EVALUATION OF FIRE BLIGHT RESISTANCE OF APPLE CULTIVARS AND BREEDING NEW RESISTANT CULTIVARS
The evaluations were conducted over a period of several years as a series of tests among cultivars; however, each single test included only a subset of the 377 cultivars evaluated. The results of these tests are combined using an analysis of variance approach, with cultivar and test as main effects. Interactions are also considered. Because of the unbalanced nature of the data, care must be taken in drawing conclusions about the cultivars. It is proposed that this type of analysis be considered for use by other researchers so that results of tests in different places and times can be directly compared. A similar statistical approach has been utilized to evaluate the fire blight susceptibility of 37 apple cultivars based upon the incidence of blossom and shoot infections resulting from 4 years of field inoculations.
Resistance to fire blight is an important criterion in the diseaseresistant apple breeding program at Geneva. Because hybrid seedlings are initially tested in the greenhouse for resistance to apple scab and cedar apple rust, growth conditions are such, that testing for fire blight resistance in the greenhouse can only identify some very susceptible seedlings for elimination. Identification of fire blight-resistant seedlings is best done during the third year in the field. The recent demonstration of differential virulence in E. amylovora mandates testing with multiple strains that have a spectrum of virulence. It also brings into question reliance on oligogenic resistance in breeding since several sources of oligogenic resistance have succumbed to infection by differentially virulent strains. Less complete but more durable polgenic resistance may be preferable. Breeding with both types of resistance is continuing at Geneva. A cooperative project with researchers in Georgia is accelerating flowering and also allowing field testing for fire blight resistance under favorable conditions for the disease and with different strains of the bacterium.