Articles
EVALUATION AND SELECTION OF FIRE BLIGHT RESISTANCE IN APPLE WITH REFERENCE TO DIFFERENTIAL VIRULENCE IN ERWINIA AMYLOVORA
Differential host X pathogen interactions occur among some of the sources of fire blight resistance used in the breeding program and strains of E. amylovora (Ea). The purpose of this study was to determine the effect of the virulence of Ea strains on the evaluation of fire blight resistance in an apple breeding program.
Two aspects of the virulence of Ea were considered, the aggressiveness (sensu Vanderplank, 1968) of the strain and the differential virulence of the strain to specific host genotypes.
In addition, the effect of mixing strains in the inoculum on the evaluation of resistance was tested at the greenhouse and field stages of the evaluation process.
Inoculation of greenhouse-grown apple seedlings from controlled crosses with a mixture of five strains of Ea resulted in a larger percentage of the seedling population being evaluated as susceptible and a smaller percentage being evaluated as resistant when compared to inoculation with a single strain.
Inoculation of field-grown apple seedlings with five individual strains of Ea and a mixture of the same strains indicated that the strain used for inoculation significantly affected the evaluation of resistance.
In general, the aggressiveness (sensu Vanderplank, 1968) of the strains most strongly affected the evaluation of resistance.
The differential virulence of the strains to specific cultivars had less effect on the evaluation of resistance.
Mixtures of strains masked the differential response of specific cultivar by strain interactions.
A major constraint in an apple breeding program is the limited number of seedlings that can be grown in the field for evaluation.
In the program at the NYSAES, attempts are made to eliminate as many disease-susceptible seedlings as possible during the first months of growth in the greenhouse before field planting.
Seedlings from controlled crosses are inoculated in the greenhouse with a mixture of highly aggressive Ea strains which include strains differentially virulent and avirulent to ‘Novole’. This inoculation identifies susceptible seedlings that can be discarded without testing in the field.
After initial pomological evaluation in the field, selections of interest are inoculated again with several individual Ea strains.
Our results indicate that the evaluation of resistance to Ea can be affected by the virulence of the Ea strains used in screening.
Highly aggressive strains are most effective in identifying susceptible genotypes.
Strains of Ea that are differentially virulent