Stability of fire blight resistance in apple

The objective of the apple breeding program in the Institute of Fruit Breeding Dresden-Pillnitz is the development of new varieties with high fruit quality combined with resistance to economically important diseases. One of these is fire blight, caused by Erwinia amylovora. The resistance of the hybrids against fire blight has been evaluated at Aschersleben since 1974. New strains of the fire blight pathogen are collected from infected areas every year. Before resistance evaluation in the greenhouse, three strains with the highest virulence on three different apple varieties are selected. The hybrids are tested as graftings on the rootstock MM 106. Thirty actively growing shoots per accession are inoculated at a length between 15 and 25 cm with the mixture of three high virulent strains (1x10⁹ cfu/ml). The resistance of the test plants is evaluated by calculating length of necrosis to total shoot length (x 100 %) 4-6 weeks post inoculation. The level of the evaluation test and the yearly differences caused by different weather conditions and different strains can be assessed by the use of standard varieties. New fire blight resistant cultivars from the Pillnitz breeding program are 'Realka', 'Reanda', 'Rebella', 'Regia', 'Regine', 'Remo', 'Rene', 'Resi' and 'Rewena'. They possess a high level of fire blight resistance in growing shoots and after flower infection. The resistance is stable after repeated inoculations with different E. amylovora strains in several years.