Commercial apple cultivars require a pollinizer for cross-pollination to ensure fruit-set yields when planted in contiguous blocks found in modern apple orchards. Pollinizers must have compatible viable diploid pollen and have bloom overlap with the variety grown in the orchard block. Climate change can influence the viability of a pollenizer cultivar by shifting the bloom times of the pollinizers and the commercial apple cultivar, depleting the source of pollen for that block, and decreasing yield potential. 4 commercially available pollenizing crabapple cultivars (Indian Summer, Mt. Blanc™, Mt. Evereste™, Snow Drift) were replicated 5 times in completely randomized design. Pollinizers were analyzed weekly for developmental stages from silver tip to petal fall. 7 apple cultivars were also analyzed for comparison. The stage progression data for each pollenizer cultivar was modeled using growing degrees days base 6.6°C. These models were used to compare the stage progression and bloom dates of the pollenizer cultivars, and compared to the production apple cultivars to assess bloom overlap and determine pollinizer feasibility. All 4 crabapples began to bloom earlier than the scion cultivars leading to little bloom overlap. Indian Summer was the first crabapple to bloom on 3/28 with cumulative growing degree days (GDD) 166.4. The latest to bloom was Mt. Blanc on 4/13 with cumulative GDD 250.1. The commercial cultivars did not begin to bloom until 4/6 with cumulative GDD 199.6 through 4/13 with cumulative GDD 250.1. While the first commercial cultivar to bloom was Gold Rush and most at full bloom around 4/13, at which time all but Mt. Blanc crabapples were past bloom. Results suggest most pollinizer cultivars have poor bloom overlap with commercial cultivars in Kentucky, with Mt. Blanc offering the best, though limited overlap. Furthermore, the crabapple cultivars are seemingly responding to environmental cues such as chilling and GDD accumulation differently to leave dormancy and progress through bloom at a more rapid rate. These early results suggest that the use of crabapple cultivars as pollinizers and their overlap with commercial apple cultivars could be an area of concern as climate change brings earlier springs in major apple producing regions.
