APS Abstracts of Presentations

Detection of Phytophthora cactorum on ginseng seed and screening pathogen isolates for resistance to mefenoxam
Presenter: S. Hill, Dept. of Plant Pathology, Michigan State University, East Lansing, MI 48824
Co-Author(s): M. Hausbeck, Dept. of Plant Pathology, Michigan State University, East Lansing, MI 48824; J. Woodworth, Dept. of Plant Pathology, Michigan State University, East Lansing, MI 48824; B. Harlan, Dept. of Plant Pathology, Michigan State University, East Lansing, MI 48824
Phytopathology 95:S42

Stratified ginseng seeds from commercial ginseng gardens were planted in sterilized silica sand in a research greenhouse at Michigan State University. Following seedling emergence, wilting, damping off and black stem lesions were observed. Phytophthora cactorum was isolated from 56 affected seedlings and Koch's Postulates were completed. All P. cactorum isolates were screened for sensitivity to the fungicide mefenoxam by measuring radial mycelial growth on 100 ppm mefenoxam-amended V8 agar. All isolates were resistant to mefenoxam. A replicated trial was conducted twice in the greenhouse to evaluate registered and unregistered fungicides including a reduced risk product and a biopesticide for the control of P. cactorum on ginseng seedlings. Disease pressure was severe with a minimum of 75% of the untreated plants dead by the final observations. Significant differences in plant death were observed among treatments. Mefenoxam, the industry standard, performed poorly likely due to the mefenoxam-insensitive isolate used for inoculation. Drenches of copper hydroxide or captan completely prevented plant death. Drenches of dimethomorph limited plant death to 25%. This is the first report of P. cactorum recovered from ginseng seed in the United States.

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