Preparation of hymexazol microspheres and its control effect of cucumber dumping-off

To develop an environmentally friendly, low toxicity, biodegradable microsphere formulation with sustained-release functionality, this study selects chitosan as the carrier material and genipin as the cross-linking agent, using water-soluble fungicide hymexazol as the model drug. The reverse emulsion polymerization method was successfully used to prepare genipin-crosslinked chitosan microspheres and the microsphere loading of hymexazol was achieved through both encapsulation and adsorption methods. The optimal loading method for hymexazol microspheres was determined through the loading tests, release tests, and soil column leaching tests. The biological activity of hymexazol microspheres was evaluated using in vitro antifungal assays and pot experiments. Meanwhile, the cytotoxicity of the genipin cross-linked chitosan microspheres and the acute toxicity of hymexazol microspheres were examined. The results showed that the particle size of hymexazol microspheres prepared by encapsulation was 6.42 µm and the drug loading rate was 15.87%. The release rate of hymexazol microspheres was 71.56% within 12 h, showing a remarkable slow-release effect and pH-responsive release characteristics. With the same content of active ingredients, the fungicidal effect of hymexazol microspheres against Pythium aphanidermatum was similar to that of traditional hymexazol aqueous solutions. Soil treatment showed that the retention of hymexazol microspheres was significantly better than that of hymexazol aqueous solutions, with only 67.69% loss in 7 days. Meanwhile, compared to the aqueous solution, hymexazol microspheres showed better control of cucumber damping-off disease caused by P. aphanidermatum at a lower concentration. In addition, the genipin-crosslinked chitosan microspheres prepared in this study showed lower cytotoxicity than the traditional glutaric dialdehyde-crosslinked chitosan microspheres. Additionally, the hymexazol microspheres formulated using them as carriers showed significantly lower acute toxicity to earthworms than the hymexazol aqueous solutions. The results indicate that the prepared hymexazol microspheres are environmentally friendly, high-efficiency, low-toxicity, and slow-release soil treatment formulation, with good prospects for development and application.