Envinia amylovora is the causative agent of apple rootstock blight. This disease is becoming more important as more susceptible and yet desirable scion cultivars are utilized in production using susceptible rootstocks. Utilization of disease resistant apple rootstocks increases the survivability of young trees infected by fire blight. Previous experiments in our breeding program showed that some apple rootstocks that derive their resistance from Mattis x rob:,sta cv. Robusta 5 show differential

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... susceptibility to differentially virulent strains of E. anyIovora. The goal of this experiment was to expand knowledge about the inheritance of the differential susceptibility in a mapping progeny of 170 individuals of the cross between apple rootstocks Ottawa 3' and Robusta 5'. Buds from each individual in the segregating population were grafted onto seedling rootstocks and trained to a single actively growing shoot. The shoots were inoculated at the same time with a differentially virulent strain of E. wnvlovora, E2002a and a virulent strain, Fa273. The same segregating population is being used to construct a genetic map with the intention of mapping any quantitative trait loci involved in the resistance. The Geneva rootstock breeding program has developed several new rootstocks that exhibit disease resistance to E. arnylovora but a few of them have shown some susceptibility to differentially virulent strains. The results from this experiment will help us develop more durable resistance to rootstock fire blight.