Competition between strains of the bacterial plant pathogen Xylella fastidiosa

Abstract

Understanding the dynamics of interactions among different strains of the bacterial plant pathogen Xylella fastidiosa (Xf) is crucial for preventing, detecting, and mitigating potential outbreaks. The potential for recombination and exchange of genetic material among these strains could lead to the emergence of new virulent strains and a wider range of susceptible hosts, making it necessary to investigate these interactions comprehensively. This study aimed to develop strainspecific quantitative PCR (qPCR) primers as a tool to identify and quantify strains of X. fastidiosa during mixed bacterial strains cultures and infections. A key consideration to select the competing strains was their classification in subspecies, plant host, and geographic location. The selected strains were X. fastidiosa subsp. fastidiosa TemeculaL (CA) and WM1-1 (GA) from grapevine, EB92-1 (FL) from elderberry, an avirulent strain, and X. fastidiosa subsp. multiplex AlmaEM3 (GA) and BB08-1 (FL) from blueberry. Co-culture experiments were performed to evaluate the impact of bacterial competition on growth and survival. Potential mechanisms of competition were assessed using in silico and dual culture methods. Results showed that strain TemeculaL had a significant inhibitory effect on the growth of WM1-1, while strains AlmaEm3, EB92-1, and BB08- 1 showed no significant inhibition. Fitness assays revealed that X. fastidiosa subsp. fastidiosa TemeculaL had an increased growth rate compared to the other strains. The study also evaluated the symptom development in a non-natural plant host, Nicotiana tabacum cv. SR1 Petite Havana, and showed high disease progression in plants infected with strains TemeculaL+BB08-1, and low symptom expression in plants infected with EB92-1 and TemeculaL+EB92-1. The latter results support the previous suggestion that the elderberry strain could be a potential biocontrol strain. The study provides new perspectives and innovative approaches for identifying distinctive genetic sequences among different strains of X. fastidiosa using novel strain-specific primers.