Biennial bearing is a major problem in many apple-producing areas of the world (Schmidt et al., 2009) and Norway is no exception. As a result of overcropping in the “on-year,” fruit size and quality are reduced. The next year, yields are markedly reduced and oversized fruit may result. This inconsistent yield pattern results in volume and quality problems both for the growers and the market.
The main horticultural practices for controlling biennial bearing are renewal pruning, growth control, and blossom or fruit thinning chemically, by hand, or a combination of the two. Blossom thinning is a standard procedure for improving fruit size, increasing return bloom, and promoting regular bearing (Byers, 2003; Jonkers, 1979). Reducing the number of fruits per tree increases the amount of leaf area per fruit and hence more photoassimilates are available to the remaining fruit. Thinning can, however, result in variable responses. Both excessive and inadequate thinning can be costly for apple growers. Consequently, it is essential to increase precision of the thinning process.
Apple seeds are a major source of gibberellic acid, which can be translocated into the tree and may inhibit flower formation for the next season (Chan and Cain, 1967; Luckwill et al., 1969). Blossom thinning results in early removal of flowers, eliminating their competitive effects on photosynthetic assimilates or on the endogenous hormonal balance, important for flower bud induction. Blossom thinners are more effective than fruitlet thinning for encouraging consistent annual bearing (Meland and Gjerde, 1993). Blossom thinning improves fruit size and fruit quality at the same cropping level (Meland, 2011). In Norway, ethephon is the only chemical registered for blossom thinning. The only other registered product, a liquid fertilizer, ammonium thiosulphate, is, however, less effective.
For blossom thinning agents to be effective, certain minimum weather requirements must be met during the application time. In the event of unfavorable weather during bloom, growers should be afforded another opportunity for thinning during the young fruitlet stage.
In Europe, ethephon is used as a flower thinner for biennial-bearing apple trees; however, it has been shown to be risky because it can easily result in overthinning (Jonkers, 1979). Ethephon also promotes flowering the next year (Buban and Sagi, 1976; Stopar and Zadravec, 2004).
‘Summerred’ apples are highly prone to biennial bearing in Norway if not properly thinned resulting in poor fruit quality and erratic yields. Ethephon can be an effective thinner; however, results are dependent on a.i. concentration, ambient temperatures, the volume of water applied per unit area, and time of application (Jones et al., 1983, 1989, 1990; Stover and Green, 2005). The objective of this study was to investigate the effects of ethephon as a blossom and fruitlet thinning agent for ‘Summered’ apple trees.
Buban, T. & Sagi, F. 1976 The possibility of regulating flowering in apple trees. II. Effects of morphactin, ethrel and bromacil sprayings on flower bud formation Fruit Science Report III 1 9 14
Bukovac, M.J., Sabbatini, P. & Schawallier, P.G. 2006 Modifying alternate bearing of spur type ‘Delicious’ apple with ethephon HortScience 41 1606 1611
Byers, R.E. 2003 Flowering and fruit thinning and vegetative fruiting balance 409 436 Ferree D.C. & Warrington I.J. Apples: Botany, production and uses CABI Publ Cambridge, MA
Byers, R.E., Barden, J.A. & Carbaugh, D.H. 1990 Thinning of spur ‘Delicious’ apples by shade, terbacil, carabaryl, and ethephon J. Amer. Soc. Hort. Sci. 115 9 13
Embree, C.G., Nichols, D.S., DeLong, J.M. & Prange, R.K. 2001 Certain chemical thinning treatments advance maturity of ‘Paulared’ apple Can. J. Plant Sci. 81 499 501
Jones, K.M., Koen, T.B., Oakford, M.J. & Bound, S.A. 1990 Thinning ‘Red Fuji’ apples using ethephon at two timings J. Hort. Sci. 65 381 384
McArtney, S., Unrath, D., Obermiller, J.D. & Green, A. 2007 Naphthaleneacetic acid, ethephon, and gibberellins A4+A7 have variable effects on flesh firmness and return bloom on apple HortTechnology 17 32 38
Meland, M. 2011 Effects of different crop loads and thinning times on yield, fruit quality and return bloom of the apple cultivar ‘Elstar’ J. Hort. Sci. Bio (In press)
Schmidt, T., Elfving, D.C., McFerson, J.R. & Whiting, M.D. 2009 Crop load overwhelms effects of gibberellic acid and ethephon on floral initiation in apple HortScience 44 1900 1906
Stopar, M. & Lokar, V. 2003 The effect of ethephon, NAA, BA and their combinations on thinning intensity of ‘Summerred’ apples J. Central Europe Agr. 4 399 404
Stopar, M. & Zadravec, P. 2004 Thinning of ‘Jonagold’ and ‘Elstar’ apples with the combination of ethephon and CPPU Acta Hort. 653 93 97
Stover, E.W. & Green, D.W. 2005 Environmental effects on the performance of foliar applied plant growth regulators: A review focusing on tree fruits HortTechnology 15 214 221
Whale, S.K., Singh, Z., Behboudian, M.H., Janes, J. & Dhaliwal, S.S. 2008 Fruit quality in ‘Cripp's Pink’ apple, especially colour, as affected by preharvest sprays of aminoethoxyvinylglycine and ethephon Sci. Hort. 115 342 351