EFFICACY OF EARLY SELECTION FOR FIRE BLIGHT RESISTANCE AND THE ANALYSIS OF COMBINING ABILITY FOR FIRE BLIGHT RESISTANCE IN SEVERAL PEAR PROGENIES
A set of progenies was tested for fire blight resistance by needle inoculation at 3 mo of age in the greenhouse and then 5 yr later in the orchard to determine if the measurements of fire blight resistance made in the greenhouse was correlated with those made in the field. The correlation of fire blight resistance at the two stages of growth was weak or absent on a single plant basis. This indicated a lack of precision in the greenhouse test but genetic gain based on the field measurements appeared to be possible if plants were selected in the greenhouse with less than 19% of blighted shoot length. In another set of progenies, the greenhouse test was observed to delay fruiting, but the delay was not greater than 1 yr. It is proposed that selection for fire blight be carried out at both stages of growth, in the greenhouse as single measurements on young plants and again in the field when repeated measurements can be made. Maximum likelihood estimates of the variance components determined on data that were transformed by arc sine of the square root and corrected for censoring more than 100% blighted shoot indicated a high general combining ability and a low specific combining ability for fire blight resistance. Genetic variance was, therefore, predominantly additive. Combining ability estimate indicated Kieffer was the superior female parent and HW 601 and Old Home were the superior male parents. If this sample of parents is representative, genetic advance for fire blight resistance should be obtained by selecting parents on the basis of phenotypic values.Key words: Pyrus, Erwinia amylovora, general combining ability, specific combining ability