Spread of Erwinia amylovora in Apple and Pear Trees of Different Cultivars after Artificial Inoculation

In the present study the correlation of visual fire blight symptoms and the migration of a virulent E. amylovora strain within the tissue of susceptible and tolerant apple and pear cultivars was investigated. Flowers and shoots of commercial apple cultivars were artificially inoculated. Three cultivars diverged in terms of symptom expression after flower and shoot inoculation, respectively: 'Fuji KIKU 8'(R) and 'Topaz'(R) developed severe symptoms on shoots but only a relatively low number of flowers were infected. Conversely, 'Golden Delicious Kl. B Laimburg'(R) displayed severe necrosis on shoots but was the least susceptible of all the cultivars after flower inoculation. Apple and pear cultivars that have a regional importance for large trees in extensive orchard management were tested for their susceptibility to artificial shoot inoculation. In our experiments the susceptibility (i.e., expression of shoot necrosis) of investigated apple cultivars for commercial and traditional orchards was compared and ranged from 17 to 50%. A lesion length of more than 60% was observed for only one apple cultivar ('Brentewinar'). The opposite situation arose in the tested traditional pear cultivars where 61% showed >= 60% lesion length. In this study the visual symptoms did not correspond with the spatial distribution patterns and the quantities of E. amylovora within the plant tissue. In apple cultivars most bacteria remained near the site of inoculation. However, in two tested apple cultivars, and in all tested pear cultivars, the pathogen could be detected even in the rootstocks. Also in pear trees, large numbers of the pathogen could be detected in non-inoculated shoots and further downwards. Even the pear cultivar 'Harrow Sweet' and the apple cultivars 'Schmiedberger Renette', and 'Rewena', which had been considered less susceptible, showed an extended endophytic bacterial dissemination. These findings may have an impact on both the evaluation of new cultivars and on sanitary measures regarding fire blight control.