Differential Effects of Chitosan–Salicylic Acid Nanocomposite and Salicylic Acid on Cucumber Mosaic Virus in Cucumber

Cucumber mosaic virus (CMV) is a destructive viral pathogen of vegetables, fruits, grains, and ornamentals across the globe. This study investigated the comparative antiviral efficacy of chitosan–salicylic acid nanocomposite (Ch/SA NC) and salicylic acid (SA) against CMV in cucumber plants. Transmission electron microscopy (TEM) analyses revealed that Ch/SA NCs can aggregate on the viral coat protein surface, suggesting direct nanoparticle–virus interaction. Greenhouse trials showed that Ch/SA NC, particularly at 90 ppm applied 24 h before CMV inoculation, was the most effective treatment in reducing disease severity and viral load. SA at the same concentration also conferred significant protection when used prophylactically. An RT-PCR analysis confirmed suppression or complete silencing of CMV coat protein gene expression, especially Ch/SA NC-treated plants. Both treatments significantly enhanced the physiological condition of infected plants, including restoration of chlorophyll a, chlorophyll b, and carotenoids, and elevated levels of total phenolics, flavonoids carbohydrates, and proteins. In addition, they boosted the key antioxidant enzymes activities (POX, PPO, SOD) and improved vegetative growth indicators such as plant height, fruit fresh weight, and number of fruits per plant. These results indicate that Ch/SA NC and SA not only inhibit CMV replication but also stimulate host defense responses, improving overall plant health. The strong antiviral effect is likely due to the dual action of Ch/SA NC: direct virus binding and induction of systemic acquired resistance (SAR). Given their efficacy and eco-friendly nature, especially the Ch/SA NC, these treatments offer a promising strategy for integrated viral disease management. Future studies should investigate long-term environmental safety, molecular mechanisms, and field-level applicability.