Fire Blight resistance in apple/pear hybrids may be related to their genomic structure

Although Malus and Pyrus are closely related, with highly co-linear genomes, the two genera are characterised by many specific differences, including disease resistances, secondary metabolites, fruit texture, flavour and shape. Hence, intergeneric hybrids between apple and pear provide a unique germplasm resource for genetic analysis, as well as new cultivar development, using advanced genomic breeding strategies. Fire blight, caused by the Gram-negative bacterium Erwinia amylovora (Enterobacterales; Erwiniaceae), affects apple and pear production worldwide. A number of resistance loci against this disease have been located on genetic maps of both apple and pear.We investigated fire blight resistance in apple-pear hybrids, by studying 31 putative hybrids raised from a ‘Cox’s Orange Pippin’ x ‘Old Home’ cross at The New Zealand Institute for Plant and Food Research Limited. We inoculated up to eight replicates of each hybrid grafted on ‘M9’ rootstock and compared these with ‘Cox’s Orange Pippin’ and ‘Imperial Gala’ grafted on ‘M9’ rootstock and ‘Old Home’ and ‘Williams’ grafted with Pyrus calleryana, using the cut-leaf method (Maas Geesteranus and Heyting, 1981) for inoculation with E. amylovora (Ea 236 at 1 x 106 cfu/ml), as it is widely applied in both apple and pear. Disease progress was observed in the period from 2 to 6 weeks after inoculation. Level of disease was quantified by expressed necrosis length as a percentage of the total shoot length, both measured downwards from the point of inoculation. The result clearly showed that all of the 31 putative hybrids were resistant to fire blight, while the parents and controls exhibited the expected range of resistance and susceptibility according to previous work. Preliminary results using high-resolution melting marker analysis of the seedling genomes indicate there is a hybrid apple/pear genomic region on LG2, while LG7 is represented by apple DNA. Interestingly, fire blight resistance has been reported on LG2 of pear ‘Old Home’ (Montanari et al., 2016), while Khan et al. (2007) have located fire blight resistance on LG7 of the apple ‘Fiesta’, which is related by descent from ‘Cox’s Orange Pippin’.Our next step is to analyse recombination events during the crossing of apple and pear, using the IRSC 9K apple/pear SNP array: this will enable us to further investigate the relationship of these reported QTL resistances to fire blight infection within the genomic structure of our 31 apple/pear hybrids