Survival and dispersal of a marked strain of Pseudomonas syringae in a maple nursery

A pathogenic Pseudomonas syringae strain resistant to rifampicin and nalidixic acid was sprayed upon and colonized maple twigs and perennial ryegrass. The inoculated twigs were sampled at intervals of 2-3 weeks from July 1985 to September 1986, and epiphytic populations of the marked strain recovered during this time ranged from undetectable to 10⁴ colony-forming units/g. The results showed that this strain of P. syringae could overwinter on maple twigs and potentially serve as a source of inoculum in the spring. Aerial dispersal was also investigated. The marked strain inoculated onto grass growing in pots was detected on medium in inverted petri plates, on maple leaves and with an Andersen sampler positioned from 12 to 100 cm above the grass. The number of detectable cells that dispersed vertically upwards was low even in the presence of wind, rain, or irrigation water. The marked strain inoculated onto maple branches was isolated from grasses under the inoculated trees, showing that P. syringae was dispersed downward. Lateral dispersal of the marked strain from inoculated to uninoculated trees was not detected. The identity of the antibiotic-resistant strain isolated from the trees and grasses was confirmed by DNA restriction-fragment profile analysis.