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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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Richard Marini

In five experiments with `Redchief Delicious' and one with `Braeburn', oxamyl (Vydate 2L) was used alone or combined with other chemicals to thin apples. The thinning response to oxamyl depended on dose. In most cases, oxamyl at 600 mg·L−1 and carbaryl at 900 mg·L−1 thinned trees similarly, but the combination of oxamyl plus carbaryl was no more effective than either chemical alone. The combination of oxamyl plus NAA (2.5 to 5 mL·L−1) was slightly more effective than either material alone. The thinning response to oxamyl and carbaryl was related to the concentration of superior oil added to the spray solution; for both chemicals, adding oil at 5 mg·L−1 or Tween 20 at 1.25 mL·L−1 gave equivalent thinning. Apples on trees sprayed with oxamyl plus oil had a dull finish. Adding Tween 20 at 1.25 mL·L−1 improved the thinning activity of carbaryl (Sevin XLR-Plus) more than oxamyl. Similar thinning occurred whether oxamyl was applied when fruit diameter averaged 4 or 10 mm. On `Braeburn' oxamyl, carbaryl, Accel, and NAA were mild thinners, but all combinations of oxamyl or carbaryl plus Accel or NAA overthinned the trees without improving fruit size. In general, oxamyl at 600 mg·L−1 (2 pints of vydate 2L/100 gal.) and carbaryl thin apple trees similarly, and the efficacy of both chemicals is improved by adding a surfactant.

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Richard Marini

Twenty-eight-year-old `Starkrimson Delicious' trees were spur pruned in 1986 and 1987 and/or treated with BA + GA4+7 in 1986 in an attempt to improve spur growth and fruit weight. Yield, fruit weight, and spur quality characters were recorded for 1986-1989. All treatment combinations failed to improve yield or fruit weight. Although spur-pruning improved spur length, spur bud diameter, leaf area per spur and leaf dry weight per spur, fruit weight was not improved. BA + GA4+7 reduced yield and fruit weight, and increased the number of pygmy fruit in 1986, but had little effect on fruiting for the three years after treatment.

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Richard P. Marini

Mature `Norman'peach [Prunus persica (L.) Batsch] trees were dormant pruned to retain a range of fruiting shoots per tree (71 to 250) during 3 years from 1997 to 1999. About 40 days after bloom each year, fruits on all trees were thinned to similar crop loads, so only the number of fruits per shoot varied. Fruit set and number of fruits removed by hand thinning were positively related to number of fruiting shoots retained per tree. Number of fruits harvested per tree was not related to number of shoots per tree, whereas average fruit weight at thinning and at harvest, and crop value per tree were negatively related to the number of shoots retained per tree. These results indicate that commercial peach producers should consider modifying pruning and thinning strategies. Rather than retaining a large number of fruiting shoots per tree and hand thinning to distribute fruits every 15 to 20 cm along each fruiting shoot, producers should first determine the number of fruits that trees of a given cultivar can adequately size and then perform the thinning operation to obtain the desired crop load. The number of fruiting shoots retained per tree during pruning should be one-fifth to one-seventh of the number of fruits desired per tree, so that five to seven fruits per fruiting shoot are retained after hand thinning.

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Richard P. Marini

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Richard P. Marini

Chemical fruit thinners were applied to limbs or whole trees of spur `Delicious' at various stages of fruit development as indicated by fruit diameter. Carbaryl, naphthalene acetic acid (NAA), and ethephon all reduced fruit set when applied at a fruit diameter of ≈4 to 15 mm. Fruit thinning for NAA and carbaryl, alone or combined, generally was greater when applied at an average fruit diameter of 8 mm, rather than at 4 mm. Repeated applications of NAA or carbaryl were no more effective than single applications. NAA + carbaryl applied at 9 mm was more effective than NAA applied at 4 mm followed by carbaryl at 8 mm. Applied when fruit diameter averaged 17 to 22 mm, ethephon and ethephon + carbaryl were effective fruit thinners. When applied at full bloom to ≈10 and 20 mm, the insecticides ethion and oxamyl, respectively, were effective fruit thinners.

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Richard P. Marini

`Norman' peach trees were trained to the central-leader or open-vase form and were planted at high (740 trees/ha) or low (370 trees/ha) density. A third density treatment was a high/low density, where alternate trees in high-density plots were removed after 6 years to produce a low-density treatment. Annual yield per hectare was ≈15% to 40% greater for high-density treatments than for low-density treatments, but tree form had little influence on yield. Fruit size tended to be greater for low-density than for high-density treatments, but cumulative marketable yield was greatest for high-density and lowest for high/low density treatments. After 9 years, cumulative crop value was higher for open-vase than central-leader treatments (P = 0.12), but tree density had less of an effect on crop value (P = 0.21). Cumulative costs were highest for high-density treatments, but were not influenced by tree form. Income minus costs was nearly $4500/ha higher for open-vase than for central-leader trees and net present value was more than $2000/ha higher for high-density than low-density trees (P = 0.20). Open-vase trees were more profitable than central leader trees and should be planted at densities of about 700 trees/ha in the mid-Atlantic region.

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Richard P. Marini

Average fruit weight from two apple-thinning experiments was estimated by sampling 20-fruit/tree or harvesting all fruit on three branches/tree. The estimated values were compared with the true average fruit weight calculated from the entire crop on a tree. The value of a fruit was calculated from packout data obtained from the two sampling methods and was compared to the true value obtained from the entire tree. Statistical techniques, typically used by biometritions in medical research, were used to assess the agreement between the values obtained with the estimation methods and the true values. Estimates of average fruit weight obtained from 20-fruit/tree may differ from the true value by about 13% and estimates obtained from weighing all fruit on three limbs/tree may be within about 11% to 19% of the true mean. Estimates of fruit value obtained from a 20-fruit sample may differ from the true value by about 4 cents per fruit and estimates from three limbs/tree may differ from the true mean by about 7 cents per fruit. Analysis of variance was performed on each data set and the resulting P values differed for the three methods of estimating fruit weight and fruit value. Thus, erroneous conclusions may result from experiments where fruit weight and fruit value is estimated from relatively small samples.

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Richard P. Marini

Counting blossoms before treatment and collecting yield data for whole trees following thinning treatments arc expensive practices. Researchers often collect data on sample branches to reduce the time and expense of data collection. How accurate are these techniques? To generate discussion concerning sampling, results will he presented for several experiments where data were collected for whole trees vs. sample branches. Data will also be presented for different ways of assessing fruit size at thinning time. Fruit diameter, fruit weight and fruit volume were recorded for several cultivars over several seasons. I hope to generate discussion about the desirability of these different measures of fruit development.