First report of <i>Sclerotinia nivalis</i> causing stem rot on winter oilseed rape ( <i>Brassica napus</i> ) in Russia

In May 2022, stem rot symptoms were observed on winter oilseed rape (Brassica napus) hybrid F1 Mercedes in Dzerzhinsky District, Kaluga Oblast, Russia (54°45′N 35°51′E). Lesions developed as brown discoloration in leaf axils, followed by white mycelium and small, dark, irregular sclerotia along lesion margins; disease incidence was approximately 5%. In April 2025, similar symptoms occurred on the same hybrid in Chistopol District, Republic of Tatarstan, Russia (55°17′N 50°31′E). These symptoms differed from typical Sclerotinia sclerotiorum stem rot by smaller sclerotia, slower colony growth in culture, and predominantly basal lesions observed in early spring soon after snowmelt. No apothecia were observed at the affected sites during the surveys. The basal position of lesions and the presence of mycelium and sclerotia at lesion margins suggest that soilborne sclerotia may have contributed to infection, whereas the role of ascospores under field conditions remains unknown. Affected stem and leaf base fragments (2 cm) were incubated in a moist chamber for 5 days at 25°C. Emerging mycelium was transferred to potato dextrose agar (PDA) supplemented with antibiotics (streptomycin sulfate and chloramphenicol, each at 100 mg/L). Colonies were white and loosely textured after 2-3 days and produced small, dark sclerotia after 7-9 days (Wang et al. 2016). Two isolates, SnR1 (Kaluga Oblast) and PL3 (Republic of Tatarstan), were obtained by subculturing colony margins. Both isolates grew slowly, with loosely organized mycelium, and formed predominantly peripheral sclerotia 1-3 mm in diameter, corresponding to the morphological description of S. nivalis (Saito, 1997). Genomic DNA was extracted using Fitosorb kit. The internal transcribed spacer (ITS), glyceraldehyde-3-phosphate dehydrogenase (G3PDH), and β-tubulin loci were amplified and sequenced using previously described primers (White et al. 1990; Staats et al. 2005). The sequences were deposited in GenBank (SnR1: PX832511, PX977137, PX977138; PL3: PX664504, PX960861, PX977136). BLASTn searches showed 100% identity to multiple S. nivalis isolates, including strains with available genome assemblies. A maximum-likelihood phylogeny of concatenated ITS, G3PDH, and β-tubulin sequences placed SnR1 and PL3 in a well-supported S. nivalis clade, distinct from other Sclerotinia spp. (1000 bootstrap replicates). Pathogenicity was tested on 20 healthy winter oilseed rape hybrid F1 Mercedes plants (10 per strain) grown for 45 days in a greenhouse under standard plant-growth conditions (25°C, relative humidity 80%, 12-h photoperiod). Leaves were inoculated with 5-mm mycelial plugs from 7-day-old cultures; control plants received sterile PDA plugs. The experiment was repeated three times. At 5 days post-inoculation, inoculated leaves developed necrotic lesions 1.2-1.6 cm in diameter with light surface mycelium, whereas control plants remained symptomless. The pathogen was re-isolated from symptomatic tissues, and the identity of representative re-isolates was confirmed by sequencing of the ITS, G3PDH, and β-tubulin loci, fulfilling Koch’s postulates. Detection of S. nivalis on winter oilseed rape is important because this species is adapted to cold, wet conditions and can grow at 0-5°C (Medvedeva et al. 2025), allowing infection to develop beneath snow cover during winter. In the surveyed fields, S. sclerotiorum remained the predominant cause of stem rot (20% incidence), whereas S. nivalis was less frequent (5%) but locally damaging in early spring. This finding indicates that S. nivalis may represent an overlooked risk for winter oilseed rape in regions with prolonged snow cover. To our knowledge, this is the first report of S. nivalis causing disease on B. napus in Russia.