1977) FRANK AND MURPHY: EFFECT OF CROP ROTATIONS 315 THE EFFECT OF CROP ROTATIONS ON RHIZOCTONIA DISEASE OF POTATOES 1

Since 1964, replicated rotation plots in Presque Isle, Maine have continually been planted and evaluated to determine the effect of various rotation crops on tuber appearance, quality, and yield. In 1974 and 1975, the saprophytic activity ofRhizoctonia solani in the soils of each rotation combination was determined, and the tubers harvested were scored for Rhizoctonia damage. Soils with a two-year rotation of oat-potato had the lowest amount of saprophytic activity by Rhizoctonia and provided the lowest incidence of disease on the stems, roots, and tubers of the cultivars Kennebec and Katahdin. With the cultivar Russet Burbank, the three-year rotations generally provided the lowest incidence of disease; however, this cultivar is more susceptible to the pathogen. The plots of potatoes that were not rotated had the highest pathogen activity and the greatest disease incidence.