Adaptation to spring frost: Evaluating late-flowering apple cultivars in southwest Germany

Spring frosts remain a major threat to apple ( Malus domestica ) production in temperate regions. We assessed historical cultivar-specific frost exposure timing in the Lake Constance fruit-growing region in southwestern Germany. Using continuous phenology records for 26 cultivars with ≥25 years of observations collected at the ‘Kompetenzzentrum Obstbau Bodensee’ (KOB) experimental station between 1963 and 2024, we linked phenology data to local temperature data and quantified frost exposure during and after flowering (accumulated frost hours below 0 °C and below −2.2 °C). In addition, we screened ∼800 traditional cultivars (2021–2025) to identify late-flowering genotypes preserved in a regional genetic resource collection. Across cultivars, frost events during or after bloom occurred frequently, with high variability in intensity and timing. The historical record revealed three major prolonged frost events during bloom (1974, 1981, 2017), which coincided with pronounced regional yield losses, with yields dropping to 47%, 38%, and 45% of the long-term mean. Full bloom advanced by ∼4 days per decade; in 2015–2024, bloom occurred 19.2 days earlier than in 1963–1972. Although cultivar-specific differences in full bloom within the modern cultivar set were comparatively small (6.6 days between the earliest and latest cultivars), bloom timing strongly explained variation in frost exposure frequency (R² = 0.64 at 0 °C; R² = 0.42 at −2.2 °C). A 5.5-day difference in bloom timing (Gravensteiner vs. Golden Delicious) translated into a threefold difference in frost-hour exposure. Screening of traditional germplasm identified 22 consistently late-flowering cultivars that remained several BBCH stages behind Golden Delicious, implying substantially improved stage-dependent frost tolerance. Overall, modern commercial cultivars show limited phenological diversity, whereas traditional germplasm offers a much wider range for breeding late-flowering genotypes to reduce spring frost exposure and thereby enhance yield stability.