First Report of Sclerotinia Rot Caused by Sclerotinia sclerotiorum on Brassica juncea var. multiceps in China

Brassica juncea var. multiceps Tsen et Lee is a vegetable crop widely cultivated across China (Zhou et al., 2018). In February 2025, a severe outbreak of Sclerotinia rot was observed on this variety in Rongjiang County, Guizhou Province, ecoChina (25°58'57"N, 108°32'33"E). In an approximately 0.67 ha fields, the disease incidence was more than 70%, resulting in significant economic losses. Initial symptoms appeared as water-soaked brown lesions, which expanded into soft rot-like necrosis on roots and stems, accompanied by the development of aerial mycelia and sclerotia on the lesions. In severe cases, leaves and stems wilted and drooped. To identify the causal agent, symptomatic stems were collected from twelve diseased plants. Tissue segments (5 × 5 mm) were surface sterilized (75% ethanol for 35 s, 2% NaClO for 2 min, followed by three rinses with distilled water) and placed onto potato dextrose agar (PDA). After incubation at 25°C in the dark for 4 days, the resulting cultures were purified, yielding 11 isolates. The isolates produced white, sparse, appressed aerial mycelia. Following further incubation in the dark at 25°C for 15 days, 15 to 32 sclerotia developed near the colony margin. The sclerotia were irregularly ovoid or curved, measuring 1.2–3.0 × 2.0–5.2 mm (mean 1.8 × 3.2 mm; n = 80). Based on morphological characteristics (Maas 1998), the isolates were identified as Sclerotinia sclerotiorum. PCR amplification of isolate DNA was performed using the primer pairs ITS1/ITS4 (White et al., 1990) and G3PDHfor/G3PDHrev (Smith 1989). The sequences of the ITS region and the G3PDH gene from two representative isolates (S2 and S4) were deposited in GenBank under accession numbers PX367226 and PX367227 (ITS), and PX573152 and PX573155 (G3PDH). BLASTn analysis showed that the ITS and G3PDH sequences of isolates S2 and S4 shared >99% nucleotide identity with those of S. sclerotiorum CBS 499.50 (ITS: KF859933; G3PDH: KF878370). Furthermore, a neighbor-joining phylogenetic tree constructed using MEGA 11 based on concatenated ITS and G3PDH sequences confirmed that these isolates belong to S. sclerotiorum. To confirm pathogenicity, Sclerotinia-infested soil was prepared separately using isolates S2 and S4 following a previously described method (Chen et al., 2016). Ten healthy 2-month-old plants were planted into the treatment soil, in which the volume ratio of infected soil (containing S. sclerotiorum inoculum at approximately 1 × 10⁵ CFU/g of slurry) to untreated soil was 1:9, while ten control plants were planted into untreated soil. All plants were sprayed with sterile distilled water and maintained in a greenhouse at 25 ± 2 °C and 75% relative humidity. After 5 days, the inoculated plants exhibited symptoms similar to those observed in the field, including leaf wilting and drooping, whereas control plants remained asymptomatic. Pure cultures were reisolated from diseased stems and confirmed as S. sclerotiorum using the methods described above, thereby fulfilling Koch’s postulates. The experiment was repeated three times. It has been widely reported that sclerotinia rot caused by S. sclerotiorum poses significant threats to economically important crops (Guan et al., 2022; Liu et al., 2024). To the best of our knowledge, this study represents the first report of S. sclerotiorum causing Sclerotinia rot on B. juncea var. multiceps. Identification of the causal agent may provide a foundation for the sustainable management of this disease in the future.