Chemical Thinners Research Articles

ETHEPHON, NAA, AND NAD AS CHEMICAL THINNERS OF PEAR FRUITLETS

ETHEPHON, NAA, AND NAD AS CHEMICAL THINNERS OF PEAR FRUITLETS

Ethylene Evolution from Detached Apple Spurs in Response to Chemical Thinners

Ethylene evolution from detached fruiting apple spurs was measured after application of various bloom and post-bloom thinning agents. Ethylene evolution from fresh detached spurs of `Splendor' apple trees increased one day after application of a bloom thinning spray of ethephon or NAA, and remained higher than rates of ethylene evolution by detached spurs from unsprayed control trees for 6 (NAA) or 10 (ethephon) days. Both ethylene evolution and fruit abscission during the initial drop period were higher on trees treated with ethephon compared to NAA, however final fruit set was similar for these two treatments. Ethylene evolution was significantly higher following NAA application onto `Fuji' trees compared with NAAm, but final fruit set was reduced by a similar amount (≈20%) for both of these materials. Application of BA to `Pacific Rose™' apple trees when the average diameter of spur fruit was either 4 mm (6 days after full bloom) or 7 mm (12 days after full bloom) resulted in a significant increase in the rate of ethylene evolution and also reduced final fruit set. When application of BA was delayed until the average diameter of spur fruit was 14 mm (24 days after full bloom) neither the rate of ethylene evolution or final fruit set was affected. Although an increase in the rate of ethylene evolution was a prerequisite for thinning in the present experiments, the magnitude of this increase was not related to the final thinning efficacy. Chemical names used: benzyladenine (BA); 2-chloroethyl phosphonic acid (ethephon); naphthaleneacetic acid (NAA); naphthalene acetamide (NAAm).

Experience with Thinning: What Factors Contribute to Contradicting Results?

POLLINATION There are two species of parasitic mites that essentially have eliminated wild honeybees in the mid-Atlantic region and have drastically reduced the number of hives available from beekeepers in Virginia. This raised some concerns about apple (Malus ×domestica Borkh.) fruit set and the activity of our chemical thinners on fruits with poor pollination. In 1996, we performed an experiment to try to determine if thinners were more effective on fruits with few seeds. During the pink stage, varying numbers of stigmas were removed from king flowers on ‘York Imperial’ trees. Some trees were sprayed with carbaryl when mean fruit diameter was ≈10 mm, and another set of trees were not sprayed with a chemical thinner to serve as a control. Fifty days after treatment, the number of treated fruits that remained on the tree was recorded and expressed as a percentage of the original number of treated flowers. Fig. 1 shows that fruit set was 30% for the control flowers with no stigmas removed and flowers with three stigmas removed set 22%. Removal of four stigmas reduced fruit set to 12%. Trees that were thinned with carbaryl had 8% to 14% fruit set regardless of the number of stigmas removed. Based on this preliminary experiment, it seems that poorly pollinated flowers set fewer fruits when not thinned, but the severity of drop following a thinning treatment may not be related to the degree of pollination. When pollination is considered inadequate, one may want to delay the application of thinners as long as possible to evaluate natural fruit drop. We have found that carbamates will provide fairly good thinning when mean fruit diameter is ≤14 mm and ethephon will provide thinning when fruit diameter is <30 mm.

Chemicals, Timing, and Environmental Factors Involved in Thinner Efficacy on Apple

