AbstractResistance in apple is evaluated by needle–inoculation of succulent shoot tips with 106–107cells of a virulent isolate ofErwinia amylovora(Burr.) Winslow et al. (the incitant of fireblight) and measurement of the resulting cortical lesion when extension is complete. Data are now available on practically all commercial cultivars, some of which have a useful level of resistance. Some newer cultivars, particularly those with resistance to scab (Venluria inaequalis[Cooke] Wint.) derived fromMalus floribunda, have good resistance toE. amylovora. Avery high level of resistance is present in AsiaticMalusspecies, includingM.xrobusta, M.xsublo–bata, M.xatrosanguinea, andM. prunifolia, and in the North American speciesM. fusca.This type of resistance appears to be inherited polygenically, and because of its detectability in young seedlings can be used conveniently in breeding.Objectives of pear breeding programs are aimed at developing superior fruit quality combined with resistance to fireblight, psylla, andFabraealeaf spot. Many high quality cultivars ofPyrus communisare extremely susceptible toE. amylovoraand sensitivity appears to be controlled by a dominant gene Se. A high level of resistance is present inP. ussuriensisbut varies considerably between clonal selections of otherPyrusspecies. Pear seedlings from controlled pollinations are artificially inoculated in the glasshouse with a similar bacterial suspension as used for apples, and only the most resistant ones are selected and planted in the field for future evaluation. In Beltsville, heritability studies of crosses between non–sensitive parents have indicated that selection for resistance within progenies results in a high degree of genetic gain. Interspecific hybridization has an advantage overP. communiscrosses mainly when insect or fungus resistance is r