Susceptibility of apple genotypes from European genetic resources to fire blight (Erwinia amylovora)

The study was conducted in greenhouses at two locations on one-year old trees of apple cultivars and clones originating from genetic resources and breeding programs conducted in Belgium, Hungary, Germany, Poland, Sweden and Switzerland. At each location different inoculation method of trees with Erwinia amylovora was used. In Poland, at the Research Institute of Horticulture in Skierniewice (IO), the method of cutting-off shoot tips with scissors immersed in a single highly virulent strain of the inoculum (10⁷ cfu ml⁻¹) was applied for testing 38 genotypes. In Germany, at the Julius Kühn-Institut in Dresden and Quedlinburg (JKI), the method of cutting-off the tips of the two youngest leaves with scissors dipped in a suspension of three highly virulent strains at concentration of 10⁹ cfu ml⁻¹ was used for testing 27 genotypes. Observations and measurements of fire blight development on inoculated shoots were performed at 2, 4 and 6 weeks after inoculation at IO and after 4 weeks at JKI. Each genotype was evaluated over the course of two or three seasons. Of the total 40 genotypes, 25 were tested at both locations. The final assessment based on results of all tests was either similar or the same for 17 genotypes. MR 10, Pi-As 12,53 and Pi-As 50,74 were the most resistant. ‘Free Redstar’ and ‘Reanda’, tested only at IO, as well as Pi-As 36,164, tested only at JKI, were also classified as highly resistant. The following cultivars were found to be very susceptible at both locations: ‘Blauacher Wädenswil’, ‘Bohnapfel’ and ‘Idared’. Among those tested only at IO, CRA Ma A40, CRA Ma A68, CRA Ma A70, CRA Ma B28, CRA Ma D30 and ‘Jacob Level’ appeared to be also very susceptible. No significant differences in shoot necrosis length appeared when comparing the two inoculation methods used at IO and JKI (i.e. application of single strain vs. mixture of three highly virulent E. amylovora strains, at two concentrations) on four genotypes.