A novel composite microbial agent controls kiwifruit bacterial canker and modulates bark bacterial communities

Kiwifruit bacterial canker (KBC), caused by Pseudomonas syringae pv. actinidiae (Psa), is an important disease limiting kiwifruit production worldwide. Management relies largely on copper-based products and antibiotics, whereas biological control options with consistent field efficacy remain limited. We evaluated the composite microbial agent "Fufangkuiyangjing" (FKYJ) against KBC when applied during two key management windows: post-harvest to pre-defoliation and pre-flowering to post-flowering. In vitro inhibition and detached-leaf inoculation assays showed that FKYJ exhibited strong antibacterial activity against Psa and significantly reduced lesion area on detached leaves of multiple kiwifruit cultivars. At a 50-fold dilution, FKYJ produced an inhibition zone 40.13% larger than that of Zhongshengmycin (ZSM), and in detached-leaf assays, an 800-fold dilution of FKYJ provided lesion-suppression efficacy comparable to that of 200-fold dilutions of ZSM and Bacillus amyloliquefaciens Ba168 (Ba168). Across two consecutive years of field trials, FKYJ provided more than 80% control efficacy against trunk canker and more than 75% efficacy against blossom blight, exceeding the efficacy of the single-agent treatments. Quantitative PCR further showed that FKYJ reduced Psa target gene abundance in bark, branches, rhizosphere soil, and flowers more effectively than ZSM and Ba168. Amplicon sequencing revealed treatment-associated shifts in bark bacterial community structure and enrichment of several genera previously reported to have plant-beneficial or antagonistic potential, such as Brevibacillus spp. The enrichment of these putatively beneficial taxa provides a possible microbiological explanation for the field efficacy of FKYJ, suggesting that changes in bark-associated microbial communities may partly contribute to its disease-control performance. Collectively, these findings further support that FKYJ has strong potential as a biological control agent for integrated management of KBC.