Enhanced Overwintering Survival of Stevia by QoI Fungicides Used for Management of Sclerotium rolfsii

Stevia (Stevia rebaudiana) is a herbaceous perennial under evaluation as a new crop in the southeastern United States. Stem rot caused by Sclerotium rolfsii is common in stevia plantings in North Carolina, with symptoms including wilting, root and stem necrosis, and plant death. Fungicide efficacy trials for management of S. rolfsii were conducted over 2 years. Fungicides evaluated included azoxystrobin, flutolanil, and tebuconazole applied at three timings. Azoxystrobin applied to transplants 1 week prior to planting had the lowest area under the disease progress curve values across all trials. Fungicide plots were also used to evaluate overwintering of stevia. End-of-season stand counts were compared with spring emergence counts to quantify overwintering survival. In spring 2015, plots treated with azoxystrobin in 2014 had greater overwintering survival (78%) than other fungicide treatments (38%) and the control (38%). Similar results were obtained at two locations in spring 2016 in plots treated with azoxystrobin or pyraclostrobin in 2015. Successful overwintering of stevia directly impacts the profitability of second- and third-year harvests and enhances the likelihood of long-term establishment of stevia as a viable crop. Future studies will be directed at elucidating the mechanism of the enhanced overwintering survival of plants treated with quinone outside inhibitor fungicides.