Stringy stonecrop (Sedum sarmentosum) belongs to the family Crassulaceae. It is a succulent plant with characteristics of creeping and ascending stems. The plant is native to China, Japan, Korea, Manchuria, Thailand, and Vietnam (Plants of the World Online, 2024), and has been cultivated as a wild vegetable in Korea. We observed unusual symptoms of Sclerotinia rot in stringy stonecrop plants cultivated in vinyl greenhouses in Icheon, Korea, during a disease survey conducted in March 2024. The symptoms were characterized as soft rot of stems and leaves at or above the soil line in the early stage (Fig. 1A). In the late stage, most of the diseased plants collapsed and rotted with white to grayish brown mycelia (Fig. 1B). Occasionally, black, spherical or irregular, small sclerotia formed on the stems and leaves of the diseased plants. We observed 100 plants at each of three sites in a vinyl greenhouse to investigate the incidence of the disease. The disease occurred in 1‒3% of the plants in three out of 20 surveyed vinyl greenhouses.

We collected diseased plants of stringy stonecrop from surveyed vinyl greenhouses. Fungal isolates were obtained from lesion pieces of the diseased plants using the method described in a previous study (Kim et al., 2022). We obtained three isolates (SLSC-2401, SLSC-2402, and SLSC-2403) of Sclerotinia sp. from the diseased stems. The isolates were transferred to potato dextrose agar (PDA) in 9 cm diameter petri dishes and incubated at 22^°C in the dark for 15 days. Colony morphology of the isolates on PDA displayed white to gray mycelia and spherical or irregular black sclerotia (Fig. 2). The isolates formed a large number of sclerotia which ranged from 800 to 1,000 per PDA dish. The sclerotia measured 0.4‒2.0 mm in diameter. The morphological characteristics of the isolates were consistent with those of Sclerotinia minor as reported in previous studies (Jagger, 1920; Kohn, 1979; Willetts and Wong, 1980).

A phylogenetic analysis was conducted using polymerase chain reaction (PCR) works to confirm the morphological identification of the Sclerotinia sp. isolates. Internal transcribed spacer (ITS) region of rDNA from the isolates was amplified using the DNA Free-Multiplex Master Mix (Cellsafe, Yongin, Korea) and the ITS1/ITS4 primer pair according to the previous study (White et al., 1990). The alignment of ITS sequences for the isolates and related Sclerotinia spp. from the National Center for Biotechnology Information (NCBI) GenBank was conducted and improved as if necessary, using MEGA version 7 software (Kumar et al., 2016). A neighbor-joining tree was generated with maximum composite likelihood model performing 1,000 bootstrap replicates with MEGA version 7 (Kumar et al., 2016). Clarireedia homoeocarpa MAFF 235854 was set as the outgroup taxon. The phylogenetic analysis revealed that the isolates from stringy stonecrop formed a single cluster with reference strains (DAOM 191806 and CBS 339.39) of S. minor (Fig. 3). The ITS sequence data for the S. minor isolates have been deposited in the NCBI GenBank with the accession numbers PP854567- PP854569.

The three isolates of S. minor from stringy stonecrop were tested for their pathogenicity to the host plant through artificial inoculation. For the inoculation test, each isolate was cultured on PDA at 22°C for 3 days. The 6 mm-mycelial disks were cut from the margins of actively growing PDA cultures of each isolate and placed on the stems at the soil surface level of 2-month-old stringy stonecrop plants grown in circular plastic pots (height 15 cm; upper diameter 17 cm; lower diameter 10 cm) in a vinyl greenhouse. The inoculated and control plant pots were moved into plastic boxes (60×43×33 cm) under 100% relative humidity in a cultivation room at 22°C. After three days, the pots were moved out of the plastic boxes and kept indoors at 20-22°C. The inoculation test was repeated in triplicate. Pathogenicity of the isolates was investigated based on formation of rot symptoms 5 days after inoculation. The tested isolates caused rot symptoms in the inoculated plants (Fig. 1C), but no symptoms occurred in the control plants (Fig. 1D). The symptoms on the plants induced by the inoculation tests were similar to those observed in the surveyed vinyl greenhouses. Re-isolation of the tested isolates from the induced symptoms was conducted and confirmed by comparing the morphological characteristics of the isolates.

S. minor, like S. sclerotiorum, causes Sclerotinia rot in various crops (Kim and Cho, 2002, 2003a, 2003b). It has been reported that S. nivalis causes white mold disease (Fan et al., 2012) in stringy stonecrop, and S. sclerotiorum causes Sclerotinia rot in the plant (Kim et al., 2022). However, there have been no reports of S. minor causing Sclerotinia rot in stringy stonecrop. This study reveals that S. minor is one of the causal fungi of Sclerotinia rot in stringy stonecrop.