Bacillus inhibits poplar leaf blight through coordination of rhizosphere microbiome remodelling and foliar immune...

Poplar leaf blight caused by Alternaria alternata represents a major biotic constraint on plantation productivity. Although beneficial Bacillus species are widely used as biocontrol agents, their field efficacy is often limited by their single application modes and unclear mechanisms of action. In this study, we conducted field experiments to systematically evaluate the effectiveness of different application methods of Bacillus spp. (root-drenching, combined root-drenching and foliar spraying) on the control of leaf blight in Populus alba × P. berolinensis and analysed the underlying mechanisms from the perspectives of rhizosphere microbiome changes and leaf immune responses. The combined treatment reduced the disease index by 86.7%. Root-drenching treatment optimized the rhizosphere microbial community structure, enriching key beneficial genera, including Rhodanobacter and Apiotrichum, whose abundance was positively correlated with leaf defense parameters. The combined treatment reshaped the defense-oriented transcriptional coregulatory network and strongly and continuously activated the SA and JA signalling pathways, leading to the significant upregulation of pathogenesis-related genes such as PR-1. Notably, the combined treatment was associated with transcriptional changes in ROS metabolism-related genes and elevated antioxidant enzyme activities, whereas root drenching mediated long-lasting resistance through rhizosphere microbiome regulation. This study describes a coordinated disease resistance pattern involving rhizosphere microbiome-mediated long-term defense and foliar application-triggered immune response." The findings underscore the importance of the coordinated action between underground microbes and aerial immune activation in field trials, providing a practical framework for optimizing control strategies against foliar diseases in trees.