Suppression of Ice Nucleation-ActivePseudomonas syringaeby Antagonistic Bacteria in Fruit Tree Orchards and Evaluations of Frost Control

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Abstract
Bacteria composing a major component of fruit tree flower surfaces were isolated and screened for in vitro inhibition of ice nucleation-active (INA) Pseudomonas syringae pv. syringae and control of frost injury in greenhouse and orchard trials. Nearly all of the inhibitory strains isolated were identified as either P. fluorescens or P. putida. Greenhouse evaluations on young corn plants showed that some strains of these bacteria significantly suppressed (by up to 98%) the INA P. s. pv. syringae population resulting in fewer ice nuclei active at -4.5 C. INA strain W4N1613 of P. s. pv. syringae was comparable to selected strains of P. fluorescens and P. putida in effectiveness, whereas strain M232A of Erwinia herbicola was inferior to these strains. Applications of the most promising strains to pear trees at Hood River, OR [Oregon], and sweet cherry and apple trees at Prosser, WA [Washington, USA], at the tight cluster stage of bud development resulted in antagonist populations ranging from 104 to over 106 colony-forming units (cfu)/g throughout bloom. Substantial decreases in populations did not occur until late May or June when the weather became warmer and drier. A 10- to 100-fold suppression of INA P. syringae was observed for the most suppressive bacterium, which was INA strain W4N1613- of P. s. pv. syringae. This strain reached populations of over 106 cfu/g to compose virtually 100% of the total bacterial flora. Populations for all P. fluorescens and P. putida strains on pear, sweet cherry, and apple were surprisingly similar; little suppression of INA P. syringae was observed because their populations sometimes exceeded that of the antagonists. None of the antagonists, however, controlled frost injury to pear following radiation frosts ranging from -4 to -4.5 C (air temperatures) in 1982 and 1983, respectively. In addition, incidence of fruit russet and Pseudomonas blossom blast were indistinguishable for treated and untreated trees. These results are discussed in relation to the presence of an intrinsic, wood-associated ice nucleus, active at about -2 C, that impedes control of frost injury to fruit trees through suppression of INA bacteria.