FIRE BLIGHT SUSCEPTIBILITY OF PEAR CULTIVARS GRAFTED ON OHF 333 ROOTSTOCK

The rootstock OHF 333 ('Old Home' x 'Farmingdale) is characterized by high yield efficiency and moderate resistance to fire blight (Erwinia amylovora) in Bulgarian environmental conditions. The aim of this study was to examine the reaction of 10 pear cultivars grafted on OHF 333 rootstock to Erwinia amylovora. Their susceptibility was determined after artificial inoculation of leaves in a greenhouse. Several vigorously growing shoots per tree were inoculated with two pathogenic local isolates of Erwinia amylovora by cutting the leaves. The two bacterial suspensions of Bulgarian strains were used single and mixed inoculum (3 x 10(8) cfu/ml). Based on the artificial inoculation the degree of attack of leaves and shoots was calculated All cultivars were grouped into 5 categories - very low susceptible, low susceptible. moderately susceptible, susceptible and very susceptible. In twenty-five days after inoculation with isolate Ea3325, 6 of the studied cultivars reacted as very low susceptible showing less than 20% shoot necrosis. Two of the cultivars, artificially inoculated with mixed inoculum showed terminal shoots necrosis over 60%.