HORTSCIENCE, VOL. 37(3), JUNE 2002 Fallahi, E., M. Kilby, and J.W. Moon. 1990. Effects of various chemicals on dormancy, maturity and thinning of peaches. Deciduous Fruit and Nut. University of Arizona, College of Agr. Rpt., Ser. P-83, p. 121–128. Fallahi, E., R.R. Lee, and G.A. Lee. 1998. Commercial-scale use of hydrogen cyanamide for blossom thinning of apple and peach. HortTechnology 8:556–560. Fallahi, E., B.R. Simons, J.K. Fellman, W.M. Colt. 1992. Use of hydrogen cyanamide for apple and plum thinning. Plant Growth Regulat. 11:435–439. Fallahi, E., M.W. Williams, and W.M. Colt. 1997. Blossom thinning of ‘Law Rome Beauty’ apple with hydrogen cyanamide and monocarbamide dihydrogensulfate. J. Tree Fruit Prod. 2:33–44. Ferree, D.C. 1996. Performance of benzyladenine as a chemical thinner on eight apple cultivars. J. Tree Fruit Prod. 1:33–50. Greene, D.W. 1984. Microdroplet application of GA4+7 + BA: Sites of absorption and effects on fruit set, size, and shape of ‘Delicious’ apples. J. Amer. Soc. Hort. Sci. 109: 28–30. Greene, D.W., K.I. Hauschild, and J. Krupa. 2001. Effect of blossom thinners on fruit set and fruit size of peaches. HortTechnology 11:179–183. Greene, D.W. and W.J. Lord. 1985. Effects of chemical thinners on ‘Delicious’ apple trees previously sprayed with GA4+7 +BA. HortScience 20:84–86. Hildebrand, E.M. 1944. The mode of action of the pollenicide, Elgetol. Proc. Amer. Soc. Hort. Sci. 45:53–58. Johnson, S. 1998 Consistent tonnage needed for profitability. Good Fruit Grower 49(7):9–10. Looney, N. 1998. Chemical thinning of ‘Fuji’ apple. Compact Fruit Tree. Proc. Annu. Conf. Intl. Dwarf Tree Assn. 30:55–57. Sanderson, P. 1998. Consistent tonnage needed for profitability. Good Fruit Grower 49(7):9-10. Southwick, S.M., K.G. Weis, J.T. Yeager, and M.E. Rupert. 1998. Blossom thinning of ‘Loadel’ cling peach with a surfactant: Effects of concentration, carrier volume, and differential applications within the canopy. HortTechnology 8:55–58. Taylor, J. 1998. Consistent tonnage needed for profitability. Good Fruit Grower 49(7):9-10. Williams, K.M. 1998. Blossom thin for consistent crops. Good Fruit Grower 49(7):8. Williams, M.W. 1993. Comparison of NAA and carbaryl petal–fall sprays of apples. HortTechnology 3:428–429. Williams, M.W. 1994. Factors influencing chemical thinning and update on new chemical thinning agents. Compact Fruit Tree 27:115–122. Williams, M.W. and Edgerton. 1981. Fruit thinning of apples and pears with chemicals. U.S. Dept. Agr., Agr. Bul. 289.

