Incorporation of Green Manure Reduces Tobacco Root Rot Incidence via Modulation of Rhizosphere Microbial Communities

Incorporating green manure into tobacco production systems significantly boosts soil fertility and suppresses soil-borne fungal diseases. However, the comparative effectiveness of different green manures against tobacco root rot and their associated microbial regulatory mechanisms is not fully understood. In this study, we evaluated the disease-suppressive capabilities of two green manures—smooth vetch (Vicia villosa) and rape (Brassica campestris)—applied at varying rates in a pot experiment on Fusarium solani-induced tobacco root rot. Results showed that smooth vetch incorporation substantially reduced Fusarium solani abundance by 62.3% and decreased tobacco root rot incidence by 20% compared to the no green manure and pathogen inoculation control. Smooth vetch increased the relative abundance of Bacillus spp. in rhizosphere soil by over two-fold compared to no incorporation. Co-occurrence network analysis detected four microbial modules, with Module 0 showing a significant negative correlation with pathogen abundance. In Module 0, bacterial taxa, particularly Bacillus spp., held central positions with extensive node interactions, while fungi displayed higher relative abundance. This module also included other taxa with potential disease-suppressive capabilities such as Paenibacillus spp., Lysobacter soli, and Chaetomium sphaerale. Importantly, smooth vetch more effectively enhanced soil-available nutrients, especially alkaline nitrogen, compared to rape. These nutrient improvements were associated with enriched potential disease-resistant microbial communities. Overall, smooth vetch shows greater potential for enhancing tobacco resistance to root rot and reducing disease incidence, presenting an effective strategy for managing soil-borne diseases in tobacco.