Blossom Thinning Research Articles

USE OF NEW BLOSSOM THINNERS TO REDUCE FRUIT SET AND IMPROVE FRUIT QUALITY IN APPLES, PEACHES, NECTARINES, AND PLUMS

ISHS XXVII International Horticultural Congress - IHC2006: International Symposium on Endogenous and Exogenous Plant Bioregulators USE OF NEW BLOSSOM THINNERS TO REDUCE FRUIT SET AND IMPROVE FRUIT QUALITY IN APPLES, PEACHES, NECTARINES, AND PLUMS

Assessment of Potential Organic Pollenicides as Apple Blossom Thinners

The potential of six products to act as pollenicides on apple pollen both in vivo and in vitro was evaluated. The six candidate pollenicides used were lime sulphur (LS) (1.15% solution), ammonium thiosulphate (ATS) (0.75% solution), Surround™ WP (S) (59.2 g/L), biodiesel (BD) (5% emulsion), sodium chloride (NaCl) (8 g/L) and Horticultural-Vinegar (HV) (2% solution). It was found that ‘Golden Delicious’ blossoms that were treated with ATS, LS, S or NaCl had significantly fewer pollen grains that germinated in vivo than the treatment receiving no spray. The effect of the six candidate pollenicides was also investigated in vitro by spraying ‘McIntosh’ pollen with the candidate pollenicides and examining pollen germination, pollen tube growth length and the proportion of unviable pollen grains. All of the treatment sprays significantly reduced pollen germination compared with the control, with ATS and LS resulting in considerable reduction in pollen tube germination and HV completely hindering germination. Among those treatments that allowed some germination, only LS, ATS and BD effectively reduced the mean length that the pollen tubes grew in vitro. These laboratory results are compared with those generated in a parallel field trial to assess what stage of pollination the candidate pollenicides are targeting. Taken together these results indicate that ATS reduces pollen viability and germination on pollinated stigmas and on germination medium and is an effective blossom thinner in the field. Lime sulphur has a greater effect on pollen tube germination and growth on germination medium and is a good thinner in the field and that NaCl reduces pollen germination on pollinated stigmas and is an effective mild thinner in the field. Finally, this research suggests that while BD was an effective thinner in the field, it has less pollenicidal and greater phytotoxic effects than the Other products tested.

Physiological and Horticultural Effects of Sweet Cherry Chemical Blossom Thinners

This article reports on the physiological effects and horticultural benefits of chemical blossom thinners on 9-year-old and 12-year-old `Bing'/`Gisela®5′ sweet cherry trees in 2004 and 2005, respectively. Chemical thinning agents were applied at 20% and 80% full bloom (FB) by air-blast sprayer and were comprised of: 2% ammonium thiosulphate (ATS), 4% vegetable oil emulsion (VOE), 2% fish oil + 2.5% lime sulfur (FOLS), 1% tergitol, and an untreated control. Leaf gas exchange, leaf SPAD meter readings, chlorophyll fluorescence parameters, fruit yield, and fruit quality were evaluated. FOLS, tergitol, VOE, and ATS suppressed leaf net CO2 exchange rate (NCER) by 33%, 30%, 28%, and 18%, respectively, over a variable length recovery period directly after 80% FB treatment. Leaf NCER recovered fully from every thinning treatment. Reductions in leaf NCER were unrelated to gS. VOE reduced estimated leaf chlorophyll content the greatest, suppressing overall levels by 11% for 23 days after treatment. All blossom thinners reduced constant fluorescence (Fo). No thinning agent reduced fruit set or yield in 2004. ATS, FOLS, and tergitol reduced fruit set in 2005. VOE was ineffective as a thinner yet exhibited significant leaf phytotoxicity. Among thinners, there was no relationship between inhibition of leaf NCER and thinning efficacy.

Crop Load, Fruit Thinning and their Effects on Fruit Quality of Apple (Malus domestica Borkh.)

