Fire blight, caused by the pathogen Erwinia amylovora (Burrill) Winslow et al, is the most devastating bacterial disease of pome fruits around the world. The primary dispersal of E. amylovora is through ooze, a mass of exopolysaccharides and bacterial cells that is exuded from infected host tissue. Over the 2013 and 2014 field seasons, 631 ooze droplets (201 in 2013 and 435 in 2014) were collected from field inoculated trees. Populations of E. amylovora in ooze droplets range from 108 to 1011 colony forming units per micro liter (cfu/μl). In the host tissue surrounding the droplets even larger populations of E. amylovora reside in the surrounding 1 cm of tissue. Three apple cultivars with varying levels of resistance were also infected with four Michigan E. amylovora strains. Using scanning electron microscopy, host tissue was examined for the origin of the ooze droplets and erumpent mounds and small (10 μm) tears were the only bacterial sources observed. Genetic expression analysis indicated that E. amylovora cells in stem sections located above ooze drops and in ooze drops were actively expressing virulence genes. If disseminated to susceptible host tissue, these cells would be primed for infection. The current study suggests the following: high populations of E. amylovora are present in ooze droplets which larger populations found in darker pigmented, smaller volume droplets. These droplets are rupturing out from the parenchyma and epidermis of the host, with evidence of immense pressure being involved from SEM observations. Ooze droplet volume and population can vary between host cultivar and the virulence of a specific E. amylovora strain. Genetic expression analysis of virulent factors in E. amylovora indicated that the bacteria in ooze were primed and ready to infect a new susceptible host.
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