Identification and Validation of QTLs Linked to Fire Blight Resistance in Malus and Their Applicability in Marker-Assisted Selection

Fire blight, a devastating disease of apple, pear, and many Rosaceae species worldwide is caused by enterobacterium Erwinia amylovora. At present, disease control mainly relies on antibiotics, prevention, and eradication-based strategies. Prevention and eradication is expensive, while resistance development for antibiotics can reduce their efficacy over time. Apple varieties do, however, exhibit a wide range of relative tolerance to fire blight. As part of our long-term strategy to design sustainable fire blight controls, we have initiated a breeding project to identify quantitative trait loci (QTLs) and markers linked to them. F-1 segregating progeny plants of 'Fiesta' x 'Discovery' were inoculated with a strain of Erwinia amylovora and studied under quarantine conditions to identify QTLs (Quantitative Trait Loci) for fire blight resistance. Progress of disease was measured at different time intervals as percent shoot lesion length (PLL) and used for QTL analysis. One significant QTL (P < 0.05) was identified on the linkage group 7 of 'Fiesta' (F7), which explained about 37.5 to 38.6% of the phenotypic variation. Two RAPD markers bracketing the QTL have been transformed into SCAR markers and a SSR marker specific for the region was developed. Stability of the effect of this QTL allele in different backgrounds and different cultivars was tested. Plants that amplified both flanking markers were significantly more resistant than those that did not amplify both markers, indicating high stability of the F7 QTL allele in different backgrounds, which makes it promising for use in marker-assisted selection (MAS). Work is currently ongoing to identify additional QTLs linked to fire blight in apple using a different cross 'Florina' x 'Novaeasygro'.