First report of sclerotinia stem rot of marsh mallow caused by sclerotinia Sclerotiorum in Korea

Marsh mallow (Althaea officinalis L.) belongs to family Malvaceae and is used both as a medicinal and ornamental plant (Shah et al. 2011). During winter 2013 and 2014, marsh mallow grown in a plastic greenhouse in Gongju (36°29′42.2″ N; 127°02′00.8″ E), Korea, exhibited typical signs and symptoms of Sclerotinia stem rot. The lower stems near soil level developed a brown rot. Leaves on rotted stems became chlorotic and wilted. Dark brown stem lesions enlarged and white cottony mycelial mats covered the affected area, followed by crown rot and wilt a few days afterward. About 10% of plants died before harvest due to the disease. Whitish aggregates of mycelia developed into sclerotia that were 2 to 8 mm in diameter outside and inside affected stems. Stem tissues were surface-disinfested with 1% sodium hypochlorite, and segments were transferred onto potato dextrose agar (PDA). Resultant colonies were white or light gray and floccose, with black sclerotia (5 to 10 mm in diameter) on the surface near colony margin. A representative isolate was deposited in the Korean Agricultural Culture Collection (Accession No. KACC47725) and used for pathogenicity tests and molecular analysis. Based on the morphology and culture characteristics, the fungus was identified as Sclerotinia sclerotiorum (Lib.) de Bary (Mordue and Holliday 1976). Fungal DNA was extracted with a DNeasy Plant Mini Kit (Qiagen Inc., Valencia, CA). The internal transcribed spacer region of rDNA was amplified using primers ITS1/ITS4 and sequenced. The resulting 558-bp sequence was deposited in GenBank (Accession No. KJ614566). A BLASTn search revealed that sequences of the Korean isolates shared 100% identity to several isolates of S. sclerotiorum (e.g., KJ614564, JN013184, KF859932, and DQ329537). Pathogenicity tests were conducted by placing PDA plugs (9 mm²) from a 7-day-old culture on the stems of three healthy plants at the soil line. Three plants inoculated with noncolonized PDA plugs served as controls. Plants were enclosed in plastic bags that were sprayed with water on the inside to maintain high humidity and kept in a greenhouse at 16 to 20°C. After 3 days, all inoculated stems became discolored, soft, watery, and covered with white mycelia, whereas control plants remained symptomless. Sclerotinia sclerotiorum was consistently reisolated from the symptomatic tissue, fulfilling Koch’s postulates. Pathogenicity tests were repeated twice with similar results. Sclerotinia stem rot of A. officinalis has never been recorded globally, though crown rot of A. rosea caused by S. sclerotiorum has been reported in the North America (Farr and Rossman 2015). To our knowledge, this is the first report of Sclerotinia stem rot on A. officinalis in Korea. Our continuous observations during winter season (December, January, and February) suggest that low temperature at night, high humidity, poor ventilation, and continuous cultivation in nonheated plastic greenhouse cultivation systems can increase the incidence and severity of Sclerotinia stem rot on marsh mallow plants.