The fire blight resistance QTL on LG7 of Malus ×robusta 5 is not dependent on the avrRpt2_EA 156 S/C amino acid switch

Malus ×robusta 5, which has been the subject of extensive fire blight resistance research over time, is highly resistant or susceptible to the fire blight-causative bacterial pathogen, Erwinia amylovora – depending on the strain. M. ×robusta 5 has been crossed with susceptible apple cultivars and rootstocks, and inoculated with several E. amylovora strains in order to study the genetics and mechanism of its fire blight resistance and susceptibility. A strong resistance QTL was first mapped on linkage group 3 (LG3) of M. ×robusta 5 using an F1 progeny derived from a cross with the apple cultivar ‘Idared’ in Germany. This QTL was confirmed in two other F1 populations derived from crossing M. ×robusta 5 with the rootstock ‘Malling 9’ in New Zealand, and with ‘Ottawa 3’ in the USA. A second QTL on LG7 was detected in the ‘Idared’ × M. ×robusta 5 population but only with strains that break the LG3 QTL. However, in the US population of ‘Ottawa 3’ × M. ×robusta 5, the LG7 QTL was detected regardless of strain-specificity, unlike in the New Zealand population of ‘Malling 9’ × M. ×robusta 5 where the LG7 QTL was not also detected. Here, we report the detection of the LG7 QTL in a different ‘Malling 9’ × M. ×robusta 5 population in Germany, and confirm the independence of the LG7 locus to E. amylovora strains.