Blossom Thinning of Pome and Stone Fruit

Kathleen M. Willemsen2 Department of Horticulture and Landscape Architecture, P.O. Box 646414, Washington State University, Pullman, WA 99164-6414 T.T. Kozlowski (ed.). Shedding of plant parts. Academic Press, New York. Edgerton, L.J. 1973b. Control of abscission of apples with emphasis on thinning and pre-harvest drop. Acta Hort. 34:333–343. Greene, D.W., W.R. Autio, J.A. Erf, and Z.Y. Mao. 1992. Mode of action of benzyladenine when used as a chemical thinner on apples. J. Amer. Soc. Hort. Sci. 117:775–779. Hartman, F.O. and F.S. Howlett. 1962. Effects of naphthaleneacetic acid on fruit setting and development in the apple. Ohio Agr. Expt. Sta. Res. Bul. 920. Knight, J.N. 1983. Translocation properties of carbaryl in relation to its use as an apple fruitlet thinner. J. Hort. Sci. 53:371–379. Luckwill, L.C. 1953. Studies of fruit development in relation to plant hormones. II. The effect of naphthalene acetic acid on fruit set and fruit development in apples. J. Hort. Sci. 28:25–40. Luckwill, L.C. and C.P. Lloyd-Jones. 1962. The absorption, translocation and metabolism of 1-naphthaleneacetic acid applied to apple leaves. J. Hort. Sci. 37:190–206. Marsh, H.V. Jr., F.W. Southwick, and W.D. Weeks. 1960. The influence of chemical thinners on fruit set and size, seed development, and pre-harvest drop of apples. Proc. Amer. Soc. Hort. Sci. 75:5–21 Murneek, A.E . 1954. The embryo and endosperm in relation to fruit development, with special reference to the apple, Malus Sylvestris. Proc. Amer. Soc. Hort Sci. 64:573–582. Murneek, A.E. and F.G. Teubner. 1953. The dual action of naphthaleneacetic acid in thinning of apples. Proc. Amer. Soc. Hort. Sci. 61:149–154. Rahemi, M. 1981. Role of ethylene and the effect of supplemental hand pollination in apple (Malus domestica Borkh.) fruit set. PhD Diss., Michigan State Univ., E. Lansing. Schneider, G.W. 1973. Translocation of C–indoleacetic acid and sucrose in excised apple pedicels. J. Amer. Soc. Hort. Sci. 98:278–281. Schneider, G.W. 1974. Ethylene evolution and apple fruit thinning. J. Amer. Soc. Hort Sci. 100:356–359. Schneider, G.W. 1975. C-Sucrose translocation in apple J. Amer. Soc. Hort. Sci. 100:22–24. Schneider, G.W. 1977. Studies on the mechanism of fruit abscission in apple and peach. J. Amer. Soc. Hort. Sci. 102:179–181. Schneider, G.W. and A.M. Lasheen. 1973. NAA and Sevin on composition, development and abscission of apple fruit. HortScience 8:103–104. Schumacher, R., R. Neuweiler, and W. Stadler. 1993. Einfluss der Fruchtausdunnung auf Fruchtansatz, Frucht-und Treibwachstum. Schweiz. Zeitschrift fur Obst– und Weinbau 129:421–426. Southwick, R.W., W.D. Weeks, E. Sawada, and J.F. Anderson. 1962. The influence of chemical thinners and seeds on the growth rate of apples. Proc. Amer. Soc. Hort. Sci. 80:33–42. Stopar, M., B.L. Black, and M.J. Bukovac. 1997. The effect of NAA and BA on carbon dioxide assimilation by shoot leaves of spur–type ‘Delicious’ and ‘Empire’ apple trees. J. Amer. Soc. Hort. Sci. 122:837–840. Struckmeyer, B.E and R.H. Roberts. 1950. A possible explanation of how naphthalene acetic acid thins apples. Proc. Amer. Soc. Hort. Sci. 56:76–78. Teubner, F.G. and A.E. Murneek. 1955. Embryo abortion as mechanism of “hormone” thinning of fruit. Missouri Agr. Expt. Sta. Bul. 590. Tromp, J. and S.J. Wertheim. 1980. Synthetic growth regulators: Mode of action and application in fruit production, p. 137–150. In: Physiological aspects of crop productivity. Proc 15th Colloq. Intl. Potash Inst., Bern. Walsh, C.S., H.J. Swartz, and L.J. Edgerton. 1979. Ethylene evolution in apple following post-bloom thinning sprays. HortScience 14:704–706. Wertheim, S.J. 2000. Developments in the chemical thinning of apple and pear. Plant Growth Regulat. 31:85–100. Williams, M.W. 1979. Chemical thinning of apples. Hort. Rev. 1:270–300. Williams, M.W. 1981. Response of apple trees to aminoethoxyvinylglycine (AVG) with emphasis on apical dominance, fruit set, and mechanism of action of fruit thinning chemicals. Acta Hort. 120:137–141. Williams, M.W. and L.P. Batjer. 1964. Site and mode of action of 1-naphthyl N-methylcarbamate (Sevin) in thinning apples. Proc. Amer. Soc. Hort. Sci. 85:1–10. Yuan, R. and D.W. Greene. 2000. Benzyladenine as a chemical fruit thinner for ‘McIntosh’ apples. I. Fruit thinning effects and associated relationships with photosynthesis, assimilate translocation, and nonstructural carbohydrates. J. Amer. Soc. Hort. Sci. 125:169–176.

