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Leaf Damage as a Predisposing Factor in the Infection of Apple Shoots by Erwinia amylovora. J. E. Crosse, Visiting Professor, Department of Plant Pathology, University of Missouri, Columbia 65201; R. N. Goodman(2), and W. H. Shaffer, Jr.(3). (2)(3)Professor, and Research Microbiologist, respectively, Department of Plant Pathology, University of Missouri, Columbia 65201. Phytopathology 62:176-182. Accepted for publication 30 August 1971. DOI: 10.1094/Phyto-62-176.

Spraying uninjured apple shoots with suspensions of Erwinia amylovora had no effect, but extensive blight developed when the leaves were damaged before inoculation by excising the apices of the laminae. The incidence of blight decreased as the interval between injury and inoculation was increased, but residual susceptibility remained after 48 hr. There was evidence that the bacteria penetrated into leaves through the severed ends of the xylem vessels, then migrated in these elements to the shoot axis. The incidence of leaf and of shoot infection increased with inoculum concentration or inoculum dose (ID) in a typical dosage:response relationship. The median ID(50) for shoot infection was estimated to be 38 bacterial cells, but some shoots were probably infected from a single cell. Infection of the shoot axis most commonly originated through the first fully unrolled leaf and the next leaf below. These leaves were also the most susceptible to infection at the higher inoculum concentrations, but at lower concentrations the position of maximum susceptibility shifted to slightly older leaves. Reasons for this interaction between leaf age and inoculum concentration are discussed.