Inoculation of Malus × robusta 5 progeny with a strain breaking resistance to fire blight reveals a minor QTL on LG5

Fire blight, caused by the bacterium Erwinia amylovora, is one of the most dangerous diseases in pome fruit growing. Two major aims in apple breeding are the selection of apple cultivars resistant to fire blight and understanding the genetics of fire blight resistance. Although no gene-to-gene relationship has been reported for the interaction between Malus and E. amylovora, a specific strain-cultivar interaction has been demonstrated in several studies. The fire blight resistant wild species accession Malus × robusta 5 (Mr5) carries a major QTL for resistance to fire blight on LG3, explaining up to 80% of the phenotypic variation. This QTL was confirmed using two different strains that did not overcome the resistance of Mr5, and no minor QTL could be detected. In this study a strain overcoming the resistance of Mr5, i.e., E. amylovora Ea3049, was used to phenotype grafted scions of the progeny 'Idared' × Mr5. From one to eight replicates/seedling were inoculated with Ea3049, by cutting the tips of the two youngest leaves with scissors dipped in inoculums containing 109 cfu/ml. Four weeks after inoculation the necrosis length was determined and susceptibility assessed as percentage necrosis length. Grafted scions of the two parents Mr5 and 'Idared' showed 97 and 95% necrosis length, respectively, and the necrosis length of the progenies varied from 7 to 100% with an average of 80%. Interval mapping of susceptibility resulted in the total breakdown of the QTL on LG3 but generated a new QTL on LG5. This minor QTL could explain up to 10% of the phenotypic variation.