Diurnal spray timing does not affect the thinning of apples with carbaryl, benzyladenine, and napthaleneacetic acid

Inclement weather during fruit set of apples [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] can restrict an orchardist's ability to apply chemical thinners in a timely fashion. While recommendations for the timing of chemical thinners are very specific to the phenological stage of development and the environmental conditions following spray application, few studies have addressed the influence of diurnal timing and environmental conditions at the time of application on treatment efficacy. This 2-yr study was designed to determine the effect of diurnal spray timing and product on the thinning of mature Empire/M.26 apple trees. Treatments of 10 mg L-1 1-napthaleneacetic acid (NAA), 75 mg L-1 N-(phenylmethyl)-1H-purine-6-amine (6-benzyladenine) (BA), and 750 mg L-1 1-napthyl-N-methylcarbamate (CB) were applied at approximately 6-h intervals over a 24-h period when the largest fruitlets were 10–12 mm in diameter. Environmental conditions at the time of spraying in both years ranged from very cool (5°C) and humid (&gt; 90% RH) causing slow drying times (&gt; 180 min), to warm (25°C) and dry (35% RH) with rapid drying times (&lt; 15 min). Fruit thinning was consistently influenced by the chemical thinning product, but was unaffected by diurnal timing in both years. This work strongly indicates that choosing a specific time of day or ideal temperature and humidity conditions for spraying is not a major factor influencing thinning results if the combination of fruitlet development stage and chemical product is otherwise suitable. Key words: Malus, spray conditions, chemical thinning, crop density

Responses of Several Apple Cultivars to Chemical Thinning Sprays

Studies were conducted in 1995 and 1996 at Pontotoc country, Mississippi, U.S.A. to find the effect of two chemical thinners namely: accel (N-phenylmethyl) (H-purine 6-amine (6-BA) and carbaryl (1-Naphthyl carbamate) sprayed two weeks after bloom on the apple fruit characteristics such as fruit set, yield, etc. The three apple cultivars used were ‘Empire', ‘Jon-A-Red' and ‘Braeburn'. The treatments comprised accel (25, 50 and 75 ppm), carbaryl (0.05, 0.1 and 0.2%) and unsprayed control. It was found that accel and carbaryl thinned the three apple cultivars and yield (total fruit weight per tree), pH, sugar content were 0.05) increased by the chemical thinning sprays. Nevertheless,&#61603;significantly (P 0.05) the number of seeds in the fruit, fruit&#61603;the treatments did not affect (P length, fruit diameter and fruit length to diameter ratio. The chemical thinner and cultivar interactions were significant for some fruit characteristics. The Journal of Food Technology in Africa Volume 7 No.1, 2002, pp. 16-20 KEY WORDS: Fruit Thinning, Fruit Set, Yield and Quality.

A Method for Assessing the Relationship Between Cropload and Crop Value Following Fruit Thinning

Analysis of apple (Malus×domestica Borkh.) and citrus thinning experiments indicates that the relationships between cropload, fruit size, and total yield can be used to assess optimal cropload for highest crop value. Mean fruit size increased and total yield declined as the cropload (number of fruit/cm2 trunk cross-sectional area) was reduced through the use of chemical thinners. Because crop value is influenced by fruit size and total yield, intermediate croploads gave the highest economic returns in all experiments evaluated. For `Empire' apple, croploads greater than those expected to provide good return bloom often produced the highest crop value for a single year. In citrus, optimal crop values resulted from a broad range of intermediate croploads. A method is described to analyze optimum cropload from thinning experiments.

