Dual mechanism of silver nanoparticles in controlling tobacco Fusarium root rot: Direct antifungal activity and induced systemic resistance

Tobacco root rot caused by Fusarium species severely threatens crop yield. Silver nanoparticles (AgNPs) offer a promising alternative to conventional fungicides for sustainable crop disease management. However, the comprehensive antifungal activity and underlying mechanism of AgNPs against multiple Fusarium species causing tobacco root rot remain unclear. Herein, chemically synthesized AgNPs with an average diameter of 20 nm exhibited broad-spectrum antifungal activity against F. oxysporum , F. solani , F. equiseti , and F. proliferatum , with EC₅₀ values of 18.93–28.01 μg/mL, and showed additive or synergistic antifungal effects with commercial fungicides. AgNPs below 40 μg/mL caused no phytotoxicity to tobacco seed germination and seedling growth, and 20 μg/mL AgNPs significantly increased leaf area, chlorophyll content, and nitrogen content by 22.32%, 13.84% and 12.60%, respectively. The 1:4 compound of AgNPs and Metalaxyl-M·fludioxonil achieved 81.10% control efficacy, significantly outperforming single agents. Notably, AgNPs reduced the Metalaxyl-M·fludioxonil recommended dosage by 25% without compromising control efficacy. Furthermore, AgNPs disrupted Fusarium cell membrane integrity, induced oxidative stress and metabolic disorder, and regulated genes involved in ABC transporters, steroid biosynthesis, and tryptophan metabolism. Beyond direct antifungal activity, AgNPs also elevated phenylalanine ammonia-lyase and peroxidase activities by 77.12% and 136.07% at 12 h and 48 h, and up-regulated PR2 and PR1 expression by 151- and 186-fold at 12 h and 24 h, respectively. This study reveals the multifunctional effects of AgNPs and supports its potential as an effective growth-promoting agent for sustainable tobacco root rot management.