Effect of Potassium on the Incidence of Diaporthe sojae in Soybean1

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Abstract
Many varieties of soybean (Glycine max (L.) Merrill) are highly susceptible to the fungus Diaporthe sojae Lehman when grown in Delaware. It has been shown, in Iowa, that potassium levels in upper pods of soybean plants decreased markedly in late season. Because the upper pods frequently had the highest incidence of D. sojae in Delaware and the infection occurs in late season, the relationship of K in soybean to the incidence of disease caused by D. sojae was studied.Two varieties of soybeans were grown in aluminum lined cylinders on soil testing low in K during the summer of 1972 in the field. This allowed control over the soil volume which the plants could use for a source of K. Potassium chloride and potassium sulfate were added to the soil at O, 2, 10, and 30 g/cylinder. The 30‐g treatment received a 10‐g fertilizer sidedress at midseason. These rates were equivalent to O, 98, 490, and 1,960 kg/ha of K as KC1 and O, 82, 410, and 1,640 kg/ha of K as K2SO4, respectively, which included rates higher than usually used in soybean production. Mature seed was evaluated for incidence of D. sojae.Significantly less diseased seed occurred as K level increased, although neither KC1 nor K2SO4 prevented incidence of the disease. The K had no significant effect on the seed yield or number of seeds per plant.

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