Manipulation of crop load with bioregulators to mitigate biennial bearing in apple

Torrance Ray Schmidt

Biennial bearing habits in apple (Malus domestica Borkh.) produce high yields of small, poor quality fruit in the “on” year and low yields of large fruit prone to physiological disorders in the “off” year. Flowering promoters such as ethephon may help bolster return bloom after an “on” year, and floral inhibitors such as gibberellic acids (GAs) may reduce bloom in the season following an “off” year, improving cropping consistency and orchard profitability. These studies conducted in commercially important cultivars prone to biennial bearing sought to: 1. Evaluate the influences of crop load, application timing, and spray concentration on the efficacy of ethephon and GA in apple. 2. Examine effective concentrations, application timings, and spray concentrations of several isomers of GA. 3. Investigate collateral effects of GA programs, including maturity of fruit present during spray application. Crop load was manually adjusted on individual whole trees to three levels (100%, 50%, 0%) in six trials. In 2004 and 2005, 400 mg/L GA4+7 were applied to trees of each crop level at one of three timings to ‘Cameo’ 45 days after full bloom. In separate 2004 and 2005 ‘Cameo’ trials, four concentrations of ethephon were applied to trees of each crop level. In the two remaining crop-adjusted trials, ‘Honeycrisp’ (2004) and ‘Fuji’ (2005) trees were also sprayed with 300 mg/L GA4+7 at one timing. Return bloom was generally inhibited by GA and promoted by ethephon in all trials, but their effects were overwhelmed by the influence of initial crop load. A 2004 ‘Fuji’ trial found a dose response relative to GA4 concentration and greatest floral inhibition (51-75%) at 10 mm timing. GA3, GA4, GA4+7, and GA7 reduced 2006 flowering by 53-90% in 2005 ‘Honeycrisp’ and ‘Fuji’ trials. Several concentrations of GA4+7 applied to ‘Cameo’ in 2004 and 2005 failed to clearly affect fruit maturity, but similar treatments accelerated ripening of ‘Honeycrisp’ in a 2004 trial by 2-5 days; fruit maturity effects from several GA isomers in a 2005 ‘Honeycrisp’ trial were not as clear. Our results indicate that bioregulators offer promise as tools for crop load management of apple.

Manipulation of crop load with bioregulators to mitigate biennial bearing in apple

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