Abstract
In the eastern USA and several other apple-growing regions, apple blossom thinning using lime sulfur is a relatively new crop load management strategy. This study sought to evaluate how application timing of lime sulfur + stylet oil blossom thinning sprays would influence thinning efficacy and crop safety of ‘Gala’ apples. This project occurred at two locations in the USA, Winchester, Virginia, and Mills River, North Carolina, during the 2019 growing season. Two main timing strategies were assessed: (1) model-guided sprays with the pollen tube growth model (PTGM), (2) fixed spray intervals with the first spray applied at a specified percentage of open bloom (20% vs. 80%), and the second spray applied at a reapplication interval (48 h vs. 72 h). Model-guided and 20% open bloom + 48-h treatments reduced fruit set and increased fruit weight, diameter, and length at both sites. Treatments with a delayed first spray at 80% open bloom or a more extended second reapplication of 72 h were generally ineffective. There was no conclusive evidence that lime sulfur + stylet oil blossom thinning spray timing influenced russet incidence/severity or leaf phytotoxicity. This study demonstrated that effective and safe blossom thinning can be obtained from applying two sprays at 20% open bloom and 48 h thereafter or using the PTGM.
Highlights
Fruit set data collected at early time points after petal fall (e.g., 2–8 Weeks after petal fall (WAPF)) followed almost the same trend observed at harvest, indicating that blossom thinning treatment was the major source of variation observed among treatments
Lime sulfur sprays applied at 20% open bloom and 48 h later performed near well as the pollen tube growth model (PTGM)
This study found that the ‘Gala’ PTGM was effective in timing blossom thinning application, which resulted in reduced fruit set and increased fruit weight and size across two locations
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 8–10% of apple blossoms develop into fruit; an ideal crop is obtained when approximately 5% of the blossoms set fruit [1]. When proper thinning is implemented, the crop load is reduced to a level which balances tree carbohydrate usage between developing fruit in the current year and flower bud development for the following crop. Thinning improves return bloom for the following year which promotes annual bearing [2,3,4]. Thinning improves fruit quality traits such as size and coloration by allowing carbohydrates to be channeled into fewer fruits and increasing fruit spacing to improve color development [5]
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