Crop load, a quantitative parameter used by industry, is generally defined as the number of fruit per tree. It is often expressed in terms of number of fruit per trunk cross-sectional area (fruit/TCSA). Crop load is the most important of all factors that influence fruit size, and the removing of a part of the crop is the most effective way to improve fruit size.The potential size of a given pome fruit is determined early in the season and growth proceeds at a relatively uniform rate thereafter. This uniform growth rate permits the accurate prediction of the harvest size of the fruit as early as mid-summer. The growth rate, once established, is not easily altered, and fruit numbers, therefore, can affect fruit size only within definite limits and maximum effectiveness requires adjustment in fruit numbers relatively early in the season. It was established, that „thinning does not change a potentially small fruit into a large fruit, but rather insures that a potentially large fruit will size properly.” Emphasis should be on estimating fruit numbers rather than fruit size.Fruit thinning can quickly reach the point of diminishing returns. Rather than a high percentage of large fruits, the objectives of thinning should be the elimination of the smallest fruits, improved fruit quality and annual production. Fruit thinning is accomplished by hand or chemical thinning. Chemical thinners are separated into categories as bloom thinners and post-bloom thinners. Early removal of potential fruit (blossom thinning) is currently used in many apple producing areas to enhance flower initiation for next year’s crop and thus, return bloom. It also results in reduced competition for photosynthates. Blossom thinners usually have a caustic effect on floral parts.The amount of fruit left on a tree should be determined by the vigor and general condition of the tree. Leaf area per fruit affects the number of spurs flowering the following season. It can be difficult to separate timing and fruit number effects in crop loading studies, as abscission rates after hand thinning of retained flowers/fruitlets tend to very with the time of hand thinning.

Tergitol-TMN-6 Surfactant Is an Effective Blossom Thinner for Stone Fruits

Effects of Tergitol-TMN-6 surfactant on blossom thinning (fruit set), fruit quality, and yield were studied in different cultivars of peach (Prunus persica [L.] Batsch) during 2003 to 2005, and in one cultivar of nectarine Prunus persica [L.] in one orchard and one cultivar of plum (Prunus domestica [L.]) in two orchards in 2004. In addition to Tergitol-TMN-6, effects of Crocker's fish oil (CFO) alone in three peach cultivars or in combination with lime sulfur in a nectarine cultivar were studied on fruit set, quality, and yield. Tergitol-TMN-6 at 5 mL·L–1 or higher rates, applied at about 75% to 85% bloom, reduced fruit set without russeting peach fruit. Peach fruit size was often increased by Tergitol-TMN-6 treatment. Applications of Tergitol at 20 mL·L–1 or 30 mL·L–1 excessively thinned peaches. Tergitol-TMN-6 at all rates burned foliage, but the symptoms disappeared after a few weeks without any adverse effects on tree productivity. Tergitol-TMN-6 at 7.5 mL·L–1 or 10 mL·L–1, applied either once at about 80% to 85% bloom or twice at 35% bloom and again at 80% to 85% bloom, reduced fruit set without any fruit russeting in nectarine. Tergitol-TMN-6 at 7.5 mL·L–1 to 12.5 mL·L–1 reduced fruit set in `Empress' plum. CFO at 30 mL·L–1 was effective in blossom thinning of some peach cultivars. A combination of lime sulfur and CFO was not effective in blossom thinning of nectarine. Considering results from several orchards in different locations in the Pacific Northwest over 3 years, Tergitol-TMN-6 is an excellent blossom thinner for peach, nectarine, and plum at rates of 7.5 to 12.5 mL·L–1, sprayed at a spray volume of 1870.8 L·ha–1 when about 75% to 85% blooms are open.

Blossom Thinners Reduce Crop Load and Increase Fruit Size and Quality of Peaches

The purpose of this research was to determine the effectiveness of three blossom-thinning compounds on crop density and fruit quality of two peach cultivars. Treatments consisted of 15 ml·L–1 and 30 ml·L–1 ammonium thiosulphate, 30 ml·L–1 and 40 ml·L–1 decyl alcohol, and 40 ml·L–1 lime sulfur. Treatments were applied to `Redhaven' and `Harrow Diamond' peach trees at two phenological stages: 80%, and 100% full bloom in 2002 and 2003. In both years, treatments reduced the crop density in both cultivars, and in 2003 the amount of hand thinning required to adjust the crop load was significantly reduced. Fruit size from several blossom-thinned treatments was comparable with that observed from hand-thinned trees. However, treatments caused significant leaf phytotoxicity to `Harrow Diamond' trees in 2003, likely a result of hand spray gun applications. These data indicate that chemical sprays at bloom can be used successfully to reduce fruit set, but are very environmentally, dose, and cultivar dependent.

