Characteristics of Erwinia amylovora bacteriophage and its possible role in the epidemiology of fire blight

Abstract

Bacteriophage (S1) of Erwinia amylovora, isolated from soil at the base of fire-blight-infected trees, was characterized by small, clear plaques on E. amylovora strain PR1 and hazy plaques of the same size on a closely related, yellow, saprophytic bacterium, Y, which was isolated together with PR1 from the diseased trees. Phage S1 plated with optimal efficiency at 10C on PR1 and at 28C on Y was relatively unstable to storage at temperatures typical of summer and to ultraviolet (UV) irradiation, and lysogenized Y but not PR1. Pathogenicity tests in pear slices demonstrated that symptom development was (i) delayed when mixtures of either PR1 and phage S1 or PR1 and Y were inoculated, (ii) delayed when a culture of a phage-resistant mutant of PR1 was inoculated, and (iii) prevented from appearing at all when PR1 and the lysogenic form of Y, Y (S1), were inoculated together. Strain Y was easily lysogenized and Y (S1) released phage spontaneously and after UV irradiation. It is suggested that the yellow saprophyte, which is invariably isolated from fruit trees with E. amylovora, may frequently occur in its lysogenic form in nature and serve as a reservoir of phage which may exert some influence on the occurrence and severity of fire-blight disease.