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James R. Schupp, Thomas M. Kon, and H. Edwin Winzeler

There is interest in developing new chemical thinners that are safe and naturally occurring. Existing thinners can cause phytotoxicity, pose hazards to beneficial and pollinating insects, and some are under review by regulatory agencies for possible

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Duane W. Greene

Several chemical thinners are available for use on apples: NAA, NAAm, carbaryll ethephon, benzyladenine and a few potential blossom thinners. Blossom thinners are generally applied at 80% bloom and it is thought that they work by damaging the style and/or pollen tube, thus preventing fertilization of the ovules. Postbloom thinners generally act by creating a localized stress at a time when developing fruit are especially vulnerable. The stress conditions are caused by elevated levels of ethylene, lowered levels of auxin or a temporary reduction in carbohydrate available to developing fruit. The most probable mode of action and site of absorption will be discussed for each thinner.

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Karen Inge Theron, Human Steenkamp, and Willem Jacobus Steyn

., 2011 ). Chemical thinning plums would reduce hand thinning substantially, but currently, chemical thinners available for stone fruit thinning are limited ( Seehuber et al., 2011 ). One chemical thinning approach for plums is to use gibberellins, e

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Duane W. Greene

YI-1066 is a new blossom thinner that may be useful as an alternative to the presently-used chemical thinners. It was applied as a dilute spray to `Royal Gala' apples at either 475 or 950 ml/379 liters when about 80% of the flowers were open. Browning of flowers and leaves was noted within 1 hour of application. The 950 ml/liter rate reduced fruit set. One YI-1066 treatment was applied at the 475 ml/379 liters rate, and rain started within 10 minutes after the completion of the spray. Although flower browning was noted, fruit set on these trees was increased above that on control trees. The recommended commercial thinning combination, 3 ppm NAA and 600 ppm carbaryl, did not thin. YI-1066 at 950 ml/379 liters caused a significant amount of thinning but it also reduced seed number and increased the number of fruit with solid russet at harvest.

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J.R. Schupp

Effects of chemical thinners on yield, fruit size, and fruit quality was studied in a commercial orchard in Milton, N.Y., on 6-year-old `Honeycrisp'/M.26 trees. The trees were planted at 1.8 × 3.6-m spacing with trickle irrigation and were trained to the vertical axis system. The treatments applied in a randomized complete-block design with four replications were an untreated control; carbaryl (Sevin XLR at 125 mL/100 L); NAA at 2.5 ppm, 5 ppm, or 7.5 ppm; NAA at 2.5 or 5 ppm plus carbaryl; and Accel (a.i. at 74 g·ha-1) plus carbaryl. Chemical thinners were applied to drip with an air-blast sprayer, when the largest fruit were 11.5 mm in diameter. Generally, thinning activity increased with increasing NAA concentration. The combination sprays of 5 ppm NAA plus carbaryl, and Accel plus carbaryl over-thinned `Honeycrisp'. Carbaryl alone was inconsistent. All thinning treatments increased fruit size relative to unthinned trees, with average fruit diameter exceeding 76 mm. `Honeycrisp' is a large-fruited cultivar that is easy to thin chemically at the traditional 10- to 12-mm growth stage. NAA at 2.5 or 5 ppm provided adequate thinning to produce fruit of good quality and size. If initial set is heavy and a stronger thinning response is desired, the combination of 2.5 ppm NAA plus carbaryl could be used. `Honeycrisp' appears to be very sensitive to Accel, when used in combination with Sevin XLR. Further research needed before Accel is used to thin `Honeycrisp'.

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Duane W. Greene

The lack of pollination and the effects of blossom thinners were simulated by enclosing selected `McIntosh' apple spurs in super-light insect barrier netting at the pink stage of flower development. Fruit set was recorded and fruit size measured at 2- to 3-day intervals from petal fall until initial set. The effects of lack of pollination or the use of blossom thinners on initial set could not be determined with any degree of accuracy until at least 8 days after petal fall. NAA was applied at 8 ppm when fruit were 8.5 mm in diameter. Fruit set and fruit size were taken at 2- to 3-day intervals until the end of June drop. Fruit set on NAA-treated trees was greater than that on check trees for 2 weeks following application. Although NAA ultimately did cause significant thinning, it was not until 3 to 3.5 weeks after application that it was possible to determine with accuracy the thinning response to NAA. However, the thinning response to NAA could be predicted within a week after application, since growth of fruitlets that ultimately abscised slowed 4 to 7 days after the application of NAA. A working model to predict effective pollination and the response to chemical thinners in apples will be discussed.

