Leaf blight, caused by Alternaria panax, is a yearly challenge for those who cultivate ginseng, requiring frequent fungicide applications to protect valuable yield. In this study, the incidence and pathogenicity of Alternaria spp. on American ginseng in Wisconsin and effective leaf blight-controlling strategies were assessed. Five hundred and ninety-two isolates of Alternaria spp. were obtained from leaves (491), drupes (88) and seeds (10) of ginseng between 2011 and 2015. Resistance to azoxystrobin was determined using an amended agar plate assay and a molecular assay that detected a common mutation responsible for strobilurin resistance in fungi. The majority (74.5%) of the isolates were identified as small-spored Alternaria spp. and their pathogenicity was confirmed using a detached leaf assay. Wounding increased the occurrence of infection by small-spored Alternaria spp. although infection occasionally (<6%) occurred without wounding. Alternaria panax was consistently pathogenic; lesion area resulting from A. panax and small-spored Alternaria spp. infection ranged from 4.34 to 28.88 mm2 and from 0.04 to 27.72 mm2, respectively. Results from the plate assay show 65.2% of A. panax isolates and 82.6% of small-spored Alternaria spp. isolates were classified as resistant to azoxystrobin at an EC50 of greater than 4.00. Leaf blight-control strategy field trials were conducted in commercial ginseng gardens over two years. One trial compared fourteen fungicides (boscalid, chlorothalonil, cyprodinil+fludioxonil, famoxadone+cymoxanil, penthiopyrad, fluazinam, pyraclostrobin, difenoconazole+azoxystrobin, pyrimethanil, fluxapyroxad+pyraclostrobin, difenoconazole, extract of giant knotweed, mancozeb, and azoxystrobin) and a non-treated control. In a second trial, a disease forecaster program, TOM-CAST, with spray thresholds of 10 or 15 disease severity values (DSVs) was tested. Penthiopyrad, difenoconazole, fluazinam, difenoconazole+azoxystrobin and pyraclostrobin limited disease incidence compared to the control (P < 0.05). Applications of pyraclostrobin and mancozeb resulted in a higher seedhead yield than the control, the highest number of healthy drupes and the lowest number of diseased drupes (P < 0.05). In the forecasting trial, the disease severity rating of TOM-CAST 10 DSV plots was similar to those treated every 10 days, but higher than for plots treated every 7 days. The TOM-CAST 15 DSV plots were severely diseased. Yields were similar among plots treated every 7 days and according to TOM-CAST 10 DSV.The establishment costs of a ginseng planting are high and seed health is an important consideration. Because of this, fungal presence on drupes, green seed coats, and endosperms was surveyed using a plate assay. A field trial to assess seed treatments in commercial ginseng gardens included stratified seed treated with the fungicides penthiopyrad, oxathiapiprolin, fluopicolide, ethaboxam, captan, mefenoxam + fludioxonil, or azoxystrobin. Alternaria spp. were commonly isolated from drupes and green seed coat and Fusarium spp. were recovered from all seed parts, including the endosperm. Cylindrocarpon destructans was only recovered in low numbers on green seed coats. Although none of the treatments significantly increased final plant stand compared to the control, oxathiapiprolin and azoxystrobin resulted in the highest plant stands throughout the season in year one (P < 0.05). In year two the control and fluopicolide resulted in the highest final plant stands (P < 0.05).
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