Chemical Blossom Thinners Vary in Their Effect on Sweet Cherry Fruit Set, Yield, Fruit Quality, and Crop Value

The commercial adoption of the relatively new rootstock `Gisela 5' (Prunus cerasus L. × P. canescens L.) has been limited in the United States sweet cherry (P. avium L.) industry despite its ability to induce precocity and productivity and reduce scion vigor compared to the standard Mazzard (P. avium). This is due in large part to inadequate crop load management that has led to high yields of small fruit. This paper reports on sweet cherry chemical blossom thinning trials conducted in 2002 and 2003. Two percent ammonium thiosulphate (ATS), 3% to 4% vegetable oil emulsion (VOE), and tank mixes of 2% fish oil + 2.5% lime sulphur (FOLS) were applied to entire 8- and 9-year-old `Bing'/`Gisela 5' sweet cherry canopies at about 10% full bloom (FB) and again at about 90% FB. In both years, ATS and FOLS reduced fruit set by 66% to 33% compared to the control (C). VOE reduced fruit set by 50% compared to C in 2002 but had no effect in 2003. In 2002, fruit yield was 30% to 60% lower from thinned trees. In 2003, fruit yield was unaffected by thinning treatment. In 2002, ATS and FOLS improved fruit soluble solids but had no effect in 2003. VOE did not affect fruit soluble solids in 2002 and reduced fruit soluble solids by 12%, compared to C, in 2003. In 2002, each thinning treatment nearly eliminated the yield of the small fruit (≤21.5-mm diameter) and increased yield of large fruit (≥26.5 mm) by more than 400%, compared to C. In 2003, ATS and FOLS did not affect yield of small fruit but increased the yield of large fruit by 60%. In 2003, VOE-treated trees yielded 4.3 kg of small fruit per tree compared to about 0.15 kg from C, suggesting a phytotoxic response to VOE beyond that which may effect thinning. Compared to C, ATS and FOLS consistently reduced fruit set and improved fruit quality. We conclude that commercially acceptable yields of excellent quality `Bing' sweet cherries can be grown on size-controlling and precocious rootstocks.

Effect of blossom stage of ‘Alpine’ nectarine on efficacy of chemical thinning using Armothin

Fruit thinning of peach and nectarine cultivars is essential to maximize profitability and stabilize production and is routinely done by hand thinning of blossoms and early fruit. Chemical thinning of blossoms may reduce the cost of manual labour, but results are inconsistent and commercialization is lacking. Blossom sensitivity to caustic agents may depend on blossom type and degree of pollination and/or fertilization. Armothin®, a registered blossom thinning agent, was evaluated at 1.5% v/v and 3% v/v applied at 11, 17 and 42% full-bloom, to evaluate the effect of blossom type and the fertilization and/or pollination of blossoms, on thinning of ‘Alpine’ nectarine. The earliest application (11% full-bloom) of 3% v/v Armothin® provided optimum thinning and fruit size. Any blossoms showing pink appear to be removed by Armothin®, while open blossoms are removed irrespective of whether they are fertilised and/or pollinated. The removal of early opening blossoms results in the potential loss in early ripening, higher value fruit especially in an early cultivar. Later maturing blossoms remain to develop into later maturing fruit which tend to be smaller. Thus an early application of 3% Armothin®, when less than 50% blossoms show pink, may provide sufficient blossom thinning to expedite later hand thinning, but does not necessarily remove the need for hand thinning.

Application of ammonium thiosulfate for blossom thinning in apples

The efficacy of ammonium thiosulfate (ATS) in blossom thinning was investigated as a function of concentration, drying time and flower development stage at the time of treatment. In 2-year-old crabapple trees, flower damage caused by ATS, applied at a concentration of 10 g/l, increased as the drying time of the ATS spray increased. Approximately 40% of flowers were damaged when the ATS spray dried in 30 min, significantly less than the 80% damage observed when the drying time was 71 min. Application of ATS at 2% resulted in high levels of flower damage regardless of drying time. Fully developed open flowers were more susceptible to ATS damage than closed flowers. Ethylene does not appear to have a major role in mediating the thinning activity of ATS, as the levels of flower and leaf damage were unaffected by the use of an ethylene inhibitor. In ‘Jonagold’ apples, ATS at either 5 or 10% provided excellent thinning but was phytotoxic unless washed off the trees. The effects of drying time and flower development stage on ATS activity can be countered by applying ATS at a concentration of 5% at petal fall then washing the tree with water within 1 h of application to avoid excessive thinning and leaf damage.

