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Ockert P.J. Stander, Johané Botes, and Cornelius Krogscheepers

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

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Duane W. Greene

purpose of this investigation was to confirm the effect of ProCa on fruit set and to examine several chemical thinning strategies that may be used to effectively and appropriately thin ProCa-treated apple trees. Materials and Methods Expt. 1

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Richard P. Marini

191 WORKSHOP 23 (Abstr. 1059-1061) Methods and Techniques for Testing Chemicals Used for Thinning Fruit Crops

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Warren C. Micke, Joseph A. Grant, and James T. Yeager

108 ORAL SESSION 27 (Abstr. 564–571) Thinning–Fruits/Nuts

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Jose R. Cartagena, Frank B. Matta, and James M. Spiers

The thinning potential of various chemicals sprayed on `Tifblue' rabbiteye blueberry was examined in the greenhouse in 1990 and under field conditions in 1991 and 1992. In the greenhouse, BA concentrations ranging from 25 to 500 mg·liter-1 and carbaryl concentrations ranging from 400 to 2100 mg·liter-1 reduced fruit set when treatments were applied 16 days after corolla drop (ACD). GA3 reduced fruit set only at 50 mg·liter-1 and NAA did not influence fruit set. In the field, BA at 75 mg·liter-1 and the combination of carbaryl at 400 mg·liter-1 and BA at 25 mg·liter-1 reduced fruit set in 1991 and 1992. Combinations of carbaryl and GA3 reduced fruit set, but the response depended on GA3 concentration and varied from year to year. GA3, NAA, and carbaryl also reduced fruit set, but the results were inconsistent. In 1991, greater thinning occurred when the treatments were sprayed 10 days ACD. BA at 25 mg·liter-1 increased fruit diameter at first harvest in 1991, and carbaryl at 400 mg·liter-1 increased fruit diameter in 1991 and 1992. Fruit diameter was increased in the presence and absence of thinning, depending on year and application time. Yield and return bloom were not influenced by any of the treatments. Chemical names used: 7 benzylamino purine (BA); gibberellic acid (GA3); 2-naphaleneacetic acid (NAA); 1-naphthyl N-methylcarbamate (carbaryl).

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Richard Marini*

Six-year-old York/M.9 trees were used to evaluate combinations of chemicals for fruit thinning. In one experiment a factorial combination of 2 levels of carbaryl (0 or 600 mg·L-1) and 5 levels of 6-BA (0, 40, 80, 120, and 160 mg·L-1) were sprayed when fruit diam. averaged 10.5 mm. Carbaryl significantly reduced fruit set, number of fruit/tree, yield efficiency, and crop density, and increased fruit weight. The main effect of 6-BA did not significantly influence any response variable. Two variables were significantly influenced by the carbaryl × 6-BA interaction. In the absence of carbaryl, fruit set was reduced and fruit weight was increased by 6-BA at concentrations less than 160 mg·L-1, but the addition of 6-BA to carbaryl was no more effective than carbaryl alone. In a second experiment, a factorial combination of 2 levels of carbaryl (0 vs. 600 mg·L-1), 2 levels of NAA (0 vs. 5 mg·L-1), and 2 levels of ethephon (0 vs. 450 mg·L-1) were sprayed when fruit when fruit diam. averaged 10.5 mm. Carbaryl and NAA reduced fruit set by about 30%, but ethephon overthinned and reduced set by 65%. When the other materials were combined with ethephon, thinning was similar to ethephon alone. The combination of carbaryl and NAA was no more effective than either material alone. The lowest values for yield, yield efficiency, and numbers of fruit per tree were associated with the combination of ethephon plus NAA. Ethephon was the only material that increased fruit weight.

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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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R.E. Byers and D.H. Carbaugh

Apple growers of different regions need different chemical fruit-thinning responses for thinning trees of different tree ages, cultural conditions, rootstocks, climates, and amounts of fruit removal desired. In this research, a range of chemical thinning responses was achieved by combinations of thinning materials or addition of potentiating agents. Superior oil, certain organic phosphates, and a light-absorbing agent (ferbam, a fungicide) increased the thinning of carbaryl. In addition, combinations of 50 or 200 ml 6-BA/liter + carbaryl + oil defruited `Campbell Redchief Delicious'/M.111 trees, and 50 ml 6-BA/liter alone over-thinned in one year (however, oil or 6-BA has been shown previously to cause russet in `Golden Delicious'). Carbaryl 50 WP and the 4L carbaryl formulations were equally effective for thinning `Golden Delicious', `Stayman', and `Redspur Delicious', and did not affect fruit russet. Three days of cloudy weather is typical at least once in most seasons in the eastern United States during the fruit set period. Two days of artificial polypropylene shading (92%) (which was nearly equivalent to 3 days of cloudy weather) caused more thinning of `Golden Delicious' and `Stayman' than carbaryl or 10 mg NAA/liter + Tween. Shading reduced viable seed numbers about 50% for `Golden Delicious' in fruit remaining at harvest, but chemical thinning agents (NAA or carbaryl) did not affect viable seed number.

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D.C. Elfving and R.A. Cline

Postbloom applications of benzyladenine (BA) thinned young fruitlets of mature `Empire' apple trees (Malus domestica Borkh.) as well as or better than NAA or carbaryl (CB). BA increased fruit weight more effectively than either NAA or CB. Promalin (PR) was less effective than BA for both thinning and fruit-weight increase. In 1990, both BA and PR reduced fruit set up to 29 days after full bloom, but PR showed less thinning activity. BA and NAA produced independent and additive thinning responses when tank-mixed. Effects of all thinners on foliar mineral-nutrient concentrations were associated with changes in fruit load. BA increased return bloom as much or more than NAA or CB. PR did not affect return bloom. Chemical names used: N -(phenylmethyl)-1 H -purine-6-amine [benzyladenine (BA)]; BA plus gibberellins A, and A, [Promalin (PR)]; 1-naphthaleneacetic acid (NAA); 1-naphthalenyl methylcarbamate [carbaryl (CB)].

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Ross E. Byers and Alson H. Smith Jr.

An analysis of daytime high temperatures for the 10-year period from 1984 to 1993 indicated that, in the 21 days after full bloom (AFB) in 7 of 10 years, there were 3 days or more above 29.5C. In the 15- to 21-day period AFB, when fruit are considered at their optimum diameter (8 to 12 mm) for thinning, only 3 days above 29.5C were recorded. In the 15 to 21 days AFB, the high temperature was only 24C for 7 out of 10 years. Thus, growers would have to spray at temperatures 5.6C degrees lower if they were to choose to spray the warmest 3 days during the 15- to 21-day period when fruit are 8 to 12 mm in diameter. NAA caused thinning of `Golden Delicious' fruit at 8-mm fruit diameter. Tank mixing of one of several pesticides (regulaid or guthion, captan, carzol, imidan, polyram, lorsban, omite, or lannate) had no effect on NAA efficacy. Comparison of identical chemical thinning treatments (carbaryl + Accel + oil) applied to `York' and `Red Delicious' apple trees indicated that more thinning occurred with the PF treatments than at 8 mm. Average 2-day high temperatures at PF were 7.1C higher at PF for the `Red Delicious' experiment and 5.6C higher for the `York' experiment. The higher temperatures at PF could account for the differences in thinning response and not the spray timing. Pollination and fertilization inhibitors caused some fruit thinning at the highest rates and multiple applications. The MYX4801 caused more thinning and more injury to fruit than other materials. Endothall gave good thinning without fruit injury. Wilthin (GWN-6592) did cause some thinning, but fruit injury was a problem in one experiment.