Assessment of the importance of similarity in carbon source utilization profiles between the biological control agent and the pathogen in biological control of bacterial speck of tomato

Bacterial speck of tomato, caused by Pseudomonas syringae pv. tomato, was used to determine whether similarity in carbon source utilization between a preemptive biological control agent and the pathogen was significant in determining the ability of the bacterium to suppress disease. Similarity in carbon source utilization was quantified as the ratio of the number of tomato carbon sources utilized in vitro by the biological control agent to the number of tomato carbon sources utilized in vitro by the target pathogen (the niche overlap index [NOI]). Suppression of the disease was quantified as the percent reduction in disease severity compared to the pathogen-only control when nonpathogenic bacteria were applied to foliage 48 h prior to the pathogen. In the collection of 36 nonpathogenic bacterial strains, there was a significant (P < 0.01), but weak (r = 0.25), correlation between reduction in disease severity and similarity in carbon source utilization, suggesting that similarity in carbon source use was significant in determining ability to suppress disease. The relationship was investigated further using catabolic mutants of P. syringae strain TLP2, an effective biological control agent of speck. Catabolic mutants exhibited lower levels of similarity (NOI = 0.07 to 0.90) than did wild-type TLP2 (NOI = 0.93). With these catabolic mutants there was a significant (P < 0.01), and stronger (r = 0.42), correlation between reduction in disease severity and similarity in carbon source utilization. This suggests that similarity in carbon source utilization was a more important component of biological control ability for the catabolic mutants than for the nonpathogenic bacteria. Together, these studies indicate that suppression of bacterial speck or tomato was correlated with nutritional similarity between the pathogenic and nonpathogenic bacteria and suggest that preemptive utilization of carbon sources was probably involved in the biological control of the disease by both the naturally occurring nonpathogenic bacteria and the catabolic mutants.