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D.L. Peterson, S.S. Miller and J.D. Whitney

Three years of mechanical harvesting (shake and catch) trials with two freestanding apple (Malus domestica Borkh.) cultivars on a semidwarf rootstock (M.7a) and two training systems (central leader and open center) yielded 64% to 77% overall harvesting efficiency. Mechanically harvested `Bisbee Delicious' apples averaged 70% Extra Fancy and 10% Fancy grade, while two `Golden Delicious' strains (`Smoothee' and `Frazier Goldspur') averaged 40% Extra Fancy and 13% Fancy grade fruit. Mechanically harvesting fresh-market-quality apples from semidwarf freestanding trees was difficult and its potential limited. Cumulative yield of open-center trees was less than that of central-leader trees during the 3 years (sixth through eighth leaf) of our study. `Golden Delicious' trees generally produced higher yields than `Delicious' trees.

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W.J. Kender, U. Hartmond, M. Salyani, J.K. Burns and J.D. Whitney

A field experiment was conducted to determine effects of concentration and spray volume of metsulfuron-methyl as an abscission aid for mechanical harvesting of citrus. Concentrations of 1, 2, and 4 mg·L–1 metsulfuron-methyl were applied to `Hamlin' orange [Citrus sinensis (L.) Osbeck] trees at 470, 1900, and 4700 L·ha–1 (0.5 to 19 g·ha–1 a.i.). Effective fruit loosening was achieved with all applications >1.9 g·ha–1 (4 mg·L–1 at all volumes, 2 mg·L–1 at 1900 and 4700 L·ha–1, and 1 mg·L–1 at 4700 L·ha–1). Heavy defoliation and twig dieback were observed on trees receiving 2 and 4 mg·L–1 at all volumes. Defoliation and dieback became more severe and flower development and fruit set were inhibited as fruit loosening increased. The use of metsulfuron-methyl as an abscission agent for `Hamlin' oranges is not recommended until conditions for its safe application can be determined. Chemical names used: methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl).

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U. Hartmond, J.D. Whitney, J.K. Burns and W.J. Kender

Two field studies were conducted to evaluate the effect of metsulfuron-methyl and 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMN-pyrazole) on abscission of `Valencia' orange [Citrus sinensis (L.) Osbeck] during the 3-month harvest season. Solutions of metsulfuron-methyl at 0.5, 1, and 2 mg·L-1 active ingredient (a.i.) were applied at 10-day intervals beginning on 13 Feb. and ending 18 May 1998. Early in the harvest season, 1 or 2 mg·L-1 metsulfuron-methyl significantly reduced fruit detachment force (FDF) 14 days after application. Metsulfuron-methyl was less effective during a 4- to 6-week period following bloom (“less-responsive period”). After this period, metsulfuron-methyl regained the ability to loosen fruit. Applications of 2 mg·L-1 a.i. were more effective than 1 mg·L-1 in reducing FDF and causing leaf drop, but 0.5 mg·L-1 a.i. had little or no effect on FDF. Flowers and leaflets on developing shoots and young fruit completely abscised with 1 and 2 mg·L-1 a.i. Defoliation and twig dieback was extensive at all concentrations and spray dates, eliminating metsulfuron-methyl as a commercially viable abscission agent for citrus. In a separate experiment CMN-pyrazole at 50 and 100 mg·L-1 a.i. and metsulfuronmethyl at 0.5 mg·L-1 a.i. were applied to `Valencia' trees to determine fruit removal with a trunk shake and catch harvesting system. Application of both abscission materials before and after the “less-responsive period” resulted in a 10% to 12% increase in fruit removal when compared to control trees. Less than a 35% reduction in FDF was sufficient to significantly increase fruit removal. Only 100 mg·L-1 a.i. CMN-pyrazole significantly increased fruit removal when applied during the “less-responsive period.” Chemical names used: Methyl-2-(((((4-Methoxy-6-Methyl-1,3,5-Triazin-2-yl)-Amino)Carbonyl) Amino)Sulfonyl)Benzene (Metsulfuron-methyl); 5-Chloro-3-methyl-4-nitro-1-H-pyrazole (CMN-pyrazole).

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T.A. Wheaton, W.S. Castle, J.D. Whitney and D.P.H. Tucker

`Hamlin' and `Valencia' oranges [Citrus sinensis (L.) Osb.], `Murcott' tangor (C. reticulata Blanco × C. sinensis), and `Redblush' grapefruit (C. paradisi Macf.) on 15 rootstock and own-rooted cuttings were planted at a 1.5 × 3.3-m spacing providing a density of 2020 trees/ha. Growth rate, productivity, and fruit quality varied among the scion and stock combinations. Combinations of moderate vigor and precocious fruiting performed better than very vigorous or dwarfing materials. Several freezes slowed canopy development and delayed production. Most trees had filled their allocated canopy space 7 years after planting. At that age, the orange trees yielded 23 to 75 t·ha-1. Scion and stock combinations with desirable vigor and fruiting characteristics were satisfactory in this high-density planting. However, there appears to be little advantage of high tree density under Florida conditions, and moderate densities of fewer than 1000 trees/ha may be preferable.

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T.A. Wheaton, J.D. Whitney, W.S. Castle, R.P. Muraro, H.W. Browning and D.P.H. Tucker

A factorial experiment begun in 1980 included `Hamlin' and `Valencia' sweet-orange scions [Citrus sinensis (L.) Osb.], and Milam lemon (C. jambhiri Lush) and Rusk citrange [C. sinensis × Poncirus trifoliata (L.) Raf.] rootstocks, tree topping heights of 3.7 and 5.5 m, between-row spacings of 4.5 and 6.0 m, and in-row spacings of 2.5 and 4.5 m. The spacing combinations provided tree densities of 370, 494, 667, and 889 trees ha. Yield increased with increasing tree density during the early years of production. For tree ages 9 to 13 years, however, there was no consistent relationship between yield and tree density. Rusk citrange, a rootstock of moderate vigor, produced smaller trees and better yield, fruit quality, and economic returns than Milam lemon, a vigorous rootstock. After filling their allocated space, yield and fruit quality of trees on Milam rootstock declined with increasing tree density at the lower topping height. Cumulative economic returns at year 13 were not related to tree density.