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Lime and Gypsum Effects on Pea-Root-Pathogen Inoculum and Related Factors in a Wheat-Peas Rotation¹

Root-disease responses to manageable soil chemical factors, such as pH or Ca saturation, can be an effective biological control strategy. In a wheat-peas rotation, single applications of lime, to adjust pH of a Walla Walla silt loam (coarse silty, mixed, mesic Typic Haploxeroll) from 5.5 to 6.2, produced less growth in peas (Pisum sativum, L.) than in wheat (Triticum aestivum L.)-a response inconsistent with greater legume responses to liming. Gypsum applied at 1 Mg/ha did not change soil pH, but was adverse for late-season pea growth while the sulfate ion remained in the upper 0.30 m of soil. Wheat responded to the added S. Propagule density of Pythium ultimum was high enough to damage pea roots, but showed no differential response to the single lime or gypsum application during the 4-yr period. Propagule density of Fusarium solani f. sp. pisi in the 0- to 0.15-m soil layer of the lime treatment decreased 37% over a 3-yr period. Gypsum reduced propagule density 22%. Increased exchangeable Ca in response to the lime and gypsum was the only soil factor that explained this reduced propagule density. Calcium may have improved the resistance of pea-root membranes to pathogen attack, impaired saprophytic growth and survival of the pathogen, or favored microbial antagonism of the pathogen. No measurements were made to determine the mechanism for the Ca effect. Reduced propagule numbers in the top 0.15 m of soil may not have reduced root disease because propagules of F. solani f. sp.pisi were abundant in the 0.15- to 0.45-m layer, which had a pH of 5.7 and showed only a negligible increase of Ca saturation.

Key Words: Calcium • Iron • Manganese • Pea growth • Phosphate • Sulfate • Wheat growth • Zinc • Fusarium solani f. sp. pisi • Pythium ultimum • Pisum sativum L. • Triticum aestivum L.

² Soil scientist, USDA-ARS, University of Minnesota, St. Paul, MN 55108; plant pathologist, USDA-ARS, Prosser, WA 99350; and soil scientist, USDA-ARS, Pendleton, OR 97801.

Received for publication March 21, 1986.

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