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R.E. Byers, D.H. Carbaugh, and L.D. Combs

Prohexadione calcium applied as a series of three applications starting soon after petal fall to `Fuji'/M.9 apple trees reduced the number of pruning cuts, pruning time, pruning weight per tree, current season's shoot length, individual shoot weights, and increased number of nodes on the lower 40 cm of shoots. Fruit diameter, soluble solids, starch, or individual fruit weights were not affected by Apogee sprays. Fruit color and firmness were slightly increased in only one experiment. Growth suppression appeared to be greater on trees cropping more heavily. When trees were more heavily thinned, less shoot growth control was achieved. Apogee applied at 250 mg/L in three applications caused a significant increase in fruit set when compared to the control. Alone Vydate, Carbaryl+Oil, or Carbary+Accel+Oil caused fruit thinning, but neither ethephon nor shading 3 days caused significant thinning. Apogee did not influence results of chemical thinners when applied between the first and second Apogee applications. The 10% and the 27.5% Apogee formulations gave similar shoot growth inhibition when applied with Regulaid or Oil+Silwet L-77. When using hard water (well water), the 27.5% Apogee formulation was not as effective as the 10% formulation. The 10% Apogee formulation has more NH4SO4 than the 27.5% formulation w/w; NH4SO4 is used to prevent inactivation of Apogee by calcium and other cations when hard water is used for spraying. The addition of CaCl (frequently used to reduce bitter pit and corkspot disorders) to the 27.5% Apogee formulation caused poorer growth control than with hard water alone. When Apogee was used at 125 mg/L, the addition of NH4SO4 restored the effectiveness of the hard water+CaCl mixture. Alone the additives NH4SO4, Ca Cl, Regulaid, and/or Oil plus L-77, had no effect on tree growth. Apogee plus L-77+Oil provided additional growth suppression when compared to Apogee+Regulaid. In 1998, three applications of Apogee (63 mg/L) or ethephon (135 mg/L) did not affected shoot growth of `Fuji'/M.9 trees at these low rates. Combinations of Apogee and ethephon gave good control of tree growth. Flowering and fruit set were not promoted by any of these applications.

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Kiyoshi Yokota, Koichi Murashita, Masanobu Nonaka, and Koichi Hirai

Since 1987 some chemicals have been examined for thinning of apple, while concidering the safety for humans, animals and bees. Ethyl 5-chloro-3 (1H) - indazorylacetate (Ethychlozate) had the same effect as Carbaryl on `Fuji' cultivor Suitable results were obtained by 40ppm solution spray at 2 to 3 weeks after full bloom. An addition of 300ppm Ethephon increased thinning effects on `Fuji'. But over thinning occured on `Tsugaru'. `Jonagold' and `Jonathan' by same concentration of Ethychlozate or Ethychlozate plus Ethephon.

Thinning effects were more severe on the lateral fruits than on the center fruits. The results were advantageous for apple growing in Japan as center fruits are very important. Epinasty, russet and other injuries caused by Ethychlozate sprayed were not recognized.

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V. E. Emongor and D. P. Murr

Benzyladenine (100 or 200 mg.litre-1) was applied to mature Empire/M.26 apple trees as dilute sprays 2, 4, 9, 11, 13, 15, 17, 20, 25, 27, 29 or 31 days after full bloom (DAFB). The most effective time of thinning was 25-29 DAFB (king fruit diameter 8.94-13.91 mm), and the thinning response to BA concentration was linear. Benzyladenine (BA) did not reduce fruit set when king fruit diameter was less than 5.35 mm, but BA significantly increased fruit weight, diameter (D), length (L) and L:D ratio compared to unsprayed controls and later BA treatments. BA - treated fruitlets had higher ethylene production, 24 hours and 7 days after spraying compared to untreated controls. We suggest that the response of apple fruitlets to BA applied as a thinner is mediated by ethylene. High fruit quality was obtained when BA was applied at 17-31 DAFB. Timing of BA sprays had no effect on seed number, though BA significantly increased seed number, fruit size, weight and L:D ratio. These results suggest that BA has the potential to substitute for the use of carbaryl as a thinner of apples in Ontario orchards.

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Duane W. Greene

An experiment was initiated on mature `Morespur McIntosh'on M.7 rootstock to document the effects of repeated yearly applications of benzyladenine (BA) and naphthaleneacetic acid (NAA) on fruit quality at harvest, the development of storage disorders following regular air storage, and on return bloom. When analyzed over the 4-year period, thinning did not significantly reduce crop load. This result was due in large part to no thinning response one year and very poor set on all trees in another year. Thinners were effective at increasing return bloom over the course of the experiment. BA increased fruit weight but reduced red color compared with NAA treated and control trees. Fruit quality differences at harvests were attributed primarily to crop load effects. There were no fruit quality, return bloom, or storage disorders that could not be explained by treatment effects on crop load or due to previously known effects of individual thinners. The results of this experiment clearly suggest that there are no direct adverse effects following repeated use of either NAA or BA.