ZnO Nanoparticles Enhance the Biocontrol Efficacy of Bacillus siamensis YZUS007 Against Sclerotinia Stem Rot of Brassica napus

Sclerotinia sclerotiorum causes destructive sclerotinia stem rot and severe yield losses in global rapeseed production. The Bacillus biocontrol strain YZUS007 serves as an eco-friendly alternative to synthetic fungicides, yet its control efficacy fluctuates under complex field conditions. This study aimed to investigate whether low-dose ZnO NPs could enhance the biocontrol efficacy of YZUS007 and to elucidate the underlying mechanisms. We isolated and identified the antagonistic strain Bacillus siamensis YZUS007 from the rhizosphere soil of healthy Brassica napus, which exhibited an in vitro pathogen inhibition rate of 78.4 ± 0.7%. Pot tests showed that YZUS007 (BC) achieved 60.79% control efficacy, whereas the YZUS007 + 20 μg/mL ZnO NPs group (BZ) reached 74.68%. Cell-free filtrate of YZUS007 (EC) had a 50.44% control efficacy, and filtrate with ZnO NPs (EZ) increased efficacy to 58.08%. LC-MS/MS identified four extracellular metabolites with increased relative abundances: cinnamic acid, sesamol, patchouli alcohol and 2-phenylbutyric acid. The combined treatment boosted defense enzyme activities and induced higher expression of the defense-related genes RBOH, PR1 and WRKY33. Collectively, low-concentration ZnO NPs significantly enhanced the biocontrol performance of YZUS007 by promoting antifungal metabolite accumulation, providing a low-dose, nanomaterial-assisted strategy for sustainable sclerotinia rot management.