A novel type of quantum-delivery system (salicylic acid conjugation based on ZnO quantum dots) alleviates the bacterial fruit blotch disease of melon plants by activating defense response

The use of nano-pesticide delivery systems to suppress crop disease has great potential in sustainable agriculture. Herein, a pH-responsive salicylic acid (SA) conjugation based on aminopropyl-functionalized ZnO quantum dots (SA-ZnO QDs) with a loading capacity of 13.4 wt% was prepared by an amidation reaction. Results indicated that SA-ZnO QDs could release SA rapidly in an acidic condition, which corresponded to the pH of the water-soaked lesions where the bacterial fruit blotch disease often spread. The antibacterial activity of the synthesized SA-ZnO QDs was about 1.6 times and 1.3 times higher than that of SA and aminopropyl-functionalized ZnO QDs, respectively. A pot experiment showed that foliar application of SA-ZnO QDs could reduce the incidence of bacterial fruit blotch disease (54.46%) by increasing the antioxidant defense systems (44.33–79.59%) and nutrient absorption (23.19–26.95%). More importantly, we observed that, after the application of SA-ZnO QDs, the systemic acquired resistance and flavone biosynthesis were activated in infected melon seedlings, which was revealed by metabolomics. This work demonstrates the important role of the quantum-delivery system in improving crop disease suppression for sustainable agriculture.