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Effects of Crop Rotation on Rhizoctonia Disease of White Potato. L. P. Specht, Former Research Graduate Assistant, Botany and Plant Pathology Department, University of Maine, Orono 04469. S. S. Leach, Research Plant Pathologist, USDA, ARS, Orono, ME 04469. Plant Dis. 71:433-437. Accepted for publication 10 November 1986. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1987. DOI: 10.1094/PD-71-0433.

Sweet corn (Zea mays), Japanese millet (Echinocloa crusagalli), buckwheat (Fagopyrum esculentum), spring oat (Avena sativa), and annual ryegrass (Lolium multiflorum) were used as 1-yr rotation crops with potato (Solanum tuberosum) to evaluate their effectiveness in controlling the Rhizoctonia disease of potato (R. solani). The effects of incorporating the rotation crop residues as green, immature amendments versus mature, partially decomposed amendments were also examined. Populations of Rhizoctonia spp. were usually significantly higher in buckwheat-rotated soils, but the resulting disease severity level was not higher. Buckwheat may have selected for species or strains of Rhizoctonia that were nonpathogenic or less pathogenic on potato. Annual ryegrass gave the lowest overall disease severity, but this was only significantly lower than that for Japanese millet (P = 0.05). The effect of tillage regime on disease severity was not significant, except for Japanese millet; in this instance, the early-tilled soils had lower levels. Increased populations of bacteria and fungi, but not actinomycetes, were found in the early-tilled plots immediately after incorporation of the green, immature amendments. Except for the Japanese millet rotation, this increase in soil microorganism population was not associated with a suppression of the pathogen or a reduction in disease severity.