On a method for direct assesment of resting spores of Plasmodiophora brassicae in soil

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A modification of BUCZACKI and OCKENDON's method was satisfactory for extracting of resting spores of Plasmodiophora brassicae from soil. In the extracting process of resting spores from soil described by BUCZACKI and OCKENDON (Fig. 1), replacing the steps of filteration of the soil suspension through 4 layers of muslin, followed by centrifugation of the filterates by wet-sieving method through 500-, 125-, 74- and 37 μm mesh sieves to remove course organic matters and grits was more successful than BUCZACKI and OCKENDON's method. Efficency of recovery of resting spores from the artificially inoculated soil was about 95.0±2.0%. The naturally infested soils where severe symptoms of clubroot developed contained more than 106 resting spores/g dry soil, but the number of spores obtained from the field treated with calcium hydroxide and pentachloronitrobenzen (PCNB) prior to cropping was less at 105 /g dry soil. Other methods tried in this study, such as floatation methods using various chemical solutions with different specific gravities, sucrose density gradients and a double layers centrifugation technique were not useful for separating resting spores from fine soil particles.
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