Influences of Some Environmental Factors on the Viability of Resting Spores of Plasmodiophora brassicae Wor. Incubated in Sterile Soil

Abstract

Influences of environmental factors such as temperature, soil moisture and soil pH on the viability of resting spores of Plasmodiophora brassicae incubated in sterile soil were examined by using the direct method based on the fluorescent staining reaction in spores. When resting spores were incubated in acid soil, the frequency of nonviable spores increased readily and profusely after incubation and reached approximately 70 to 80% after a 30-day incubation. The rate of the increase was not appreciably affected by temperatures between 15 and 30°C and by soil moisture contents between 35 and 100% during the 30-day incubation, though affected slightly by low temperature and moisture content. The temperatures and soil moistures in the range tested were not so important factor as affecting the viability of resting spores. On the other hand, soil pH distinctly affected spore viability. A remarkable increase in the nonviable spore frequency was found in acid soils with pH values of approximately 4.5-6.5. In alkali soils the rate of increase was conspicuously low in the beginning of incubation period as compared with those in acid soils, but then the nonviable spore frequency increased continuously at an almost constant rate during a 7-month incubation. The soil pH is one of important factors affecting the viability of resting spores, but may not be the determinate factor in their survival due to lack of ability to suppress independently the decline of the viability during a long period of time.