Effects of Chloride and Ammonium Salts on the Incidence of Phytophthora Root and Stem Rot of Soybean.

Abstract

In growth chamber experiments using field soil naturally infested with Phytophthora sojae, addition of KCl at 150 to 400 μg of K per gram of dry soil increased the incidence of Phytophthora root and stem rot on susceptible soybean (Glycine max) compared with a distilled water control. Other potassium salts at 150 to 400 μg of K, phosphorus salts at 20 to 317 μg of P, and sulfate salts of K, Ca, Mg, Na, and Al at 491 μg of SO4 per gram of soil did not affect disease incidence. Application of chloride salts of K, Na, Mg, Ca, NH4, Al, Fe, and Sr at 250 μg of Cl per gram of soil all increased disease incidence compared with the distilled water control. Addition of other ammonium salts also increased disease incidence. Disease incidence increased when KCl at 300 μg of K per gram soil was applied to naturally infested Blount silt loam but not to naturally infested Hoytville silty clay. The effects of chloride salts on plant growth and on natural inoculum of P. sojae were investigated. At 450 μg Cl/g of soil, addition of NaCl increased plant fresh weight but had no effect on germination, emergence, transpiration, height, and dry weight of plants grown in pasteurized, pathogen-free soil. When soybean plants were grown without a deplasmolytic shock and without promoting zoospore release by saturating the soil, the addition of chloride salts at 304 μg of Cl per gram of soil increased disease. In a leaf-disc-baiting bioassay, leaf infection by P. sojae zoospores decreased when 0.01 or 0.02 M KCl was added to soil extracts from flooded soil but was unaffected by KCl at 150 to 600 μg of K per gram of soil applied to the soil 5 days before baiting. Energy-dispersive X-ray analysis of taproots of 5-day-old soybean plants indicated a reduction in the calcium content of the outer cell layers with KCl at 400 μg of K per gram of soil but not with K2SO4. Disease increases with addition of KCl appear to be due to the presence of chloride and may be associated with changes in the micropartitioning of root calcium.