Management strategies for Alternaria leaf blight on American ginseng

Cultivated American ginseng (Panax quinquifolius) is a perennial herb valued as a traditional Chinese medicine. The crop is commercially produced under polypropylene shade cloth, creating a climate favorable for leaf blight caused by Alternaria panax; frequent fungicide applications are required annually to protect yields of the root and seed. Our objective was to improve Alternaria leaf blight tactic by identifying effective products and application intervals using a calendar or the TOM-CAST disease forecaster. Product efficacy trials were conducted at two sites over two years and included 14 products and an untreated control. The fungicides pyraclostrobin and azoxystrobin + difenoconazole were consistently effective in limiting leaf blight compared to the untreated control. Fluxapyroxad + pyraclostrobin was also effective both years across both sites with one exception. Mancozeb, fluazinam, chlorothalonil, difenoconazole, and penthiopyrad were effective when disease pressure was not severe. Applications of pyraclostrobin or mancozeb resulted in a higher seedhead yield than the untreated control. The TOM-CAST disease forecaster, with spray thresholds of 10 or 15 disease severity values (DSVs) was tested at four field sites for each of two years and compared to fungicides applied every 7 or 10 days or not applied (untreated control). All treatments at three of the four sites limited disease severity compared to the untreated control in both years. According to the area under the disease progress curve (AUDPC) data, fungicide treatment programs in 2013 were similar and better than the untreated control. The AUDPC data from 2014 indicated that the 7-day application treatments had fewer diseased plants than the untreated control at all sites; the TOM-CAST 10 and 15 DSV treatments limited disease compared to the untreated control at three of the four sites. The TOM-CAST 10 DSV treatment yielded similarly to the 10-day treatment at one site for both fresh and dried root; the TOM-CAST 10 and 15 DSV treatment yields were similar. At a second site, fresh and dried root yields were similar for the 7-day and TOM-CAST 10 DSV treatments; 10-day and TOM-CAST 15 DSV treatments were similar. TOM-CAST 10 DSV may provide a balance between reducing fungicide applications while protecting valuable ginseng root yields.