Waterlogging and Sclerotinia stem rot are major constraints on rapeseed yield and quality in the rice-rapeseed rotation system of the Yangtze River Basin. This study aims to elucidate the physiological mechanisms by which pydiflumetofen, a novel succinate dehydrogenase inhibitor (SDHI), synergistically controls Sclerotinia stem rot and promotes growth, stress tolerance, yield, and quality in rapeseed (Brassica napus).

Field and laboratory experiments were conducted from 2023 to 2025 in Jingzhou, Hubei Province, using the B. napus cultivar ‘Changshuang 1’. This study investigated the effects of pydiflumetofen, prochloraz, tebuconazole, boscalid, and their mixture (pydiflumetofen + prochloraz) on seedling growth and physiological indices under normal growth, waterlogging and polyethylene glycol (PEG)-simulated drought conditions; endogenous hormone contents and balance at the full flowering stage; and Sclerotinia stem rot resistance, yield components and grain quality at the maturity stage.

(1) Under normal growth conditions, pydiflumetofen significantly promoted seedling growth, with root length, lateral root number, seedling height, and fresh weight increasing by 65.8%, 84.2%, 39.0%, and 20.0%, respectively, compared with the control. Chlorophyll a, chlorophyll b, and carotenoid contents increased by 19.3%, 28.1%, and 28.9%, respectively. The promoting effects were significantly greater than those of other fungicide treatments. (2) Under stress, pydiflumetofen pre-treatment significantly enhanced seedling tolerance. Under waterlogging, relative root length significantly increased by 48.3% compared with the control, and the activities of superoxide dismutase (SOD) and peroxidase (POD) were increased by 26.2% and 26.3%, respectively. Under PEG-simulated drought, relative seedling height, relative fresh weight, and vigor index significantly increased by 40.3%, 36.0%, and 113.8%, respectively, compared with the control. (3) At the full flowering stage, foliar application of pydiflumetofen significantly optimized endogenous hormone balance, indole-3-acetic acid (IAA) and gibberellic acid (GA₃) contents increased by 27.5% and 45.5%, respectively, while abscisic acid (ABA) content showed no significant difference compared with the control. (4) At the maturity stage, pydiflumetofen treatment achieved synergistic disease control and yield increase. The disease index of Sclerotinia stem rot was decreased by 58.3%-59.9%, while silique number per plant, seeds per silique, yield, and seed oil content were increased by 13.9%-14.1%, 7.0%-7.3%, 34.4%-34.9%, and 5.1%, respectively, outperforming other fungicide treatments. The mixture of pydiflumetofen and prochloraz exhibited antagonism, reducing yield by 16.1%-16.9%, compared with pydiflumetofen alone.

Pydiflumetofen synergistically improves rapeseed growth, stress tolerance, disease resistance and grain quality by optimizing root architecture, activating the antioxidant system and regulating the endogenous hormone balance. Its integrated benefits are superior to those of conventional fungicides such as prochloraz, tebuconazole and boscalid.

As a new Plant growth regulator, 14-hydroxylated brassinosteroids (14-HBR) increase biological activity by 50% than the traditional Brassicin sterols, while the relation effect of 14-HBR on rapeseed growth, yield and seed coating pesticide were not clear.

In this study, 14-HBR regulator and pesticide are used to treat the seeds in mid-duration winter rapeseed variety (Yangguang 2009) and early-duration variety (Yangguang 131), investigated germination, seedling growth, insect resistance, and yield, to reveal the interaction effects of environment, variety genotype and 14-HBR.

0.0075 and 0.015 mg·L^-1 14-HBR treated seed increased significantly germination potential in medium duration rapeseed, but decreased germination potential and germination rate significantly as mixed with pesticide. 14-HBR treated seed had no significant effect on germination rate and germination potential in short duration rapeseed. The 14-HBR showed better biological activity in seedling growth and yield than that of Brassinolide, 0.0075 and 0.015 mg·L^-1Brassinolide increased by an average of 5.19% and 8.15%, 0.0075 and 0.015 mg·L^-1 14-HBR increased by an average of 11.98% and 5.50%, respectively. 14-HBR mixed with seed pesticide of Clothianidin and Thiamethoxam, also increased seedlings weight and yield. The yield of Thiamethoxam and Clothianidin seed treatments increased by 4.7% and 4.6% independently. The yield of mixed with 0.0075 and 0.015 mg·L^-1 14-HBR to Clothianidin increased 6.8% and 3.3%, mixed to Thiamethoxam increased by 3.5% and 8.2%, respectively. 14-HBR did not affect insecticidal activity of Thiamethoxam and Clothianidin to peach bud nymphs and phyllotreta striolata fabricius.

The study revealed seed treatment with 14-HBR regulator has a positive effect on early growth of rapeseed and increased yield of rapeseed significantly, interacted with planting environment, pesticide type and varieties genotype as traditional regulators, it is necessary to optimize seed treatment technology of 14-HBR regulator to obtain higher harvest yield.