Do Morphological Variations in Sclerotinia sclerotiorum Strains Indicate Differences in Aggressiveness?

White mold (Sclerotinia sclerotiorum de Bary) is one of the most important fungal diseases of winter oilseed rape (OSR). Since the pathogen can persist in the soil for a long time with its sclerotia, prevention and non-chemical methods (specifically biological agents) are important pillars in the integrated plant protection strategy against this pathogen. Mapping the intraspecific variability of the pathogen is an important step in the development of resistance to S. sclerotiorum. S. sclerotiorum isolates were collected from different OSR growing locations in Hungary during the 2020/21 and 2021/22 growing seasons. The morphological characteristics of sclerotia obtained from infected OSR stems were studied in the laboratory, and seedlings of four OSR hybrids were infected in vitro with isolates. The strains from four locations have different morphological characteristics. Significant differences in the level of aggressivity were also observed between strains; a correlation was also found between mycelial growth after 24 h, weight of sclerotia, and aggressivity. Among the four tested hybrids, OSR PT271 proved to be the most susceptible to most S. sclerotinia strains.