Bacterial wilt in South Carolina tobacco production system: Winter cover - summer crop rotation, microbial communities, and the diversity of the causal agent, Ralstonia solanacearum, in the southeastern United States

The direct extraction, and analysis, of fatty acids from soil was used to study microbial communities in agricultural soils without reliance on traditional microbiological culturing methods. Using soil microbial community fatty acid retention time profiles collected over a two-year winter cover - summer crop rotation trial, changes in the soilborne microbial community due to particular rotation crops were tracked. Two months after planting the summer crop had altered the soilborne microbial community. Similarly, changes in the soil microbial community were detected two months after incorporation the winter cover crop. Effects of the summer crops, differentiating the soilborne microbial communities of corn and soybean from the community of tobacco, lasted approximately one year. In contrast, the effect of winter cover crops in differentiating the soilborne microbial communities was brief, two months. This potentially explains the major crop benefit and bacterial wilt suppression effects of the summer rotation of corn and soybean, while winter cover crops had less effect on tobacco yields. Within two to 14 weeks post planting, tobacco, corn and soybean generated a unique rhizosphere microbial community determined by both direct extraction and dilution plating. FAME analysis also revealed that the soil microbial community could rapidly change under dry conditions. Comparison of fatty acid profiles from 93 Ralstonia solanacearum isolates from the Southeastern United State revealed three distinct populations. These three unique clusters represented the pathogen's geographic origin (North Carolina, South Carolina, and Georgia and Florida) and not due to the host source. The clustering of the strains by state suggests that the spread of a particular strain, that give rise to a unique population, may have been limited by tobacco production regulations and farming practices where dissemination of the pathogen is through contaminated equipment within the state's production area.