The role of ooze exudation in the migration of Erwinia amylovora cells in pear trees infected by fire blight

The bacterium Erwinia amylovora causes a severe disease called fire blight in pear trees. In pear trees that were artificially inoculated with E. amylovora cells the bacteria were found neither in the conducting elements of the xylem nor in the phloem (vessels and sieve tubes). They were found only in the intercellular spaces of the parenchyma cells of the bark. Therefore, the bacteria were not migrating through the conducting systems. Exopolysaccharides secreted by the bacteria in the intercellular spaces of the parenchyma cells decreased the water potential in the spaces, resulting in water and solute leakage from the living cells into the spaces. This led to cell plasmolysis and disintegration. We suggest that when ooze (the bacteria, materials secreted by the bacteria, water, and solutes leaked from the host cells) is accumulated in the intercellular spaces, it drives the air outside, blocks the oxygen supply, and inflicts additional stress on the dying cells. The outcome of the accumulated ooze in the intercellular spaces was the formation of pressure that pushed the bacteria and their secreted substances upwards and downwards throughout the continuum spaces. The ooze exuded to the outside throughout stomata in young tissues or lenticels in branches with secondary thickening.