Stability of fire blight resistance in apple

The objective of the apple breeding programme in the Institute for Fruit Breeding Dresden-Pillnitz is the development of new varieties with high fruit quality combined with different resistances to economically important diseases. The fire blight, caused by Erwinia amylovora, is one of these. The resistance of the hybrids 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 had been grafted on the rootstock MM 106. Thirty actively growing shoots per accession were inoculated by a length between 15 and 25 cm with the mixture of three (1996 and 1997: five) highly virulent strains (1×10⁹ cfu/ml). The resistance of the test plants is evaluated by calculating length of necrosis to total shoot length (x 100%) 4 weeks p.i. Blossoms were inoculated in the field with a local strain by spraying. The spread of the infections into older parts of the plants is noted. The level of the evaluation test and the yearly differences caused by different weather conditions can be assessed by use of standard varieties. New fire blight resistant cultivars from the Pillnitz breeding programme are 'Remo', 'Rewena', 'Rene', 'Realka', 'Reanda', 'Rebella' and 'Regine'. They possess a high level of fire blight resistance in growing shoots and after flower infections. The resistance is stable after repeated inoculations with different Erwinia amylovora strains in several years.