Lime and the Control of Clubroot of Crucifers: Effects of pH, Calcium, Magnesium, and Their Interactions

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Abstract
The effects of pH and nutrients on the development of Plasmodiophora brassicae in broccoli were studied in a nutrient solution-sand culture system. At pH > 7.2, primary infections were reduced and clubbing prevented, apparently because primary thalli aborted before they released secondary zoospores. As the concentration of calcium or magnesium was increased from 0.5 to 2.5 mM, infection and clubbing were inhibited at a limiting concentration that varied inversely with the pH. These results may be related to calcium uptake which was less at pH 6.2 than at pH 6.8 or 7.2 with intermediate or low amounts of calcium in the nutrient solution. Calcium and hydrogen ions have low toxicity to resting spores in that incubation of resting spores in 1.0 M CaCl2 at pH 6.2-7.2 for 1 wk reduced infectivity only slightly from that of NaCl or buffer controls.