Effect of moisture on thermal inactivation of soilborne pathogens under structural solarization

Structural solarization of greenhouses for sanitation by closing them involves dry heating to 60°C and higher with a consequent low relative humidity (RH) (≈15%), thus requiring an extended period for thermal inactivation of pathogens. In an attempt to enhance pathogen control by increasing moisture during the hot hours of the day, various regimes of inoculum moistening were studied. However, wetting inoculum of Fusarium oxysporum f. sp. melonis and F. oxysporum f. sp. radicis-lycopersici resulted in less effective pathogen control compared with that of dry heating. Fifty percent effective dose (ED₅₀) values of thermal inactivation of wetted and dry inoculum for the former pathogen were 18 and 7 days, respectively, and for the latter, a respective 9 and 4 days. This was because wetting resulted in inoculum cooling due to evaporation, which eventually led to its drying. A model describing the drying of wet inoculum in a wetted greenhouse, based on the fact that there was an ≈=10°C difference between greenhouse and ambient temperatures, was proposed. A double-tent system reduced this difference to 1 to 2°C, reduced moisture loss, and led to improved inoculum inactivation of F. oxysporum f. sp. radicis-lycopersici. Thus, the ED₅₀ value of thermal inactivation was reduced from 15 days to 1 day, because this system provided both high temperature (≈60°C) and high RH (≈100%), resulting in effective wet heating.