Abstract
A resurgence of interest has been focused on the search for a chemical which would evolve ethylene after foliar application and thus induce plant growth responses. One such compound, 2-chloroethylphosphonic acid (Ethrel), has been under study as a peach thinner at the University of California orchards at Davis. It has been assumed that the peach thinning induced by Ethrel was caused by the ethylene gas which is released after application. As an adjunct to the work with Ethrel, we were interested in the thinning action of ethylene gas and the resultant gross effects on the treated tree parts.
In South Africa and elsewhere in the world, the use of synthetic plant growth regulators (PGRs) as chemical fruit-thinning agents is a common cultural practice of citrus fruit production to increase fruit size and other important quality attributes
Shading (92%) of `Redchief Delicious' apple (Malus domestics Borkh.) trees for 10-day periods from 10 to 20, 15 to 25, 20 to 30, and 25 to 35 days after full bloom (DAFB) caused greater fruit abscission than shading from 5 to 15, 30 to 40, 35 to 45, or 47 to 57 DAFB. Fruit 8 to 33 mm in diameter (10 to 30 DAFB) were very sensitive to 10 days of shade, even though fruit sizes of 6 to 12 mm are considered the most sensitive to chemical thinners. In a second test, shading for 3 days caused fruit thinning; 5 days of shade in the periods 18 to 23, 23 to 28, and 28 to 33 DAFB caused greater thinning than 11 to 16 or 33 to 38 DAFB. Shading reduced photosynthesis (Pn) to about one-third that of noncovered trees. Terbacil (50 mg·liter-1) + X-77 surfactant (1250 mg·liter-1) applied with a hand-pump sprayer 5, 10, or 15 DAFB greatly reduced fruit set and caused some leaf yellowing, particularly in the earliest treatments. Terbacil reduced Pn by more than 90% at 72 hours after application. Shoot growth of trees defruited by shade or terbacil was equivalent to defruited or deblossomed trees; ethephon (1500 mg·liter-1) inhibited tree growth and defruited trees. No terbacil residues were dectected in fruit at harvest from applications made 5, 15, 20, 25, or 30 DAFB. Eleven of 12 photosynthesis-inhibiting herbicides were also found to thin `Redchief Delicious' apple trees. Shading caused more thinning than terbacil at the later applications, which may reflect poorer absorption and/or lesser photosynthetic inhibition than when terbacil was applied to older leaves.
Mechanical methods of thinning fruit trees such as high-pressure spray guns, tree shakers, club thinning, rope thinners, drum shakers, and string thinners can produce a thinning response in stone fruits and some nut crops ( Dennis, 2000 ). There has
from excessive thinning, and planting errors cause situations in which additional forms of growth control are needed. Daminozide was used successfully for many years to control growth, but its registration was withdrawn for use on apples in 1989
Asian pears bloom profusely and require early hand-thinning to produce large saleable fruit. In 1998, `Shinko' and `Hosui' trees were sprayed with Accel, Ethrel, NAA or carbaryl when average fruit diameter was ≈9 mm. Superior oil was added to all treatments at the rate of 2.5 mL/L. Fruit set was reduced 40 to 70% by Accel at 84 ppm and ethephon at 678 ppm, but NAA at 8 ppm and carbaryl were less effective. In 1999, Accel was applied at a range of 0 to 105 ppm and ethephon was applied at a range of 0 to 678 ppm. All treatments contained carbaryl and oil. Fruit set/100 flower clusters declined linearly with increasing ethephon concentration on `Shinko', but was not tested on `Hosui'. Fruit/100 blossom clusters declined linearly with increasing Accel concentration on `Shinko'. On `Hosui' Accel reduced fruit set by more than 40%, but treatments did not differ significantly (5% level) from the control. In both years `Shinko' was easier to thin than `Hosui'.
noir’, cluster thinning is a standard practice used to ensure yield targets are met in the range of 4.5 to 6.2 t·ha −1 to hasten ripening and achieve quality in a cool climate ( Uzes and Skinkis, 2016 ). Although there are numerous studies that
Hand-thinning (Prunus persica L. Batsch) “Y”-trained peach trees at bloom and 51 days after full bloom (DAFB) was compared to mechanical fruit thinning 51 DAFB using a spiked-drum and an impact shaker. The spiked-drum shaker removed more fruit from horizontal branches than from vertical branches, yet did not selectively remove either large or small fruit. Bloom thinning by hand increased fruit size compared to postbloom thinning 51 DAFB, and both postbloom mechanical thinning techniques were as effective as postbloom hand thinning. The spiked-drum shaker may be a better thinning technique than the impact shaker because it transfers less shaking energy to the fruit, can be used in high-density plantings, and does not contact the trunk, lessening the potential for tree damage.
. One method to combat alternate bearing and improve nut quality during large crop years is mechanical fruit thinning ( Smith and Gallott, 1990 ). Reid et al. (1993) demonstrated that fruit thinning while the fruit were between one-half and full ovule
axillary shoots in free-branching cultivars was identified to be an unculturable phytoplasma ( Lee et al., 1997 ). In free-branching cultivars, thinning of surplus shoots could be an additional procedure to control growth and to achieve crop uniformity