Population dynamics and diversity of Pseudomonas syringae on maple and pear trees and associated grasses

The number of epiphytic Pseudomonas syringae isolated from maple twigs and leaves between July 1985 and September 1986 was erratic (undetectable to 10 ⁵ cfu/g), whereas the number isolated from pear was more stable and often higher (10³ to 10⁶ cfu/g). P. syringoe was isolated consistently (about 10⁴-10⁷ cfu/g) from perennial rye, orchard, red fescue, annual rye, and brome grasses growing among trees in the maple nursery and from perennial rye grass in the pear orchard. In greenhouse pathogenicity tests, 87% of the P. syringae isolates from maple trees was pathogenic in maple seedlings, whereas 15% of the isolates from pear trees was pathogenic in young pear trees. Of the isolates tested from grasses, 55% from the maple nursery was pathogenic in maple seedlings, and 29% from grass in the pear orchard was pathogenic in young pear trees. These data indicate that grasses and trees support reservoirs of inoculum of pathogenic P. syringae. Indigenous isolates from a maple nursery were variable relative to pathogenicily and DNA restriction-fragment analysis, indicating that epiphytic populations of P. syringae from the grasses and trees were heterogenous.