Isolation and Pathogenicity of Phytophthora cactorum from Forest and Ginseng Garden Soils in Wisconsin. T. W. Darmono, Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706. M. L. Owen, and J. L. Parke. Research Specialist, and Associate Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 75:610-612. Accepted for publication 28 November 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0610.

Experiments were conducted to determine if Phytophthora cactorum is present in forest soils in ginseng (Panax quinquefolius) growing areas of Wisconsin and to determine whether the isolates recovered are pathogenic to American ginseng. An apple cotyledon bioassay was used to bait P. cactorum from three of 10 forest soils and three of 10 ginseng garden soils. Twenty-two isolates from infected cotyledons were tested for pathogenicity on 3-wk-old ginseng seedlings. All 10 isolates of P. cactorum from ginseng garden soils were pathogenic to ginseng, but only one of 12 forest soil isolates was pathogenic, and it was only weakly virulent. The remaining 11 forest soil isolates failed to cause disease. Results of pathogenicity tests were unchanged when forest isolates were tested in the soils from which they originated. P. cactorum indigenous to forest soils does not appear to be a significant source of primary inoculum for disease in cultivated ginseng.