Low input fast-track (LIFT): an approach for fast introgression and stacking of (R-)genes into advanced apple selections.

Main conclusion: Using the "LIFT" method can halve generation time to two years, thereby reducing the breeding cycle by 50%, and accelerate the development of disease-resistant apple cultivars for sustainable production. Good sources of resistance to pests and pathogens are often found in wild relatives or ornamental apples, which are mainly small-fruited and poor-tasting. Introgressing these resistance genes via classical breeding into new apple cultivars with good tree and fruit qualities requires a series of four to five pseudo-backcrosses, which takes at least 25 years. This study aimed to develop a low-input protocol to shorten the time between generations and thereby substantially reduce the timeline for the development of cultivars with introgressed resistance. The "LIFT" method combined forced growth of seedlings carrying the fire blight resistance genes from 'Evereste' or from Malus × robusta 5 in a standard greenhouse with artificial vernalization periods in a cold room. This allowed flower induction in about 20% of the seedlings as early as after the second vernalization period, i.e., about two years after sowing. The fire blight resistance, the increase in single fruit weight, and the reduction of the unadapted part of the genome derived from 'Evereste' or Malus × robusta 5 were monitored across generations. No erosion of the resistance was observed, but there was an increase in single fruit weight and a progressive reduction of the unadapted genome. Therefore, the developed protocol proved to be efficient and reliable and can be applied with or without molecular markers to introduce or combine traits from wild apple relative into advanced pre-breeding selections. These pre-breeding selections are urgently needed to develop new apple cultivars with broad disease resistance, ensuring long-term sustainable apple fruit production.