Non-structural carbohydrate status and CO 2 exchange rate of apple fruitlets at the time of abscission influenced by shade, NAA or BA

Four days of 90% shading at the time of fruit setting (1–5 days after petal fall, DAPF) of mature ‘Jonagold’/M.9 apple trees was combined with the application of thinning agents to establish the possible interaction of both main treatments on fruit retention, and the effects of the main treatments on fruitlets CO 2 exchange and their carbohydrate concentration. Shading alone reduced yield by up to 35%. When shading was followed by the application of 15 ppm 1-naphthaleneacetic acid (NAA) or 50 ppm 6-benzyladenine (BA), the interaction between shading and chemical thinners was on the border of significance. Following the assumption that abscission was induced by shading before the application of chemical thinning agents, the same process of diminished dark respiration rate ( R d) was found on fruitlets sampled 3 days after the end of shading and 7 days after NAA and BA spraying, i.e. in the fruitlets that were considered prone to drop. A week after the end of shading the abscission of fruitlets due to shading was completed and on the remaining fruitlets that were not prone to abscission any more, larger negative net CO 2 assimilation rates ( A) and enhanced R d were recorded. Fruitlets prone to abscission sampled 2 days after the end of shading showed a lower concentration of glucose and ascorbic acid and a higher concentration of starch. Eight days after the end of shading, the fruitlets from shaded trees that were not prone to abscission any more contained less fructose while the fruitlets from NAA and BA sprayed trees had a higher concentration of fructose a week after application. The results do not support the hypothesis that a good assimilate supply is needed for the retention of apple fruitlets, because the fruitlets retained on trees expressed even stronger CO 2 loss and because the concentration of non-structural carbohydrates did not decrease in fruitlets prone to drop with the exception of glucose.

Efeito de diferentes volumes de calda no raleio químico de frutos da macieira 'Gala'

O objetivo deste trabalho foi comparar a eficiência de diferentes volumes de calda de raleantes químicos em plantas de macieira (Malus domestica Borkh), cultivar Gala. Foram utilizados o ácido naftalenoacético (ANA) (15 g/ha de i.a.) e o carbaryl (1naftilNmetilcarbamato) (1.000 g/ha de i.a.) aplicando-se 16, 300, 430, 950, 1.300 e 1.900 L/ha de calda. O ANA foi aplicado cinco dias após a plena floração (DAPF), e o carbaryl, aos 15 DAPF. O volume de calda não parece ter sidoo fator mais importante para a eficiência do raleio: os tratamentos utilizados provocaram respostas semelhantes nas plantas. O peso e a porcentagem de frutas &gt;66 mm, em todos os tratamentos, com exceção de 430 L/ha, foram maiores do que os da testemunha, mas não diferiram entre si.

684 Thinning and Crop Load Effects on Apple Fruit Growth

Chemical thinners can be classified as either blossom thinners or postbloom thinners. Blossom thinners act by inhibit further pollination, pollen germination, or pollen tube growth. At petal fall it is not possible to distinguish between fruit that have been injured by blossom thinners, and those that will persist and continue to grow. The receptacles of blossom thinned fruit do not grow, whereas fruit that has not been treated and that also contain viable seeds, resumes growth within 4 to 6 days, depending upon temperature. Abscission of fruit treated with postbloom thinners does not usually occur until 1.5 to 3 weeks after application. Frequently, it is possible to identify fruit that will abscise and to make an initial assessment of thinning efficacy, within 4 to 6 days following application by measuring fruit growth rate. A reduction in fruit growth by as little as 15% to 20% less than rapidly growing fruit is usually sufficient to assume that the fruit will abscise sometime during the June drop period. The effects of specific chemical thinners on fruit growth and subsequent thinning will be discussed.

270 Effects of Chemical Thinners on Yield, Fruit Size, and Fruit Quality of `Honeycrisp' Apple

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'.

163 The Influence of Apogee and Its Combination with Ethephon, Chemical Thinners, Cations, and/or Adjuvants for Apple Tree Growth Control and Return Bloom

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.

