Stevia (Stevia rebaundia) is an emerging crop in the United States. Once established, the crop is grown for 3 to 5 years and is typically harvested twice per growing season. Stevia leaves contain multiple glycosides that are used as a natural noncaloric sweetener that was approved by the USDA in 2008 as a sugar substitute. In commercial plantings of Stevia in North Carolina, wilting and death of plants in first- and second-year plantings were observed in 2012 and 2013. Diseased plants were observed in multiple counties in the state, with first symptoms observed in May of each year and continuing through the summer months. Prior to Stevia, these fields had been planted primarily in a corn-soybean rotation. Symptoms began as moderate to severe wilting of young shoots and chlorosis of leaves, rapidly followed by death of stems and rotting of roots. White mycelial growth was frequently observed at the base of stem tissue. Theses characteristic hyphae of Sclerotium rolfsii were often accompanied by the presence of abundant white to brown sclerotia. Isolations from infected root and stem tissue were made on potato dextrose agar amended with 50 μg/ml of streptomycin sulfate and penicillin G. Isolations from diseased tissue yielded characteristic white hyphae of S. rolfsii (1,3). Numerous sclerotia 0.5 to 2 mm in diameter developed following 4 to 7 days of mycelial growth. Sclerotia were initially white and melanized turning brown with age. To verify pathogenicity, 10-week-old Stevia seedlings were transplanted in 10-cm diameter pots containing sterile 1:1:1 sand, loam, media mix. Inoculum consisted of oat grains infested with one isolate obtained from the field plants. Oats were sterilized on three consecutive days and then inoculated with colonized agar plugs of S. rolfsii. Oats were incubated at room temperature to allow the fungus to thoroughly colonize the oats. Three infested oat grains were added to each test pot and plants were then observed over a 3-week period. Symptoms were observed within 5 days on most plants and included chlorotic leaves, bleached stems, wilting, and necrotic roots. White mycelium and abundant sclerotia were found at the base of plants. Uninoculated plants did not develop any symptoms. This is the first report of S. rolfsii on Stevia in the United States. Kamalakannan et al. (2) reported a root rot disease of Stevia in India and confirmed S. rolfsii as the causal agent.
References: (1) R. Aycock. N.C. Agr. Exp. St. Tech. Bull. No. 174, 1966. (2) A. Kamalakannan et al. Plant Pathol. 56:350, 2007. (3) J. E. M. Mordue. Corticium rolfsii. CMI Descriptions of Pathogenic Fungi and Bacteria No. 410. CAB International, Wallingford, UK, 1974.
