First report of sclerotinia stem rot caused by Sclerotinia sclerotiorum on Chinese chives in Korea

Chinese chives (Allium tuberosum Roxb.) is the fourth most important allium crop in Korea after bulb onion (A. cepa), Welsh onion (A. fistulosum), and garlic (A. sativum). During winter of 2014-15, symptoms similar to Sclerotinia stem rot on other Allium hosts were observed in commercial crops of Chinese chives cv. Green Belt in polyethylene tunnels in Iksan, Korea (35°56′37″N; 126°59′33″E). Disease incidence ranged between 10 and 20%. White mycelial mats containing sclerotia of 2 to 8 mm in diameter were observed on dead plants and the nearby soil surface. Symptomatic tissue showing fungal growth were surface-disinfested in 1% sodium hypochlorite for 1 min and placed on water agar plates. Hyphal tips growing out of the tissue were subsequently transferred onto potato dextrose agar (PDA). Pure cultures consistently yielded abundant white mycelia. Two-week-old cultures on PDA at 16 to 20°C developed sclerotia on the peripheral edges of the plate. The pathogen was morphologically identified as Sclerotinia sclerotiorum (Lib.) de Bary (Mordue and Holliday 1976). A voucher specimen was deposited in the Korea University Herbarium (KUS-F28606). A representative isolate was deposited in the Korean Agricultural Culture Collection (KACC47723). Genomic DNA was extracted from the mycelia of S. sclerotiorum using a DNeasy Plant Mini DNA Extraction Kit (Qiagen Inc., Valencia, CA). The internal transcribed spacer (ITS) region of rDNA was amplified with the primers ITS1/ITS4 and sequenced using an ABI Prism 337 automatic DNA sequencer (Applied Biosystems, Foster, CA). The resulting sequence of 558 bp was deposited in GenBank (accession no. KJ614564). A BLASTn search revealed that sequence of the Korean isolate showed 100% identity with those of S. sclerotiorum (DQ329537, KF859932, KF859933, JN013184, etc.). Pathogenicity was confirmed by inoculating agar segments (9 mm²) from a 7-day-old culture grown on PDA onto the lower stems of three healthy Chinese chives cv. Green Belt near the soil line and wrapping with moistened sterilized cotton. Noncolonized PDA plugs were used as controls. Plants were covered individually with plastic bags and kept in the greenhouse at 15 to 20°C and relative humidity >90%. After 3 days, all inoculated stems became discolored, soft, watery, and covered with white mycelia. S. sclerotiorum was consistently reisolated from the symptomatic tissue, fulfilling Koch’s postulates. No symptoms were observed on control plants. Pathogenicity test was performed twice with the same results. To our knowledge, this is the first report of Sclerotinia stem rot on Chinese chives globally as well as in Korea (Farr and Rossman 2017). Our field observations suggest that low temperature (5°C at night), high humidity (RH 90 to 100% at night), poor ventilation, and continuous cropping in nonheated polyethylene tunnels accelerate the incidence of Sclerotinia stem rot on Chinese chives in winter. This crop is mostly grown with limited chemical control options and a good agricultural practices (GAP) model for producing eco-friendly Chinese chives is under development (Park et al. 2015).