Sclerotium Rot on Arctium lappa Caused by Sclerotium rolfsii in South Korea

Arctium lappa, commonly called greater burdock, edible burdock, beggar’s buttons, or happy major, is one kind of Eurasian plant species and a member of the Asteraceae family. It is cultivated in gardens for its roots, which are used as a vegetable. In addition, roots and young leaves of edible burdock are popular food supplements in China, Japan, and Korea. In folk medicine, dried burdock roots are exploited for their diuretic, diaphoretic, and blood-purifying properties (†;†). In September 2015, severe Sclerotium rot (∼50% incidence) was observed on edible burdock in Jinju, South Korea. Initial symptoms were water-soaked lesions at the stem base; blight and rots were clearly evident on plant stems near the soil line, resulting in plant withering and death. White mycelial mats with numerous sclerotia were produced on diseased stems near the soil surface. Fungal isolates were cultured on potato dextrose agar (PDA) as described previously (†). Briefly, the sclerotia were collected and disinfected by immersion in 1% sodium hypochlorite for 30 s and in sterilized distilled water for 1 min. They were then placed on PDA plates and incubated at 30°C. Mycelial tips of the fungal isolates grown on PDA were transferred to new PDA plates, which resulted in the formation of numerous globoid sclerotia after 18 days of growth. The optimum temperature for mycelial growth and sclerotia formation on PDA was 30°C and hyphal width was 4 to 8 μm. Sclerotia (1 to 3 mm in diameter) were initially white but gradually turned dark brown. The white mycelium formed a typical clamp connection after 5 days of growth. One of the representative fungal isolates has been deposited with the Korean Agricultural Culture Collection (KACC 48128), National Agrobiodiversity Center, Jeonju, South Korea. To confirm identity of the fungal isolate, the complete internal transcribed spacer (ITS) rDNA region of KACC 48128 was PCR-amplified using ITS1/ITS4 primers (†). The products of the PCR reaction were cloned into the pGEM-T Easy vector (Promega, Madison, WI) and the resulting plasmid was sequenced using universal primers. The ITS rDNA sequence of isolate KACC 48128 (GenBank accession no. KU760984) was homologous (99% identity) with S. rolfsii strains SR001 (HQ420816) and Sc-03 (KX186998), infecting Asiatic dayflower and sweet potato, respectively. Isolate KACC 48128 was then used in pathogenicity tests on potted edible burdock seedlings. Two-month-old healthy seedlings were transplanted individually into pots (10 × 10 × 9 cm) containing autoclaved soil. After 7 days, three seedlings were inoculated with colonized agar discs (6 mm diam.) directly on the base of plants. As the control (noninoculated) treatment, five seedlings of edible burdock were similarly treated with noncolonized PDA discs. The inoculated and noninoculated plants were incubated in a humid growth chamber at 28°C for 24 h and then maintained in a greenhouse. Eight days post inoculation, edible burdock seedlings that had been inoculated with the fungus died, whereas control seedlings were symptomless. Fungus reisolated from artificially inoculated plants had the same morphological features as the original isolates. To our knowledge, this is the first report of S. rolfsii-mediated Sclerotium rot on edible burdock in South Korea. The recent occurrence of the disease in edible burdock suggests that Sclerotium rot is spreading widely and posing serious threats to its production in South Korea. © 2017, American Phytopathological Society. All rights reserved.