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Frank T. Yoshikawa, G.C. Martin, D. Ramos, and J.T. Yeager

Various rates of Wilthin were applied at full bloom to limbs carrying 150 to 250 flowers to study their activity on blossom thinning of `Loadel' peaches. Wilthin applied at 0.75% and 1.0% significantly reduced fruit set to 29% and 30%, respectively, while the control produced 94%. The effectiveness of the 0.75% rate was dramatic, but it is interesting to note that the 1.0% rate did not lead to excessive thinning nor phytotoxicity on foliage or fruit. More extensive studies need to be done to fully determine the potential of this material. However, these results suggest that further testing of Wilthin on a larger scale is warranted.

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Ejaz Mohammad Ansari, Frank B. Matta, Taqueer Abbas, and Mohammad Baquir

The influence of bloom chemical thinner Wilthin 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 Wilthin at 0%, 0.5%, 1.0%, and 1.5% on fruit length, fruit diameter, fruit weight, yield, juice pH, acidity, SSC, sucrose, glucose, fructose, fruit firmness, and fruit set. Wilthin reduced fruit set of `Royal Gala', `Ultra Gold', and `Blushing Golden' and increased yield of all cultivars. In general, Wilthin increased fruit weight of `Royal Gala', `Ultra Gold', and `Blushing Golden' both years. Wilthin increased fruit length and diameter of `Royal Gala' and `Ultra Gold', but it did not affect fruit length of `Blushing Golden'. Wilthin increased fruit juice pH of `Royal Gala', `Ultra Gold', and `Blushing Golden' during both years. Wilthin decreased fruit juice acidity of `Royal Gala', `Ultra Gold', and `Blushing Golden'. In 1995, Wilthin increased SSC of `Royal Gala' and `Ultra Gold'. In 1996, Wilthin did not effect SSC of `Royal Gala' and `Blushing Golden', but increased SSC of `Ultra Gold'. Wilthin increased sucrose concentration of `Royal Gala', `Ultra Gold', and `Blushing Golden'. Wilthin did not effect fruit juice glucose concentration of `Royal Gala' but increased fruit juice glucose concentration of `Ultra Gold' and `Blushing Golden'. Wilthin did not influenced fructose content of `Royal Gala' and `Blushing Golden' apples. Wilthin increased fructose contents of `Ultra Gold'. Accel increased fruit firmness of `Royal Gala', `Ultra Gold', and `Blushing Golden' and increased fruit firmness of `Royal Gala'.

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Stephen C. Myers, Amy T. Savelle, D. Stuart Tustin, and Ross E. Byers

Partial thinning of peach (Prunus persica L. Batsch) during bloom to 50% of the necessary level by hand, and followed by adjustment hand thinning at 42 days after full bloom (DAFB) was compared to a similar degree of thinning accomplished entirely at 42 DAFB by hand. Partial flower thinning altered the distribution of fruit by diameter, increasing the percentage of large diameter (≥62.0 mm) fruit harvested compared to unthinned trees or trees thinned entirely at 42 DAFB. Although shoot number per limb was not altered by thinning time, the distribution of shoots by length was affected, increasing the percentage of long shoots (≥20.0 cm). Compared to unthinned trees and trees thinned at 42 DAFB, partial flower thinning increased the subsequent development of flower buds per shoot and the number of flower buds per node. Number of flower buds on the proximal five nodes of shoots 15.0-30.0 cm in length was increased, although not on shoots 5.0-7.0 cm in length. Additional trials established that airblast spray application of AMADS was effective in achieving a similar level of thinning as that accomplished by partial flower thinning by hand in previous experiments. The degree of flower removal exhibited a linear response to chemical concentration. Fruit diameter on chemically flower-thinned trees was greater at adjustment thinning time, when compared to trees thinned by hand at 42 DAFB only. Distribution of fruit at harvest indicated a larger percentage of fruit >65.0 mm in trees which received partial flower thinning in comparison to trees thinned at 42 DAFB only. As a result, overall crop value was increased, based on the commercial processing peach price structure at the time of harvest. Chemical name used: 1-aminomethanamide dihydrogen tetraoxosulfate (AMADS)

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Stephen S. Miller and Thomas Tworkoski

slow and difficult. Numerous chemical flower thinners of apple and peach have been tested, including DNOC (Na 4,6-dinitro- ortho -cresylate; Elgetol), ethephon (2-chloroethylphosphonic acid), pelargonic acid, monocarbamidedihydrogen sulphate (Wilthin

