Synergistic efficacy of chitosan nanoparticles and salicylic acid in mitigating charcoal rot (Macrophomina phaseolina) in soybean

Macrophomina phaseolina is a devastating fungal pathogen of soybean, causing charcoal rot and resulting in significant reductions in both yield and grain quality. Conventional control approaches are limited by environmental concerns and the development of fungal resistance, necessitating the development of biocompatible, sustainable alternatives. This study evaluated the individual and interactive efficacy of chitosan nanoparticles (CNPs) and salicylic acid (SA) in managing M. phaseolina infection. CNPs exhibit strong antifungal activity and enhance plant basal immunity, whereas SA acts as a signaling molecule to activate defense responses. Two soybean cultivars, Golestan and Saba, were evaluated under greenhouse conditions using a randomized complete block design. Foliar applications of CNPs, SA, or their combination were administered 72 h before fungal inoculation. CNPs were synthesized and fully characterized for particle size, crystal structure, and FTIR spectra, and their in vitro inhibitory concentration against M. phaseolina was established. Morphological and physiological parameters, including fresh/dry biomass, plant height, greenness index, and the activities of peroxidase and polyphenol oxidase, were comprehensively assessed. Enzyme assays revealed significantly higher peroxidase and polyphenol oxidase activities 72 h after inoculation in treated plants compared to the control. Three weeks post-inoculation, treated plants showed increased shoot and root biomass and plant height, alongside reduced disease severity. Importantly, the optimal CNPs–SA combination for disease suppression differed between cultivars, with 100 mg L⁻¹ CNPs + 250 mg L⁻¹SA being most effective in Saba and 250 mg L⁻¹ CNPs + 100 mg L⁻¹ SA in Golestan. Both individual and combined applications of CNPs and SA significantly suppressed fungal growth and enhanced soybean resistance. These results indicate that CNP- and SA-based treatments offer a sustainable, safe, and effective strategy for controlling charcoal rot in soybean.