Disease Dynamics of Sclerotinia sclerotiorum on Stevia in North Carolina

Stevia (Stevia rebaudiana) is a perennial plant native to Paraguay that is currently emerging as a new crop in the United States. Stevia leaves contain multiple glycosides extracted for use as a natural, low-calorie sweetener (Gutierrez and Shew 1998). Production of stevia in North Carolina began in 2011. In current U.S. production practices, stevia seeds are sown in a greenhouse float-tray system, seedlings are transplanted to the field eight to 10 weeks after seeding, and plants are harvested once during the first year of growth. Stevia harvest typically occurs before flowering in early September. At frost, all aboveground parts of the plant die, while roots and crowns remain alive over the winter. New shoots emerge in March to April, with two harvests per season in the second and third years of production. In 2013, white mold caused by Sclerotinia sclerotiorum was observed on young second-year plants (Kinghorn 2003). Symptoms included wilting, chlorotic leaves, necrotic leaves, bleached stem lesions, and dead stems. Symptomatic stems often had tufts of white hyphae and large, irregularly shaped, black sclerotia present on the base of the stem (Fig. 1). Spore trapping was conducted from 2014 to 2016 using a semiselective pH-indicator medium to detect and quantify ascospores in stevia fields and help determine if ascospores serve as primary inoculum for the disease (Koehler and Shew 2014). In October to November 2014, Petri dishes of the semiselective medium were set out weekly across a field of first-year plants in Kinston, NC, and very few ascospores were trapped. In spring 2015, trapping in the same field detected ascospores through the entire month of March, often with multiple spores caught per plate after a 1-h exposure (Fig. 2). A frost event on 28March damagedmany of the newly emerging second-year stems. No ascospores were trapped in April and first symptoms of S. sclerotiorum were observed on 23 April (Fig. 3). Individual stems within the cluster of stems from each crown were