Effect of prewetting on thermal inactivation of soilborne pathogens during structural solarization of greenhouses

Structural solarization of greenhouses, a complementary disinfestations approach for the sanitation of greenhouse structures, involves dry heating to 60°C and higher at a consequently low relative humidity (ca. 15%). Dry heating is less effective than wet heating in thermal inactivation of organisms. However, increasing inoculum moisture during the hot hours of the day results in less effective pathogen control by structural solarization due to inoculum cooling. We investigated the effect of prewetting of inoculum, prior to the heating process, on thermal inactivation of soilborne pathogens. Two types of propagules were tested: 1) sclerotia of Sclerotium rolfsii and macroconidia of Fusarium oxysporum f. spp. radicis-lycopersici, lycopersici and basilici (multicellular propagules); 2) chlamydospores of F. oxysporum f. spp. radicis-lycopersici and melonis (unicellular propagules). The inocula were exposed to various prewetting durations and matric potentials of wetted soil. Prewetting treatments significantly reduced heat sensitivity of the multicellular propagules and increased heat sensitivity of the unicellular ones. Treating S. rolfsii sclerotia with cycloheximide, which suppresses protein biosynthesis, significantly increased heat sensitivity, suggesting the involvement of metabolic processes during prewetting, e.g. heat-shock proteins synthesis. Scanning electron microscope observations revealed cracks in the rind of heated S. rolfsii sclerotia, whereas untreated sclerotia appeared smooth, with fewer cracks. Prewetting treatments significantly reduced the number of cracks on heated sclerotia. Cracked sclerotia have been shown to be more easily invaded by soil organisms. Heated macroconidia of F. oxysporum f. sp. basilici were distorted, while prewetted ones were not. Elucidating the effects of moisture regimes on pathogen control might provide tools for improving structural solarization.