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A primary focus of the apple rootstock breeding and evaluation program at USDA-ARS/Cornell Univ. has been to develop screening protocols to identify genotypes resistant to the fire blight bacterium ( Erwinia amylovora ). Direct inoculation is a simple technique, but does not represent the only mode by which rootstocks become infected in the orchard. Selection based on direct inoculation screens may, however, enrich the population for resistant genotypes. Large breeding populations from controlled crosses are shoot-tip inoculated with  E. amylovora , and the fraction showing the highest levels of resistance are retained for further evaluations. These survivors are again screened through direct inoculation in the field, and the less-resistant genotypes are discarded. Following selection for other pathogen tolerance and horticultural characters, elite genotypes are multiplied through asexual propagation. Replicated tests using direct inoculation with multiple strains of  E. amylovora  are then used to estimate the level of fire blight resistance of elite genotypes. A final screen utilizes mature, grafted orchard trees to verify that the resistance of rootstock genotypes to fire blight is maintained under conditions simulating natural infection. Direct inoculation screening and selection have resulted in a high frequency of strong resistance to severe fire blight epidemics in recent orchard inoculation trials.

480 Screening for Fire Blight Resistance in Apple Rootstock Breeding