VIEW ARTICLE

Resistance

The Effects of Virulence of Erwinia amylovora on the Evaluation of Fire Blight Resistance in Malus. John L. Norelli, Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462; Herb S. Aldwinckle(2), Steven V. Beer(3), and Robert C. Lamb(4). (2)Department of Plant Pathology, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462; (3)Department of Plant Pathology, Cornell University, Ithaca, NY 14853-0331; (4)Department of Horticultural Sciences, Cornell University, New York State Agricultural Experiment Station, Geneva 14456-0462. Phytopathology 77:1551-1555. Accepted for publication 12 May 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1551.

Inoculation of greenhouse-grown apple (Malus) seedlings derived from controlled crosses with a mixture of five strains of Erwinia amylovora resulted in a larger percentage of the seedling population being evaluated as susceptible than when seedlings were inoculated with a single strain. Inoculation of field-grown progenies with five individual strains of E. amylovora and a mixture of strains indicated that the strain used for inoculation significantly affected the evaluation of resistance to E. amylovora. 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. Methods for selecting cultivars resistant to E. amylovora in an apple breeding program are discussed.