RESPONSE OF TWO APPLE CULTIVARS TO POTASSIUM THIOSULPHATE AS A BLOSSOM THINNER

RESPONSE OF TWO APPLE CULTIVARS TO POTASSIUM THIOSULPHATE AS A BLOSSOM THINNER

INFLUENCE OF NEW BLOSSOM THINNERS ON FRUIT SET AND FRUIT QUALITY OF APPLES

INFLUENCE OF NEW BLOSSOM THINNERS ON FRUIT SET AND FRUIT QUALITY OF APPLES

Solution Characteristics Affect Pollen Germination and Growth, and Pistil Viability: Implications for Developing Chemical Blossom Thinners for Apple

A strategy of chemical crop load control has been to use chemical desiccants to prevent fertilization and cause fruitlet drop. However, little is known of the solution characteristics that reduce pollen viability, inhibit pollen germination and growth, and cause pistil damage. This project was established to determine the solution characteristics effecting those results. Apple pollen was dispersed on germination media mixed with PEG (MW 10,000) to attain osmotic tensions from 0 to -5.0 MPa to evaluate effect on pollen germination and growth. Similarly, apple pollen was dispersed on germination adjusted to a range of pH from 2.3 to 12.0 with acids and NaOH. Excised apple pistils were place on filter paper supports saturated with solutions with osmotic tension adjusted by PEG in the range of 0 to -5.0 M Pa, and pH from 3.0 to 12.0. Solutions of osmotic tension in the range of 0 to -5.0 M Pa were applied by brush to intact pistils on apple flowers in a greenhouse and under field conditions. Pollen germination decreased with increasing osmotic tension and no pollen germinated at tensions greater than 4.0. Pistils, either excised or intact, had significant desiccation and death when treated with solution osmotic tensions greater than 4.0. Fruit set of individual spurs of the cvs Jonagold, Gala, and Arkansas Black were highly related to pistil survival 48 h after treatment with PEG. When solution osmotic potential exceeded 4.0, fruit set was reduced by more than 80%. Pollen germination was reduced by more than 50% at solution pH below or equal to 4.0 and greater than 10.0 and completely inhibited at solution pH below or equal to 3.0 and greater than 11.0. Similar results were observed for excised pistil and intact viability.

AUXINIC BLOSSOM THINNERS (MCPB-ETHYL AND NAA) INHIBIT RETURN FLOWERING ON FUJI/M.26 APPLE TREES

AUXINIC BLOSSOM THINNERS (MCPB-ETHYL AND NAA) INHIBIT RETURN FLOWERING ON FUJI/M.26 APPLE TREES

PRESENT STATUS OF ORGANIC FRUIT GROWING IN EUROPE

Organic fruit growing in Europe has experienced remarkable growth rates since the mid 1990's. Southern states, especially Italy, Spain and France have the largest land area with organic fruit, are also growing olives, citrus and chestnuts. Mainly increasing interest of supermarket chains is responsible for this buoyancy, but also the availability of better plant protection products e.g. granulosis virus and mating disruption against codling moth, and Neem oil against Rosy Apple Aphid. State subsidies varying from 600 to more than 1600 Euro /ha/y in the EU-countries (15) are less decisive for the conversion of top fruit production. Market share of organic table fruit is only 1 to 2 %, reaching 4 to 5 % in Switzerland. For Switzerland, we estimate a market potential of around 12 to 15 %, which is already achieved with organic vegetables. In order to reach that percentage, better solutions for several key problems have to be found, e.g. control of scab, fire blight, sooty blotch, brown rot, weed management, fertilisation and crop load regulation. Also the assortment of organically produced “modern-standard” varieties is not satisfactory, in particular with stone fruit. The economics of organic fruit growing is comparatively healthy, however, it depends on receiving a one third higher farm gate price for the product. In Switzerland the benefit of organic orchards is 16 % higher compared to integrated fruit production; but labour hours exceed those of IFP by 7%, due to blossom thinning by hand, manual weed control and mice control. Supermarkets have a tendency to just “substitute” conventional with organic fruit if requiring organic fruit from disease susceptible varieties with no cosmetic blemishes. This can/does feed back to the growers resulting in “substitutional” production with disease and pest sensitive orchards managed with intensive “organic” spray and fertilisation programs. This certainly does not correspond with either the original concept of organic farming or with expectations of organic consumers. Thus, still a lot of development - also on the marketing side - has to be undertaken.

