Brassica napus L., winter oilseed rape (OSR), is a major crop worldwide, with a wide range of uses and high profitability. Viruses, such as turnip yellows virus (TuYV), are becoming increasingly important, and in certain years, they can cause significant infestations in OSR. It is difficult to detect the presence of the virus during visual field inspections, as the symptoms it causes can be confused with either those caused by abiotic factors (e.g., low winter temperature, soil compaction, nutrient deficiencies, etc.) or by other viruses. The objective of this study was to determine the susceptibility of four commercial hybrids of oilseed rape to TuYV in Hungary and to determine the effect of the virus on phenotypic and yield parameters. The results showed that infection with the TuYV can be significant in OSR in some growing seasons. It was found that the appearance of visual symptoms (e.g., anthocyanin leaves) does not always confirm the presence of the virus (based on the ELISA (enzyme-linked immuno assay)), and it does not always detect a negative effect of TuYV on all phenotypic and yield parameters of the tested hybrids based on the results in one growing season.

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.