Virulence and Fungicide Sensitivity of Phytophthora cactorum Isolated from American Ginseng Gardens in Wisconsin and Michigan.

Phytophthora cactorum causes foliar blight and root rot on cultivated ginseng (Panax quinquefolium). Commercial ginseng gardens in Wisconsin and Michigan and research gardens at Michigan State University were sampled and P. cactorum isolated. In all, 114 and 96 P. cactorum isolates were recovered in 2003 and 2004, respectively. The majority (82%) of the isolates screened (210 in total) were resistant to the fungicide mefenoxam. P. cactorum isolates were screened for pathogenicity using 'McIntosh' apple fruit (206 isolates) and ginseng seedlings (38 isolates). Most isolates (98%) produced necrotic lesions on apple fruit and nearly all (97%) isolates resulted in 40 to 100% plant death. Apple fruit and ginseng seedlings were also used to test fungicide efficacy against P. cactorum. Mefenoxam, the industry standard, was ineffective in both the apple fruit and seedling studies when using a mefenoxam-resistant P. cactorum isolate. Only copper hydroxide provided consistent control against P. cactorum when tested on apple fruit and ginseng seedlings.