Koehler A, Shew H
North Carolina State University.
Plant Dis. 2014 Jul;98(7):1005. doi: 10.1094/PDIS-12-13-1238-PDN.
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.
甜叶菊（Stevia rebaundia）是美国一种新兴作物。一旦定植，这种作物可生长3至5年，每个生长季通常收获两次。甜叶菊叶子含有多种糖苷，用作天然无热量甜味剂，2008年被美国农业部批准作为糖替代品。在北卡罗来纳州的甜叶菊商业种植中，2012年和2013年观察到第一年和第二年种植的植株出现萎蔫和死亡现象。该州多个县都发现了患病植株，每年5月开始出现最初症状，并持续到夏季。在种植甜叶菊之前，这些田地主要种植玉米和大豆，实行轮作。症状开始表现为嫩梢中度至重度萎蔫和叶片黄化，随后茎迅速死亡，根部腐烂。在茎基部组织经常观察到白色菌丝生长。这些立枯丝核菌的特征性菌丝通常伴有大量白色至棕色菌核。从感染的根和茎组织中分离菌株，培养基为添加了50μg/ml硫酸链霉素和青霉素G的马铃薯葡萄糖琼脂。从患病组织中分离出立枯丝核菌的特征性白色菌丝（1,3）。菌丝生长4至7天后形成大量直径0.5至2毫米的菌核。菌核最初为白色，随着老化黑色素化并变为棕色。为了验证致病性，将10周龄的甜叶菊幼苗移植到直径10厘米的花盆中，花盆中装有无菌的1:1:1沙子、壤土、培养基混合物。接种物由感染了从田间植株分离得到的一个菌株的燕麦粒组成。燕麦连续三天进行灭菌，然后接种立枯丝核菌的定殖琼脂块。燕麦在室温下培养，使真菌充分定殖于燕麦。每个试验花盆中添加3个感染燕麦粒，然后在3周内观察植株。大多数植株在5天内出现症状，包括叶片黄化、茎部变白、萎蔫和根部坏死。在植株基部发现白色菌丝和大量菌核。未接种的植株未出现任何症状。这是美国关于立枯丝核菌感染甜叶菊的首次报道。卡马拉坎南等人（2）报道了印度甜叶菊的一种根腐病，并确认定枯丝核菌为病原体。参考文献：（1）R. 艾科克。北卡罗来纳州农业试验站技术通报第174号，1966年。（2）A. 卡马拉坎南等人。植物病理学56:350，2007年。（3）J. E. M. 莫尔杜。立枯丝核菌。英联邦真菌研究所病原真菌和细菌描述第410号。英国沃灵福德国际应用生物科学中心，1974年。