Benzyladenine as A Chemical Thinner for `McIntosh' Apples. I. Fruit Thinning Effect1s and Associated Relationships with Photosynthesis, Assimilate Translocation, and Nonstructural Carbohydrates

BA was applied at 50 or 100 mg·L-1 to `More-Spur McIntosh'/Malling 7 (M.7) apple trees [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.] at the 10 mm stage of fruit development. BA thinned fruit and increased fruit size. There were two distinguishable peaks of fruit abscission during `June drop'. BA accentuated the naturally occurring waves of fruit abscission, and enhanced translocation of 14C-sorbitol from leaves to fruit when applied directly to the fruit, but not when applied directly to the leaves. Net photosynthesis was decreased and dark respiration was increased when temperature following BA application was high (30 °C), whereas there was no effect when temperature was lower (20 °C). Total nonstructural carbohydrates, total soluble sugars, and starch in the leaves decreased dramatically over the 12- or 13-day observation period, regardless of BA treatment. These carbohydrate concentrations in the leaves were lowered further by BA application. Abscising fruit, based on specific reddening of the pedicel, had higher carbohydrate levels than persisting fruit, regardless of BA application. We conclude that BA thins fruit, at least in part, by increasing dark respiration and decreasing net photosynthesis. Chemical name used: N-(phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)].

Benzyladenine as A Chemical Thinner for `McIntosh' Apples. II. Effects of Benzyladenine, Bourse Shoot Tip Removal, and Leaf Number on Fruit Retention

Experiments were conducted to evaluate the effects of BA, removal of bourse shoot tips including only folded leaves and growing point, and different numbers of leaves per fruit on fruit retention and fruit development in `More-Spur McIntosh'/Malling 7 (M.7) apple trees [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.]. Removal of the bourse shoot tip increased fruit retention, whereas BA thinned fruit regardless of whether shoot tips were removed or not. There was no interaction between BA application and shoot tipping. BA thinned fruit only when one leaf per fruit was on a girdled small fruiting branch, but not when leaf number per fruit was two or greater. Fruit weight and soluble solids concentration increased dramatically with increasing leaf number per fruit. BA reduced fruit growth rate when &lt;16 leaves per fruit were present on the girdled branches between 3 and 7 days after treatment, but it did not affect fruit growth rate when 32 leaves per fruit were on the girdled branches. Increasing leaf number also increased viable seed number per fruit while decreasing the number of aborted seeds, but it had no effect on the number of total seeds per fruit. BA reduced the number of viable seeds per fruit only when the number of leaves per fruit was less than four. Results suggest that BA thins apple fruit mainly by reducing carbohydrates available to developing fruitlets. Chemical name used: N-(phenylmethyl)-1H-purine-6-amine [benzyladenine (BA)].

Lower-bud formation in pome fruits as affected by fruit thinning

Fruit thinning is commonly practiced in many fruit bearing woodyperennials to improve fruit quality and to prevent ‘biennial bearing’, thesevere alternation of fruit load in successive ‘on’ and ‘off’ years. Biennialbearing has its origin in the negative effect of the presence of fruits onflower production (return bloom), and, thus, the yield for next year. Forreasons of labour cost, fruit thinning is usually done by using chemicalcompounds such as ethephon and ammonium thiosulphate at the blossomingstage, and for fruit thinning NAA, NAAm, carbaryl, and a few cytokinins.Apart from indirect effects of the various chemicals on return bloom viareduction of fruit load, they may also influence flower formation directly.In view of the supposedly negative relationship between flowering andshoot growth, chemical thinners may even affect bloom via interference withthe vegetative development of the tree. In the present paper, in additionto a short discussion of the essentials of the flower-formation process inpome fruits, the effect of a number of chemical thinners on flower formationis reviewed. It is argued that in most experimental studies the data on theeffect of thinning on return bloom is insufficiently detailed, and a betterunderstanding especially of the early phases of the flower-formationprocess is badly needed.

516 Predicting Fruit Set and Evaluating the Effects of Chemical Thinners on Apples

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.

