Assessment of Environmental Factors Influencing Growth and Spread of Pantoea agglomerans on and Among Blossoms of Pear and Apple

We evaluated effects of both physical and biological components of the environment on growth of Pantoea agglomerans on inoculated pear and apple blossoms and on spread of the bacterium to blossoms on non-inoculated trees. The center three rows of 0.35- to 0.5-ha blocks of four pear cultivars and four apple cultivars were sprayed with a suspension of streptomycin-resistant P. agglomerans strain C9-1S (C9-1S) at 20 to 60% and 60 to 90% bloom. Cultivars were chosen to create a sequence of continuous bloom from late March (d'Anjou pear) through mid-May (Red Rome apple). Each cultivar block was quartered into plots; two plots were treated twice with streptomycin sulfate near mid- and full bloom to suppress populations of indigenous bacterial epiphytes and the other two plots were treated with water. Colonization of blossoms by C9-1S and by indigenous bacterial epiphytes were monitored on inoculated trees and along transects of noninoculated trees. Immediately after spraying, C9-1S was detected principally on blossoms sampled from inoculated trees. As bloom progressed, trees up to 18 m from inoculated trees had high proportions of blossoms colonized by C9-1S. Streptomycin significantly (P ≤ 0.05) reduced incidence of isolation and size of detectable populations of culturable bacteria (indigenous bacteria plus C9-1S) from pear blossoms in 1998 and from apple blossoms in both 1998 and 1999, but the antibiotic treatment did not affect incidence of isolation, size of detectable populations, or spread of C9-1S compared to the water-treated control in any experiment. Across all cultivars, relative area under the curve for size of detectable populations of C9-1S on inoculated trees and for incidence of isolation of C9-1S from noninoculated trees was positively correlated with mean degree hours per day during bloom (r= 0.61 to 0.73) and negatively correlated with the proportion of days with rain (r = -0.79 to -0.84). The results indicate that establishment and growth of C9-1S on pome fruit flowers was not strongly affected by streptomycin or by competition from indigenous bacterial epiphytes and, as with Erwinia amylovora, temperature is an important environmental variable affecting successful spread of this biological control agent from blossom to blossom.