Infection Courts and Systemic Movement of32P-LabeledErwinia amylovorain Apple Petioles and Stems

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Abstract
The rate of migration of 32P-labeled virulent (E9) and avirulent (E8) strains E. amylovora was monitored in apple petiole and stem tissue. 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 2-4 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. Virulent strain E9 moves quickly and significant distances (34 mm/h) in xylem vessels. Wounding of leaves that exposes xylem vessels to the pathogen could be an important factor in the infection process in nature.