Author(s)

Todd Einhorn^1*, Denise Ruwersma¹, Mokhles Elsysy^1,2, Drew Hubbard³

¹Department of Horticulture, Michigan State University, East Lansing, MI, USA.
²Department of Pomology, Assiut University, Assiut, Egypt.
³Fine Americas, Inc., Walnut Creek, CA, USA.

Newly planted apple orchards in the USA comprising the highly biennial cultivar, “Honeycrisp”, are prone to flower within the first two years from planting and set fruit. These processes limit canopy development and subsequent yield potential. GA_{4 + 7} can inhibit floral formation processes of apples. The timing and dose for eliminating return bloom of young “Honeycrisp” trees, however, is unclear. A factorial experimental design to test GA_{4 + 7} application timing and rate produced significant reductions in return bloom for both factors and their interaction. Treatment responses demonstrated that florigenic processes in “Honeycrisp” occurred early. The most pronounced reduction in return bloom followed the 2-week after full bloom (WAFB) application timing, increasing with increasing rate. The effect on return bloom was progressively diminished over the next two weeks of applications but remained significantly lower than the control. Three successive applications timed one week apart eliminated return bloom of spurs at the highest rate. Both factors and their interaction also significantly reduced return bloom of terminal buds of leaders; in contrast to floral buds on spurs, the response of terminal buds on leaders improved with delayed application timings. Timing exerted a stronger effect than rate on return flowering of the terminal buds of leaders. Leader growth was positively affected by GA_{4 + 7}, the year of application, when three successive applications were made. Tree height, overall growth, expressed as the increase in trunk cross-sectional area, and limb number were also significantly improved by GA_{4 + 7} but varied in their responses to application timing and rate, and were inconsistent. Overall, successive applications of GA_{4 + 7} had good efficacy for inhibiting floral initiation of meristems on spurs and terminals while improving the vegetative growth of young “Honeycrisp” trees.

Floral Inhibition, Floral Initiation, Floral Induction Gibberellins, Flowering, Fruit Set, Crop Load Management

Einhorn, T. , Ruwersma, D. , Elsysy, M. and Hubbard, D. (2025) GA_{4 + 7} Rate and Timing Interact to Suppress Return Bloom of Young “Honeycrisp” Apple Trees and Improve Tree Growth. American Journal of Plant Sciences, 16, 275-286. doi: 10.4236/ajps.2025.162022.