppressing growing substrates and the addition of beneficial microorganisms.
Compost teas (water extracts from the fermentation of compost materials) have been reported to act as natural pesticides and may contain various biopesticidal microbes and organic chelators (Scheuerell and Mahaffee, 2002). Using composted organic materials such as municipal wastes, hardwood bark, and vermicompost as soil amendments have been shown to reduce root rot diseases (Dissanayake and Hoy, 1999; Hoitink et al., 1991; Szczech, 1999; Trillas-Gay et al., 1986).
More than 10 yeast genera were used to control soilborne plant diseases (El-Tarabily and Sivasithamparam, 2006). Yeasts are required for beer brewing and are often removed, in conjunction with other unwanted solids, at the end of the brewing process by filtration through powdered DE. We are not aware of any work designed to investigate the potential of using liquid from DE slurry to control plant pathogens.
During a survey of the status of greenhouse nutrient solution recirculation in Ontario, Canada (Richard et al., 2006), it was found that some greenhouses had been reusing their nutrient solutions for more than 20 years without increased disease incidence. Microbiological analysis of some of these nutrient solutions did not detect any plant pathogens (data not shown).
The objective of this study was to evaluate the inhibitory potential of three compost teas (pine bark, manure, and vermicasting), Root Rescue Landscape Powder® (a mix of mycorrhizae and other beneficial microbes), waste DE from beer brewing, and a greenhouse nutrient solution that had been reused for more than 20 years on six water- or soil-borne plant pathogens commonly found in Ontario greenhouses.
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