Agronomic management strategies and physiological factors affecting biennial bearing in apple

Thiago de Botton Campbell

doi:10.7273/000007852

Biennial bearing in tree fruit can negatively affect yield and fruit quality. It occurs when there is excessive flowering and fruit set in one year (‘on’ year) and is followed by little to no flowering and fruit set (‘off’ year). Since the fundamental physiological mechanisms underlying these processes are poorly understood, few strategies to address this challenge other than crop load management are available. In the first experiment, the goal was to understand the contributions of stored carbohydrate and nitrogen towards biennial bearing of ‘Honeycrisp’ and ‘Gala’ apple (Malus domestica) with either high or low biennial bearing, respectively. Samples were taken from both ‘on’ and ‘off’ trees in December 2022, 2023, and 2024 to assess non-structural carbohydrate (NSC) and nitrogen concentration when reserves are the highest. In a second experiment, six different agronomic strategies were studied to minimize biennial bearing risk in apple trees in 2023 and 2024: two for years when the crop load was low (less than 4 fruit per cm2 trunk cross-sectional area (TCSA)) and four for years when the crop load was excessively high (greater than 7 fruit per cm2 TCSA). ‘Honeycrisp’ trees in their high-cropping year were root pruned, girdled, sprayed with ethephon, or fertilized with ammonium nitrate in the fall and compared to an untreated control. Low-cropping trees were defoliated or sprayed with ProGibb® (GA3, Valent BioSciences, Libertyville, IL) and compared to an untreated control. All treatments were applied two to three weeks after full bloom, except for nitrogen fertilization, which was applied in late August. NSC concentration was higher following the ‘on’ year than the ‘off’ year in ‘Honeycrisp’, while ‘Gala’ showed a greater degree of variability. Higher NSC concentration may be associated with elevated leaf functionality and carbon assimilation later in the season. Although stored nitrogen and NSC contribute to vegetative vigor and fruit set the following year, there is no clear connection between biennial bearing and stored reserves following either ‘on’ or ‘off’ years in ‘Gala’ or ‘Honeycrisp’ apple. In high-cropping trees, ethephon significantly increased return bloom in both years. In low-cropping trees, defoliation successfully decreased return bloom in both years. Ethephon and defoliation are potentially effective agronomic strategies to use in the “on” and “off” years, respectively, to combat biennial bearing.

Agronomic management strategies and physiological factors affecting biennial bearing in apple

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