Mapping and genetic dissection of QTL influencing bitter pit symptoms in apple (Malus ×domestica)

Bitter pit is the most economically important physiological disorder affecting apple fruit production. Brown pits develop in the cortical apple flesh – due to the breakdown of the cell plasma membranes – especially during storage, rendering the fruit unmarketable. Environmental conditions and cultural practices play a role in symptom expression, and there is a link between the severity of symptoms and relative concentrations of calcium in the fruit, leading to the application of calcium sprays and dips; however, these are not completely effective. Cultivars vary in susceptibility and thus there is scope for breeding for resistance. In this investigation, we have identified two major QTL controlling bitter pit symptom expression in four large apple mapping populations. The QTL intervals were defined using SSR and SNP markers, and candidate genes from both intervals were characterised. The markers identified will be useful for marker assisted selection in programmes for the genetic improvement of cultivated apple.