659 Reducing Floral Initiation and Return Bloom in Fruit Trees— Applications and Implications in Fruit Production: Pome Fruit

Pome fruit display a biennial bearing tendency that is characterized by heavy flowering and fruit set one year followed by a year with reduced bloom and fruit set. This tendancy results in a year with heavy cropping with small fruit, and a subsequent year with large fruit and a small crop. Both situations are undesirable. Chemical thinners in the “on” year are frequently used to modify this cropping behavior. Alternative methods to control cropping by flower bud inhibitions will be discussed. Gibberellin application in the “off” year at or soon after bloom will inhibit flower bud formation and encourage moderate flowering. This method of crop regulation has been used infrequently. Gibberellins differ in their ability to inhibit flowering. Therefore, selection of a specific gibberellin and an effective concentration range may provide greater flexibility in controlling flowering. The cytokinins CPPU and thidiazuron inhibit flower bud formation, increase fruit size, and also thin fruit. Appropriate application of these cytokinins will be discussed where beneficial effects on fruit size may be achieved while maintaining a moderate level of flower bud formation.

224 Effect of Accel and Ethephon on Fruit Set and Sugar Content of Three Apple Cultivars

The influence of chemical thinners Accel and ethephon on three apple cultivars Royal Gala, Blushing Gold, and Ultra Gold was investigated. Two experiments were conducted in 1995 and 1996 to determine the effect of Accel at 0, 25, 50, and 75 ppm and ethephon at 0, 100, 200, and 300 ppm on fruit juice SSC, fruit juice sucrose, fruit juice glucose, fruit juice fructose, and fruit set. Accel and ethephon reduced fruit set of `Royal Gala', `Ultra Gold', and `Blushing Golden'. In 1995, chemical fruit thinners Accel and ethephon increased SSC of `Royal Gala' and `Ultra Gold' and did not affect SSC of `Blushing Golden'. In 1996, Accel and ethephon did not effect the SSC of `Royal Gala' and `Blushing Golden'. However, Accel increased SSC of `Ultra Gold'. Accel and ethephon increased sucrose concentration of `Royal Gala', `Ultra Gold', and `Blushing Golden'. Accel increased fruit juice glucose concentration of `Royal Gala', `Ultra Gold', and `Blushing Golden'. Ethephon did not effect fruit juice glucose concentration of `Royal Gala', `Ultra Gold', and `Blushing Golden'. Accel did not effect fruit juice fructose concentration of `Royal Gala'. However, Accel increased fructose levels of `Ultra Gold' and `Blushing Golden' in 1995. Accel did not effect fruit juice fructose concentration of `Blushing Golden' in 1996. Ethephon did not effect fruit juice fructose concentration of `Royal Gala'. Ethephon increased fruit juice fructose concentration of `Ultra Gold'. Ethephon did not effect fruit juice fructose concentration of `Blushing Golden'.

Chemicals, Timing, and Environmental Factors Involved in Thinner Efficacy

Chemical thinning of apples may be done over a 3- to 4-week period, starting at bloom and extending through the time fruit are 20 mm or larger in diameter. Potentially useful thinners at bloom include ammonium thiosulfate, Wilthin, endothall, and pelargonic acid. These thinners reduce set by preventing pollination, inhibiting pollen germination, or slowing pollen tube growth. Ethephon can be an effective thinner at bloom, but it is erratic. Petal fall application of thinners has become widely adopted by the industry, primarily because it reduces risks associated with one time thinner application, routinely done in the past. Carbaryl is often the thinner of choice at this time, but where more aggressive thinning is desired, NAA is used either alone or in combinations with carbaryl. Naphthale-neacetamide is effective when used at bloom but it less frequently used. Apple fruitlets are very vulnerable to the stress imposed by chemical thinners at the 8- to 12-mm stage of fruit development. NAA and Accel are particularly effective at this time, but carbaryl is frequently used because of its mild thinning action and its ability to reduce clustered spurs to one fruit. Unless weather conditions after bloom have been cool, effective thinning after the 18-mm stage of fruit development is less effective. Although erratic, ethephon is the most effective thinner on large fruit. Warm temperature during and following thinner application favor thinner uptake and response. Longer spray drying times favor thinner uptake into the plant. Environmental influences and interactions influencing thinning will be discussed.