Cross-infection by multiple soil-borne pathogens alters the physicochemical properties and microbial community structure of tobacco rhizosphere soil

Soil-borne diseases of tobacco roots and stems, caused by pathogens including Ralstonia solanacearum (bacterial wilt), Phytophthora parasitica var. nicotianae (black shank), and root-knot nematodes (Meloidogyne spp.), represent critical threats to tobacco cultivation. However, information regarding alterations in rhizosphere soil properties and microbial communities following co-infection by multiple soil-borne pathogens remains limited for tobacco-growing regions in Luxi County, Honghe Prefecture, Yunnan Province, China. This study therefore investigated rhizosphere soils collected from healthy and diseased tobacco plants in Baishui Town. Eighteen soil physicochemical parameters and six enzyme activities were measured. Bacterial and fungal community compositions were analyzed using high-throughput sequencing targeting the 16S rDNA V4 region and the ITS1 region, respectively. Relationships between environmental factors and microbial community shifts were further explored using redundancy analysis (RDA) and Spearman correlation analysis. Results demonstrated that, compared with healthy tobacco plants, various physicochemical factors and enzyme activities in the rhizosphere soils of diseased plants were significantly elevated. Bacterial community diversity and species richness in diseased tobacco rhizosphere soils were significantly altered by pathogen infection. Specifically, the relative abundance of the phylum Proteobacteria significantly decreased by 18.20%, whereas Chloroflexi and Gemmatimonadetes significantly increased by more than two-fold. The fungal phylum Chytridiomycota significantly decreased by 72.68%. At the genus level, the beneficial bacterium Sphingomonas significantly decreased by 36.13%, and the beneficial fungus Penicillium significantly decreased by 90.59% in relative abundance. Conversely, pathogenic fungi Fusarium, Phoma, and Plectosphaerella significantly increased by 3-fold, 5-fold, and 3-fold, respectively.RDA and Spearman correlation analyses revealed that variations in rhizosphere microbial communities were significantly and positively correlated with elevated soil pH, organic matter, total nitrogen, available nitrogen, total phosphorus, manganese content, and the activities of peroxidase and protease, while significantly and negatively correlated with decreased total potassium, available potassium, and boron contents. Our findings indicate that the imbalance (or alternatively: dysbiosis of microbial communities coupled with the disruption) of soil physicochemical properties, enzyme activities, and microbial community structure constitutes a typical characteristic of tobacco rhizosphere soils subjected to multiple pathogen co-infections in this region.