Factors affecting pollen dispersal in high-density apple orchards

Knowledge of pollen dispersal is essential for maximizing cross-fertilization in apples (Malus xdomestica Borkh.) and achieving optimal orchard design. Using allozyme markers, we examined dispersal of pollen from trees of a single cultivar ('Idared') throughout two apple orchards. In each orchard, the percentage of seeds sired by 'Idared' was estimated for trees sampled at regular intervals along three transects, extending up to 18 rows (86 m) from the closest donor trees. The percentage of seed sired by 'Idared' pollen ranged from 76% to 1% of seed sampled for a row. No differences in pollen dispersal were found among transects, despite differences in proximity to the bee colonies. Variation in 'Idared' siring success was attributable to the cultivar of the fruit-bearing trees as well as their distance to the nearest 'Idared' tree. Cultivar effects were associated with differences in flowering overlap, but not cross-compatibility with the pollenizer. Furthermore, flowering overlap was a good predictor of siring success only when the flowering times of competing pollenizer cultivars were also considered. The implications for orchard design are discussed.