Thermal inactivation of Plasmodiophora brassicae Woron. and its attempted control by solarization in the Salinas Valley of California

Thermal inactivation of resting spores of P. brassicae Woron. in glasshouse soil depended on temperature, duration of treatment, inoculum concentration, and soil moisture. At 42, 44 and 50°C, the relationship between temperature and the time for thermal inactivation plotted on a semi-log scale was linear. Treatment times up to 45 days at 30°C and 37°C did not reduce infectivity. The detection threshold in the system was 10^0·5 spores/g of soil. At all temperatures tested, inactivation was achieved more rapidly in soil infested with 10² spores/g than with 10⁶ spores/g. Heat treatment was more effective in saturated soil than in half-saturated soil. Soil temperatures in the field in the northern Salinas Valley were increased 11–14°C by tarping with clear, polyethylene plastic. The average weekly maximum temperature and minimum temperature at a 10 cm depth under tarps were 38°C and 29°C respectively. Solarization reduced disease development after a 10-week treatment but not after a 5-week treatment.