First Report of Sclerotinia Rot Caused by Sclerotinia sclerotiorum on Artemisia capillaris in China

Artemisia capillaris (Asteraceae) is an annual herb found in ˃10 provinces in China. It is cultivated on ˃670 ha, with annual production around 2,500 tons. Its shoot is used in traditional Chinese medicine (Liu et al. 2021). From April to May 2023, Sclerotinia rot symptoms were seen at the Institute of Medicinal Plant Development (40.04°N, 116.28°E), Beijing, China. Disease incidence was up to 10% in the field through investigation of 300 plants. Initial symptoms were irregular tan-brown lesions (0.5 to 5.0 mm) that expended to circumferential necrosis on the roots and basal stem, aerial mycelia and sclerotia were developed on them. The leaves and stem tips were withered and droopy in severe cases. Twelve symptomatic primary roots of 12 plants from two sites were cut into 5 × 5 mm pieces, surface sterilized with 75% ethanol for 30 s and 5% NaClO for 60 s, rinsed with distilled water for three times, dried with sterile filter paper, put on potato dextrose agar (PDA), and incubated at 25°C in the dark for 2 days. Two Sclerotinia-like isolates were obtained using the hyphaltip method. White aerial mycelia were sparse and appressed for isolate YC1-3 and dense for isolate YC1-7. After incubated at 25°C in the dark for 15 days, 10 to 25 sclerotia were developed near the colony margin. Sclerotia of isolate YC1-3 were 1.0 to 3.9 × 1.2 to 4.5 (mean 1.8 × 2.2) mm (n = 60), ovoid or arc-shaped. Sclerotia of isolate YC1-7 were 1.5 to 3.4 × 2.7 to 9.2 (mean 2.3 × 4.3) mm (n = 60), ovoid, dumbbell shaped or curved. The isolates were identified as Sclerotinia sclerotiorum based on morphology (Maas 1998). To further identify the pathogens, molecular identification was performed with isolates YC1-3 and YC1-7. DNA of the two isolates were extracted by the cetyltrimethylammonium bromide (CTAB) method. Polymerase chain reaction was performed with primers ITS1/ITS4 for the internal transcribed spacer (ITS) region (Choi et al. 2020; White et al. 1990) and primers G3PDHfor/G3PDHrev for the glyceraldehyde 3-phosphate dehydrogenase (G3PDH) gene (Garfinkel. 2021). BLAST search analysis revealed that the ITS sequence (GenBank OR229758 and OR229762) was ≥99% similar to S. sclerotiorum (MN099281, MZ379265, KX781301, etc.), and the G3PDH sequence (OR778388 and OR761975) was too (MZ493894, JQ036048, OQ790148, etc.). Phylogenetic trees were computed with ITS and G3PDH sequences using the Maximum Likelihood in MEGA 11. Nine two-month-old seedlings of A. capillaris were used to test pathogenicity. The epidermis layer of each primary root was slightly wounded (2 × 2 mm, 1 mm deep) using a sterile dissecting blade. Three plants were inoculated with mycelial plugs (5 mm in diameter) of YC1-3 and YC1-7 that cultured on PDA for 7 days. Control plants were inoculated with sterile PDA plugs. All seedlings were then incubated at 25oC and 90% relative humidity. After isolate YC1-7 inoculation 3 days and isolate YC1-3 inoculation 5 days, inoculated roots had symptoms like those in the field, controls had no symptoms. S. sclerotiorum was consistently re-isolated from diseased roots, fulfilling Koch's postulates. Diseases caused by S. sclerotiorum have been reported threatens several important economical crops (Marin and Peres 2020; Guan et al. 2022). To our knowledge, this is the first report of S. sclerotiorum causes Sclerotinia rot on A. capillaris. To avoid of significant economic losses, it is urgent to establish an effective disease-management strategy.