Improving Flower Production and Reducing Pre-Harvest Fruit Drop of Gold Kiwifruit (Actinidia chinensis)

Abstract

Abstract The Actinidia chinensis cultivars, ‘AU Golden Sunshine’ and ‘AU Gulf Coast Gold’ produce heavy crop loads of large, commercially acceptable kiwifruit in central Alabama. However, due to fluctuations in annual winter temperatures, these vines may not always receive sufficient amounts of chilling hours or units or portions they require to break bud and flower effectively. When these vines do not receive 700–900 chilling hours, they can exhibit nominal and intermittent bud break and flowering. This leads to insufficient cropping and negative economic impacts for growers. Experiments were conducted over 2 years on ‘AU Golden Sunshine’ and ‘AU Gulf Coast Gold’ to determine the effectiveness of the dormancy breaking chemical, hydrogen cyanamide (HC). Hydrogen cyanamide is known to help overcome lack of chilling accumulation, increase bud break and bloom intensity, and to reduce unwanted lateral flower buds when applied to kiwifruit. Applications of HC were made ~28 (1 Feb.) and ~14 (15 Feb.) days before natural bud break to assess the effect on timing of application. When HC was used, an advancement in bud break and flowering was observed compared to unsprayed vines. Applications made ~28 days before natural bud break were most advanced and broke bud before the other treatment and control vines. In year one, vines receiving applications ~14 days before natural bud break showed increased bud break and flowering intensity as well as a reduction of lateral flower buds over the vines treated at ~28-day before natural bud break. The unsprayed controls had the fewest lateral buds throughout the study. During year two, all vines were injured by a late frost event that occurred 7 Mar. 2019. Vines receiving the earliest HC applications were damaged the most, followed by the vines receiving the second HC application, which led to reduced flower production compared to unsprayed vines. Control vines had the greatest number of floral shoots after the frost event because they were the last to experience bud break. iii Hydrogen cyanamide may be a beneficial tool to growers experiencing lower than normal chilling, however caution is advised due to the possibility of late spring frost. ‘AU Golden Sunshine’ exhibits a pre-harvest fruit drop that has never been reported in kiwifruit literature before. This drop occurs just before harvest and could cause economic hardship to growers if it is not curtailed. A two-year study was conducted to determine the effectiveness of 88mg/L-1 NAA (Fruitone®), 264mg/L-1 AVG (ReTain®), and 158mg/L-1 1-MCP (Harvista®) at preventing pre-harvest fruit drop of this kiwifruit cultivar. Treatments consisted of each chemical alone, AVG + NAA, and 1-MCP + NAA applied at approximately 7, 14, and 21 days before anticipated harvest (though this timing varied between the 2 years of this study). During year one of the study, pre-harvest fruit drop ranged from 5.6–13.6% on average with no conclusive efficacy of the treatments to deter fruit drop. During year two, the incidence of pre-harvest fruit drop was substantially higher than the previous year with averages ranging from 32–66%. No differences were observed between treatments. Fruit maturity and quality were largely unaffected by treatment applications throughout the entire study. Further research is required to better understand the causes of the pre-harvest fruit drop behavior that ‘AU Golden Sunshine’ displays as recommendations cannot be made at this time.