Differential susceptibility to fire blight in commercial and experimental apple rootstock cultivars

The Geneva rootstock breeding program has developed several new rootstocks that exhibit disease resistance to Erwinia amylovora. Utilization of disease resistant apple rootstocks increases the survivability of young trees infected by fire blight. The goal of this experiment was to further investigate the possibility of differential susceptibility of numerous commercial and experimental apple rootstock varieties to four diversely virulent strains off. amylovora (E2002a, E4001a, E2017, and Ea273). Ungrafted potted rootstock liners were inoculated with the bacteria and the resultant necrotic lesions were used as an indication of susceptibility. Results showed that the E. amylovora strain E2002a was the most aggressive, followed by E4001a. E2017a and Ea273 were similar and less aggressive. The six rootstocks with greatest severity of infection in descending order were 'MM.106', 'Supporter 4', 'M.9', 'M.26', 'B.118% and 'B.9'. In this experiment B.9 had less severe infection than 'M.9' and 'M.26% in contrast with a previous experiment by Norelli et al. (2000). Most selections from the Geneva breeding program had little infection by strains E2017 and Ea273 and moderate infection by strains E2002a and E4001a. Most screenings for fire blight resistance in the seedling progenies of the Geneva® rootstocks employed only strain Ea273. 'Geneva® 3041', '7707', and '5179' and 'Geneva®16', had least severe infections. Significant rootstock by strain interactions were identified through mixed models analysis. We are utilizing this information to breed for durable rootstock resistance to fire blight. However, since it is known that a limited number of apple rootstock genotypes react significantly differently to E. amylovora infection as ungrafted potted liners and as rootstocks of flowering orchard trees, the results reported here must be extrapolated to performance in the field with caution.