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Factors Related to Control of Clubroot of Crucifers in the Salinas Valley of California. R. N. Campbell, Department of Plant Pathology, University of California, Davis 95616; A. S. Greathead(2), D. F. Myers(3), and G. J. de Boer(4). (2)Cooperative Extension Service, Salinas, CA 93901; (3)Formerly Department of Plant Pathology, University of California, Davis, now Agricultural Research & Education Center, Belle Glade, FL 33430; (4)Cooperative Extension Soils Laboratory, University of California, Davis. Phytopathology 75:665-670. Accepted for publication 10 January 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-665.

Control trials were started after clubroot, caused by Plasmodiophora brassicae, was discovered on broccoli in the Salinas Valley in 1978. Single applications of lime (CaCO3) controlled clubroot for up to 3 yr and were more effective than pentachloronitrobenzene or calcium cyanamide. Some parameters affected by lime application were evaluated to better understand the mechanism of action of lime and the variable control of clubroot reported in liming trials in which only pH was measured. Lime applied at 5?33 metric tonnes per ha (t/ha) to severely infested Placentia sandy loam soil gave control for 2?3 yr in two trials. Soil pH increased as a curvilinear response to lime and continued to increase each year so that control was obtained at pH 6.7 in the first year and at pH 7.2 in the third year. Extractable calcium increased as a linear response to lime and control was obtained with 12?14 meq/100 g of soil. In contrast, a single application of 2.7?10.8 t/ha to a lightly infested field of Elder sandy loam soil controlled clubroot in the first crop, but not in the second crop when the pH and extractable calcium fell below 7.1 and 14 meq/100 g, respectively. In one trial, lime was as effective when incorporated into the soil 1 day before planting as when added 6 wk before planting. Lime was effective in another experiment in a commercial field and in numerous infested fields treated by growers. We postulate that successful control by liming depends on an interaction between pH and extractable calcium plus magnesium which must exceed approximately 14 meq/100 g of soil from native minerals plus the lime treatment.