Seedling progenies from controlled crosses involving Malus sp. clones were tested for resistance to Erwinia amylovora (Burrill) Winslow et al. by shoot tip inoculation. In progenies from crosses between fire blight-susceptible M. pumila selections, over 90% of the seedlings were killed. In most progenies from crosses between resistant and susceptible selections, regardless of species source, few resistant seedlings occurred. Progenies from crosses between resistant parents generally had the most resistant seedlings, but segregated some highly susceptible offspring. Distribution patterns suggested quantitative control of resistance, with resistant parents heterozygous for resistance genes. In progenies from crosses of the highly resistant M. × robusta No. 5 (R5) and M. × sublobata PI 286613 (613) with susceptible parents, about 1/8 of the seedlings exhibited the fire blight reaction of the highly resistant parent. All progenies of 613 and R5, including those obtained by selfing and by crossing the two with each other, segregated some susceptible offspring. In a replicated nursery test involving seedlings of a 613 progeny, various fire blight resistance classes in the population were distinguished statistically. Resistance in 613 and R5 was interpreted as oligogenic (conditioned by few genes), with dominant, additive genes carried in the heterozygous condition.
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Gardner et al. (1980) studied this question.