Temperatures near or below freezing during bloom in apple orchards occasionally results in significant reductions in fruit set and cropping due to death of the ovule/embryo before fertilization. Apple flowers are most sensitive to freezing temperatures at the full bloom stage, when air temperatures of 28 and 25 °F are predicted to kill 10% and 90% of the blooms, respectively (Ballard et al., 1998).
Foliar gibberellin (GA) sprays during bloom increase fruit set in pear (Pyrus communis) by stimulating parthenocarpic fruit development (Deckers and Schoofs, 2002; Dreyer, 2013; Luckwill, 1960; Zhang et al., 2008). This strategy is used in commercial practice to increase fruit set and cropping in pear cultivars that naturally have poor fruit set (Lafer, 2008; Vilardell et al., 2008), and to increase fruit set of pear after frost damage during bloom (Ouma, 2008, Yarushnykov and Blanke, 2005). Deckers and Schoofs (2002) suggested that GAs need to be applied to pear within 4 d after a frost event to alleviate damage, and that gibberellic acid (GA3) increased fruit set more effectively than GA4+7.
Gibberellins, and in particular GA4, induce and sustain parthenocarpic growth of apple fruit (Bukovac, 1963; Davison, 1960; Luckwill, 1960). Bukovac (1963) reported that gibberellins “induced parthenocarpic fruit growth and promoted fruit development to maturity at growth rates and with final size, color, and general quality comparable to seeded fruits” in the cultivars Sops-of-Wine, Wealthy, and Delicious. However, Wertheim (1973) showed that GA4+7 only temporarily increased fruit set of ‘Cox’s Orange Pippin’ apple flowers after effemination. Thus, the efficacy of GA4+7 to promote parthenocarpy may vary between apple cultivars. Application of gibberellins together with the synthetic cytokinin N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU) had a positive synergistic effect on set of parthenocarpic fruit and fruit size in apple (Bangerth and Schröeder, 1994; Watanabe et al., 2008).
While GA sprays are commonly used to increase fruit set and cropping of pear following a freeze, there is no information describing the efficacy of this treatment in apple. The objectives of the current studies were to investigate the potential for a proprietary formulation of GA4+7 plus 6-BA (Promalin®; Valent BioSciences Corp., Libertyville, IL) to increase fruit set and cropping of apple following freeze events during bloom. A series of spring freezes throughout major apple production regions in the eastern United States in 2012 provided the opportunity to evaluation GA4+7 plus 6-BA applications as a frost-rescue treatment.
BallardJ.K.ProebstingE.L.TukeyR.B.1998Critical temperatures for blossom buds: Apples. Washington State Univ. Bul. 0913. 5 Apr. 2013. <treefruit.yakima.wsu.edu/weatherbuds/budhardiness/BudCharts/Apple%20Bud%20Chart%20EB0913.pdf>
BangerthF.SchröderM.1994Strong synergistic effects of gibberellins with the synthetic cytokinin N-(2-chloro-4-pyridal0-N-phenylurea on parthenocarpic fruit set and some other fruit characteristics of applePlant Growth Regulat.15293302
BlanpiedG.D.SilsbyK.J.1992Predicting harvest date windows for apples. Cornell Coop. Ext. Bul. 221
BramlageW.J.WeisS.A.GreeneD.W.1990Observations on the relationships among seed number, fruit calcium, and senescent breakdown in applesHortScience25351353
DreyerC.2013Fruit set and fruit size studies on ‘Forelle’ and ‘Abate Fetel’ pear (Pyrus communis L.). Stellenbosch Univ. South Africa M.Sc. Thesis. 13 Dec. 2013. <http://scholar.sun.ac.za/handle/10019.1/79870>
GreeneD.W.1984Microdroplet application of GA4+7 + BA: Sites of absorption and effect on fruit set, size, and shape of ‘Delicious’ applesJ. Amer. Soc. Hort. Sci.1092830
LaferG.2008Effects of different bioregulator applications on fruit set, yield and fruit quality of ‘Williams’ pearsActa Hort.800183188
LuckwillL.C.1960The effect of gibberellic acid on fruit set in apples and pears. Long Ashton Res. Stn. Annu. Rpt. 1959:59–64
RufatoL.KretzschmarA.A.BrighentiA.F.MachadoB.D.LuzA.R.Marcon FilhoJ.L.2011Plant growth regulators increase productivity of ‘Packham’s Triumph’ pear in southern BrazilActa Hort.909429434
WatanabeM.SegawaH.MurakamiM.SagawaS.KomoriS.2008Effects of plant growth regulators on fruit set and fruit shape of parthenocarpic apple fruitsJ. Jpn. Soc. Hort. Sci.77350357
WertheimS.J.1973Fruit set and June drop in Cox’s Orange Pippin apples as affected by pollination and treatment with a mixture of gibberellins A4 and A7Sci. Hort.185105
ZhangC.LeeU.TanabeK.2008Hormonal regulation of fruit set, parthenogenesis induction, and fruit expansion in japanese pearPlant Growth Regulat.55231240