‘WA 38’ abscission and fruit development in an open pollination scenario

The ‘WA 38’ apple tree (Malus × domestica Borkh.) is characterized by a self-thinning trait whereby most fruitlets abscise following a profuse bloom, and usually only one or two fruitlets set in an inflorescence. The main objectives of this study were to investigate natural fruitlet shedding in open-pollinated ‘WA 38’ clusters, growth of the retained fruit throughout the growing season, and their seed set after harvest. This preliminary study focused on a) understanding the timing and intensity of ‘WA 38’ natural flower/fruitlet shedding process, b) surveying the proportion of single and double fruit/cluster at harvest, and c) assessing apple size, shape, and seed set in relation to the flower position in the cluster (king vs. lateral apples) and cluster occupancy pattern (single apple vs. double). Tracking clusters from pre-bloom to harvest provided insight on the natural abscission of ‘WA 38’: after 7 weeks from full bloom, the fruit drop ceased with 17% of total fruitlets retained on trees in 2020. Moreover, 51% of the total surveyed clusters resulted in single apples (which originated from king flowers in 78% of the cases), while only 12% bore two apples, 35% were unfruitful, and 2% broken/out of trial. Seed set was significantly higher, and the number of underdeveloped seeds was significantly lower in single apples than double apples, but no other significant comparisons in seed set or distribution were found.