Core–Shell Microcapsules of Pyraclostrobin and 24-Epibrassinolide for Controlled Release, Enhanced Fungicidal Activity, and Reduced Acute Toxicity

To combat fungal plant diseases and induce plant resistance, this study employed w/o/w double emulsion technology to fabricate composite microcapsules using poly(lactide-co-glycolide) and hydroxypropyl methylcellulose phthalate as wall materials, coencapsulating pyraclostrobin of the outer layer and plant hormone 24-epibrassinolide of the inner layer. The microcapsules exhibited an average particle size of 2.12 μm, pyraclostrobin encapsulation efficiency (EE) of 74.1%, and 24-epibrassinolide EE of 64.8%. Cumulative release profiles showed 62% pyraclostrobin release at 10 h, reaching a maximum of 82% by 72 h, while 24-epibrassinolide achieved 52% release at 10 h and 60% by 48 h. Compared with the technical pyraclostrobin, the microcapsules significantly enhanced the photostability of 24-epibrassinolide by 4.36-fold, thereby facilitating the long-term utilization of active ingredients. The microcapsule formulations exhibited superior control efficacy against rice leaf blast and significantly reduced acute toxicity to zebrafish (20-fold higher LC50 than technical).