Modeling thermal inactivation of soilborne pathogens under Structural (Dry) and soil (Wet) solarization

Solarization relies on light being converted to heat for the killing of plant pathogens in soil, artificial growth substrates and greenhouse structures. Temperatures achieved during solarization follow the diurnal cycle, with maximum temperatures occurring during the day and the minimums during the night. However, most of the information on rate of thermal inactivation of pathogens has been collected in the laboratory under constant temperature conditions. Such data are not readily useful for modeling solarization efficacy. Accurate prediction of the effective level of pest control that one can obtain during a solarization requires that we assess thermal inactivation of pathogens under fluctuating climatic conditions. By modeling, we attempt to separate the effect of heat at varying temperatures on pest control. This can be done either empirically or by utilizing thermal inactivation data, which were accomplished by exposing the pathogens to heat at constant temperatures. Structural solarization is carried out by closing the greenhouse for a certain period of time during the hot season. Air temperature inside the structure during the hot hours can reach levels above 60°C. The consequent relative humidity (RH) drops to 15%; thus, structural solarization can be regarded as dry heating. Low humidity values lead to reduced effectiveness of thermal inactivation of pathogens. Thus, a model of structural solarization should consider both diurnal fluctuating temperatures and RH values. Using a modified Weibull model based on temperature and RH, we were able to obtain good agreement between calculated and observed rates of kill of inoculum of Fusarium oxysporum f.sp. radicis-lycopersici (FORL) and of sclerotia of Sclerotium rolfsii (SR). The R2 values for FORL control ranged in most experiments from 0.86 to 0.98 and for SR control from 0.82 to 0.94. Soil solarization is carried out in wet soil. Therefore, a model of pest control needs to only consider diurnal temperature fluctuations. The data on pathogen control under constant temperatures were used to develop equations in which the weighting of the obtained temperatures was normalized to degree-hours (NDH). The rate of pathogen control was positively correlated with heating intensity, as expressed in NDH above certain temperatures. The R2 values for correlations between NDH and pathogen survival during soil solarization at 10 to 40- cm depths was 0.53 to 0.86 for FORL and 0.82 to 0.95 for SR. At lower soil depths (40 cm), R2 values were lower for FORL, suggesting the involvement of additional factors, e.g. biocontrol. The above modeling approaches can provide tools for quantifying solarization effectiveness under various climatic conditions.