The overlapping continuum of host range among strains in the Pseudomonas syringae complex

Pseudomonas syringae is the most frequently emerging group of plant pathogenic bacteria. Because this bacterium is ubiquitous as an epiphyte and on various substrates in non-agricultural settings, there are many questions about how to assess the risk for plant disease posed by strains in the environment. Although P. syringae is considered to have discrete host ranges in defined pathovars, there have been few reports of comprehensive comparisons of host range potential. Here we present results of host range tests for 134 strains, representing eight phylogroups, from epidemics and environmental reservoirs on 15 to 22 plant species per test conducted in four separate tests to determine the patterns and extent of host range. We sought to identify trends that are indicative of distinct pathotypes and to assess if strains in the P. syringae complex are indeed restricted in their host range. We show that for each test, strains display a diversity of host ranges from very restricted to very broad regardless of the gamut of phylogroups used in the test. Overall, strains form an overlapping continuum of host range potential with equal representation of narrow, moderate and broad host ranges. Groups of distinct pathotypes, including strains with currently the same pathovar name, could not be identified. The absence of groupings was validated with statistical tests for pattern recognition. The extent of host range was positively correlated with the capacity of strains to swarm on semi-solid agar medium and with the abundance of genes in biosynthetic clusters and was inversely correlated with the abundance of genes for proteins with transmembrane domains in their genomes. Our results are consistent with the current paradigm that disease symptoms are the result of multiple molecular interactions between P. syringae and its plant host that are modulated by abiotic and biotic conditions. This leads us to propose that pathovar denominations do not correspond to the underlying biology of P. syringae. A new concept of pathogenicity that accounts for the continuum of pathogenic potential in P. syringae would open new perspectives to understand the evolution of pathogenicity in this bacterium and new insights to anticipate disease and to manage plant health