| Part I. This study analyzes the pathotypes or races of Plasmodiophora brassicae occurring in crucifer production areas along the west coast of the USA. Using the European Clubroot Differential set (ECD), all collections were found to show similar virulence characteristics. ECD code designations of the collections were either 16/02/31 or 16/03/31, both corresponding to race 7. The need of collecting spores from one ECD host showing limited infection and reinoculating these spores onto the same and other ECD hosts was important to verify the presence of virulence genes at low frequencies.;Part 2. Lime has historically been recommended and used for clubroot control, but it has not always been effective. This work was undertaken to evaluate variables that can influence the effectiveness of lime, such as, the degree of mixing lime with soil, the fineness of limestone (CaCO(,3)), and the residual basicity or acidity of nitrogen sources in the rhizosphere. Variations of pH within limed soil decreased with thorough mixing and consistently gave the best control both in greenhouse and field trials. Similarly, control was directly related with the fineness of the limestone (CaCO(,3)) and with use of Ca(NO(,3))(,2), a fertilizer reported to induce a residual basic reaction in the rhizosphere.;Part 3. In terms of zoospore movement, soil matric potential is a more appropriate measure of soil moisture than is percent maximum water holding capacity. In this study, matric potentials required for root-hair and cortical infection were determined using both tension plates and a semipermeable membrane/polyethylene glycol system. In a silt loam and sandy loam soil from western Washington, cortical infection required moisture levels greater than -150 mbars whereas in a muck soil infection was found at -200 mbars. In contrast, root-hair infection in the silt loam soil was observed at close to -800 mbars. The ability of root-hair infection to occur in these dry soil conditions is discussed in relation to the life cycle of this pathogen. |
