Wide distribution of airborne ice-nucleation active Pseudomonas syringae in agricultural environments.

Pseudomonas syringae, a phytopathogen with a wide host range, inhabits various environments. Among these habitats, air is an important transport environment for microbes, allowing them to disperse over long distances; however, information on the distribution and composition of airborne P. syringae is limited. Here, we report the isolation of several groups of airborne P. syringae in Japan. Since 2013, we have trapped 43 strains of airborne P. syringae species complex, most of which were ice-nucleation active (INA), suggesting an important role for INA in atmospheric environments. Airborne P. syringae were isolated mainly between March and May, when the temperature is optimal for the epiphytic growth of P. syringae strains, indicates that plants are the main sources for airborne INA P. syringae. Multilocus sequence typing analyses categorized these strains into six clades. The most abundant clade, KID0033, clustered with strain UMAF0158, a pathogen of mango isolated in Spain, and Cit7, isolated in the USA. The second most abundant clade, KID0001, did not clustered with any pathovar or environmental strain reported previously. In addition, we isolated 14 P. syringae strains belonging to these two clades from asymptomatic and symptomatic leaves of kiwifruit, okra, and tea from different sites over the past decade. These strains are less virulent on originally isolated plants and other plant species under our inoculum conditions. These data indicate that certain groups of INA P. syringae, which were not previously recognized as pathogens, exist both in the air and on plants, and may be quite common throughout the world.