EFFECTS OF PEAR TREE PHYSIOLOGY ON FIRE BLIGHT PROGRESSION IN PERENNIAL BRANCHES AND ON EXPRESSION OF PATHOGENICITY GENES OF ERWINIA AMYLOVORA

The extent of symptom progression and the final length of fire blight cankers in perennial branches were variably affected by the vigor of the trees and the season of infection. Monographs taken by transmission electron microscope (TEM) showed that Erwinia amylovora cells secreted unidentified material to the intercellular spaces; the form and quantity of this material varied according to the season of infection and the vigor of the trees. The interaction between E. amylovora and the physiological status of pear trees was examined under orchard conditions. Qualitatively, tree response to fire blight was governed by their vigor and their phenological stage at the time of infection. Quantitatively, the rate of symptom progression in perennial branches (SPR) was significantly (P≤0.001) related to the absolute value of the rate of sorbitol content change (|SCR|). |SCR| in annual shoots fluctuated markedly over time, in line with the phenological stage of the trees, their vigor and the horticultural practice employed, viz., application of a growth regulator, pruning or post-harvest irrigation quantity. Pruning enhanced E. amylovora virulence (estimated according to hrpE expression) by 1.2 to tenfold as compared with non-treated trees, whereas application of a growth regulator decreased E. amylovora virulence by factors of 1.5 to 100. The conclusion is that horticultural practices that readily disrupt the physiological status of the host enhance its susceptibility to E. amylovora whereas practices that stabilize the physiological status of the host increase its resistance. The latter may be used as basis for disease management.