Biocontrol of Fungal Pathogens of Winter Oilseed Rape (Brassica napus L.) Using Plant Extracts (Allium sativum, Helianthus tuberosus) and Bacterial Fermentation Supernatants (Paenibacillus, Enterobacter)

Winter oilseed rape (Brassica napus L.) production is constrained by a complex of fungal pathogens, including Alternaria alternata, A. brassicicola, Botrytis cinerea, Phoma lingam and Sclerotinia sclerotiorum, which are conventionally managed with synthetic fungicides such as prothioconazole. This study evaluated the biocontrol and biostimulant potential of two plant extracts (garlic, Allium sativum; Jerusalem artichoke, Helianthus tuberosus) and two bacterial fermentation supernatants (Enterobacter sp. and Paenibacillus sp.) against these pathogens under in vitro, greenhouse and field conditions (2022–2024, two locations: Winna Góra and Rzeszów, Poland). In vitro, the Paenibacillus sp. supernatant showed the broadest and strongest activity, with inhibition of mycelial growth comparable to prothioconazole for several pathogens, particularly A. brassicicola and B. cinerea. Garlic extract was most effective against S. sclerotiorum and B. cinerea, with efficacy increasing with concentration and reaching complete inhibition at 10%; because each concentration was analysed in a separate one-way model , this concentration trend is reported descriptively and was not formally tested as a product × concentration interaction. Jerusalem artichoke extract was consistently the most effective agent against P. lingam (88–92% inhibition, largely independent of concentration), while Enterobacter sp. showed a comparatively narrow spectrum, with no measurable activity against B. cinerea and only negligible activity against S. sclerotiorum (≤ 6% inhibition at all tested concentrations). In the greenhouse, all four bioproducts significantly stimulated seedling growth, with fresh shoot weight increases exceeding 100% relative to the untreated control; fresh root weight showed a similar but non-significant trend. Under field conditions, garlic extract and Paenibacillus sp. gave the most consistent statistically significant and, in several site-years, fungicide-comparable control of Phoma stem canker (Phoma lingam) and Alternaria black spot (Alternaria spp.), occasionally matching the fungicide Protikon 250 EC; on the other hand, Enterobacter sp. was more variable and Jerusalem artichoke extract intermediate. Seed yield was increased over the untreated control at Rzeszów in both seasons, with the exception of Enterobacter sp. in 2023/2024, but not at Winna Góra, and thousand-seed weight was unaffected by treatment.