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Infection Courts and Systemic Movement of 32P-Labeled Erwinia amylovora in Apple Petioles and Stems. C. G. Suhayda, Department of Plant Pathology, University of Missouri, Columbia 65201, Present address of the senior author: DOE/ MSU Plant Research Laboratory, East Lansing, MI 48824; R. N. Goodman, Department of Plant Pathology, University of Missouri, Columbia 65201. Phytopathology 71:656-660. Accepted for publication 8 December 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-656.

The rate of migration of 32P-labeled virulent (E9) and avirulent (E8) strains Erwinia amylovora was monitored in apple petiole and stem tissue. Both the capsulated E9 and capsule-deficient E8 were rapidly drawn into vascular elements of petiolar tissue and moved 5 mm within 20 min. Although most of the inoculum remained at the site of inoculation, enough cells moved into the stem from the petiole within 24 min to initiate systemic infection. The data from inoculation of stem tissue through cut bases with mixed inocula of virulent encapsulated E9 and avirulent capsule-deficient E8 strains suggest that the E8 strain is precluded from migrating freely in stem xylem vessels by an immobilizing phenomenon previously described. The study provides evidence that virulent strain E9 moves quickly and significant distances (34 mm/hr) in xylem vessels. The data suggest that wounding of leaves that exposes xylem vessels to the pathogen could be an important factor in the infection process in nature.