BLOSSOM THINNING AS A STRATEGY IN CHEMICAL THINNING OF APPLES IN NEW ENGLAND

BLOSSOM THINNING AS A STRATEGY IN CHEMICAL THINNING OF APPLES IN NEW ENGLAND

COMPARISON OF NEW BLOSSOM THINNERS FOR APPLES UNDER CONDITIONS OF INTERMOUNTAIN WEST OF THE UNITED STATES

ISHS XXVI International Horticultural Congress: Key Processes in the Growth and Cropping of Deciduous Fruit and Nut Trees COMPARISON OF NEW BLOSSOM THINNERS FOR APPLES UNDER CONDITIONS OF INTERMOUNTAIN WEST OF THE UNITED STATES

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.

Thinning `Fuji' Apple Blossoms with Synthetic Auxins (MCPB-ethyl or NAA) and Ethephon with or without Postbloom Thinning with Carbaryl

Three experiments were conducted at two locations, two at Summerland, British Columbia, Canada and one at Corvallis, Ore., to evaluate synthetic auxins (MCPB-ethyl or NAA) and ethephon as blossom thinners for `Fuji' apple [Malus sylvestris (L.) Mill var. domestica (Borkh.) Mansf.]. These experiments also involved application of carbaryl at 1000 mg·L-1 in the postbloom period. All blossom thinners were sprayed at 85% full bloom while carbaryl was applied at 11-mm fruit diameter. Within these experiments, MCPB-ethyl at up to 20 mg·L-1 or NAA at up to 21 mg·L-1 increased whole flower cluster removal linearly with rate; however, with the Corvallis experiment MCPB-ethyl failed to result in any thinning. Neither auxin treatment consistently reduced fruit set on the remaining clusters, resulting in “clustering”. Bloom-time application of ethephon at 100 mg·L-1 with NAA further reduced crop load. Carbaryl reduced total crop load by increasing both whole cluster removal and number of sites with a single fruit. Return flowering was not improved by the auxin treatments except where there was very excessive crop reduction. Ethephon or carbaryl promoted return flowering with the carbaryl effect being more pronounced. However, this carbaryl effect was significantly countered by the bloom-time auxin whereas ethephon overcame the negative effects of the auxin treatments. The combined use of ethephon and carbaryl was effective in terms of both crop reduction and return flowering benefits. Chemical names used: 1-naphthyl N-methylcarbamate (carbaryl); 2-chloroethylphosphonic acid (ethephon); ethyl 4-(4-chloro-2-methylphenoxy) butanoate (MCPB-ethyl); and 2-(1-naphthyl) acetic acid (NAA).

Effect of Blossom Thinners on Fruit Set and Fruit Size of Peaches

Endothall (mono-N,N-dimethylalkylamine salt, 15.9%, endothallic acid 3.8%), ammonium thiosulfate (ATS, 55%), and Wilthin (1-aminomethanamide dihydrogen tetraoxosulfate, 79%) were evaluated over 3 years for use as blossom thinners on mature `Garnet Beauty' and `Red Haven' peaches (Prunus persica). Endothall and ATS were the most effective blossom thinners and reduced set in all 3 years. Wilthin reduced initial set in 2 of 3 years. Hand thinning was reduced by 50% to 80% on endothall and ATS treated trees. The reduction in crop load at bloom resulted in significant increases in fruit size at harvest. We suggest the use of ATS at a rate of 35 to 45 L·ha-1 (3.5 to 5 gal/acre) and endothall at 1.8 L·ha-1 (1.5 pt/acre), applied in 935 L·ha-1 (100 gal/acre) on mature trees. The use of Wilthin at rates higher than 18.6 L·ha-1 (8 qt/acre) may be required for adequate thinning, but phytotoxicity at higher rates was not tested. Other important components for successful blossom thinning include applying materials before the majority of flowers have been pollinated (slightly before full bloom) and to apply the spray when there is very littlewind to assure good coverage and to prevent localized areas displaying increased phytotoxicity.