Effects of Lime Particle Size and Distribution and Fertilizer Formulation on Clubroot Disease Caused by Plasmodiophora brassicae. R. L. Dobson, Research Assistant, Western Washington Research and Extension Center, Puyallup 98371. R. L. Gabrielson, Plant Pathologist, A. S. Baker, Soil Scientist, and L. Bennett, Agriculture Research Technologist, Western Washington Research and Extension Center, Puyallup 98371. Plant Dis. 67:50-52. Accepted for publication 17 May 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-50.

Lime has historically been recommended and used for clubroot control, but it has not always been effective. This work was done to evaluate variables that can influence the effectiveness of lime, such as the degree of mixing lime with soil, the fineness of limestone, and the residual basicity or acidity of nitrogen sources in the rhizosphere. Thorough mixing of limed soil resulted in a more uniform pH distribution; the pH from one 0.5-g soil microsample to another varied as much as 2 pH units when not thoroughly mixed to as little as 0.2 pH units when thoroughly mixed. This variation in soil pH was not evident using 15-g soil macrosamples. Control was consistently best using thoroughly mixed limed soils in both greenhouse and field trials. Control also improved with decreased particle size of limestone and with use of calcium nitrate, a fertilizer reported to induce a residual basic reaction in the rhizosphere.