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Esmaeil Fallahi, Brenda R. Simons, and Max W. Williams

Effects of hydrogen cyanamide and Wilthin on blossom thinning and the consequences of thinning on fruit set, yield and fruit quality of `Rome Beauty' was studied. A full bloom application of hydrogen cyanamide at the rate of 0.25% (Dormex formulation) or 0.25% of Wilthin both followed by a fruit thinning by Sevin + NAA effectively thinned mature trees of `Rome Beauty' and had a similar effect on fruit set, yield and fruit quality. The effects of these two chemicals at these rates on several aspects of fruit set, yield and quality were similar to the effects of Elgetol. Hydrogen cyanamide, Elgetol and 0.25% Wilthin at full bloom resulted in a higher percentage of single fruit set, thus, less labor for hand thinning. Application of 0.37% Wilthin at 20% bloom or at full bloom resulted in larger fruit size, but induced fruit russetting. Soluble solids of fruit from trees with Elgetol, 0.37% Wilthin at 20% bloom or at full bloom were higher than fruit from other treatments. Hydrogen cyanamide at 0.50% resulted in a satisfactory level of blossom thinning in `Friar' plums.

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Joshua D. Klein and Shlomo Cohen

Y. Fluksman of Efal Industries, Herzliya, Israel, for assistance in obtaining WILTHIN. Contribution from the ARO–Volcani Center, no. 1400-E, 1994 series. The cost of publishing this paper was defrayed in part by the payment of page charges. Under

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Max Williams

Chemicals being tested for bloom thinning of apples are effective for bloom thinning of stone fruit. Sulfcarbamide (Wilthin) and Endothall applied to peaches, nectarines, and apricots at 90% of bloom open reduced fruit set by 50%. Fruit size and quality of crop were improved. Slight phytotoxicity occurred on leaves and twigs, but no injury occurred on fruit. Two years of data will be presented and comparisons will be made with other new thinning agents.

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Max W. Williams

Two chemicals being tested as blossom thinners for apple are effective for bloom thinning of peaches and nectarines. Monocarbamide dihydrogen sulfate (Wilthin) and Endothall applied at 90% of bloom open reduced fruit set by 50%. Fruit size and quality of the crop were improved. Both of the above chemicals gave adequate fruit removal without serious phytotoxic effects on leaves, buds or shoots. With chemical rates which over thinned, no fruit marking occurred on either nectarines or peaches.

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Jason L. Osborne and Terence L. Robinson

In 2004, we conducted a chemical thinning field study in Appleton, N.Y., on 5-year-old `Rising Star' peach trees on Lovell rootstock. Treatments included soybean oil or petroleum oil applied at 8% about 30 days before budbreak. Ammonium thio-sulfate (ATS) 3.5 gal/acre, ATS 5.0 gal/acre, lime sulfur (1%, 3%) plus Crockers fish oil 2 gal/acre, and Wilthin 6 pt/acre were applied at FB; and the grower standard hand-thinning treatment at 45 DAFB. Trees treated with thinning agents were not given supplemental hand thinning. The high rate of ATS, 5.0 gal/acre and Wilthin 6 pt/acre had the greatest thinning effect and reduced fruit set by 55% and 61%, respectively, compared to the untreated control. The high rate of ATS also increased fruit size 25%, but reduced yield by 45%. Soybean and petroleum oil treatments did not significantly reduce fruit set. Lime sulfur plus fish oil treatments 1% and 3% also did not significantly reduce fruit set. Although a significant reduction in yield was observed in the high rate ATS and Wilthin treatments, a greater proportion of the crop was in the larger size categories. In 2005, treatments included soybean oil 8% plus Latron B 1956 applied 18 days and 25 days before FB, Lime sulfur (2%, 4%) plus Crockers fish oil (2%) applied at FB, Ammonium thio-sulfate (ATS) 3.5%, 5.0%, Wilthin 1.9, 2.8 L (Entek, Inc.), plus Regulaid 473 mL per 935 L/ha applied at FB, Entry 1.5, 3.0%, Tergitol TMN-6 0.75, 1.5%, hand-thin flowers to a crop load of seven fruits per cm2 at FB and hand-thin fruit to 7 fruits per cm2 applied 45 days after FB.

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Duane W. Greene, Karen I. Hauschild, and James Krupa

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.