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  • Author or Editor: Jacqueline K. Burns x
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Two formulations of the plant growth regulator dikegulac (2,3:4,6-di-O-isopro-pylidene-α-L-xylo-2-hexulofuranosoic acid), consisting of dikegulac-sodium (Atrimmec) or dikegulac:ascorbic acid (1:1) (DAA), as well as 5-chloro-3-methyl-4-nitro-pyrazole at 200 mg·L-1, were applied as foliar sprays to `Hamlin' and `Valencia' orange trees (Citrus sinensis L. Osbeck) at two dates during the harvest season for each cultivar (11 Nov. and 10 Jan. for `Hamlin', 22 Mar. and 25 May for `Valencia'). Fruit detachment force was evaluated 10 days after application, whereas cumulative leaf abscission was monitored up to 60 days after application. In both cultivars, Atrimmec and DAA at 3,000 mg·L-1 induced moderate fruit loosening when applied at the earlier application date, but fruit loosening improved when applied at the later application date. In `Hamlin', both formulations caused higher leaf abscission when applied at the later date. DAA applications resulted in low leaf loss in `Valencia' regardless of application time, whereas Atrimmec caused unacceptably high leaf loss at either application date. No differences in internal fruit quality were found as a result of any abscission material treatment. The results indicate that DAA could be a promising option to induce fruit loosening in late harvested `Valencia' orange trees with minimal undesirable side effects.

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The effects of 2 consecutive years of annual defoliation during the harvest season on fruit size, yield, juice quality, leaf size and number were examined in trees of the midseason cultivar `Hamlin' and the late-season cultivar `Valencia' orange [Citrus sinensis (L.) Osb.]. In `Hamlin', removal of up to 50% of the leaves in late November had no effect on fruit yield, fruit number, fruit size, soluble solids yield, juice °Brix, and °Brix to acid ratio of juice the following year. In `Valencia', removal of 50% of the leaves in late March decreased fruit yield and soluble solids yield but did not affect Brix or the Brix to acid ratio of the juice. Leaf size of new flush was reduced by removal of 50% of the leaves in both cultivars but there was little effect on total canopy size. There were no measured effects of removing 25% of leaves from tree canopies. Thus, canopy growth, fruit yield, fruit quality, and leaf size were not negatively impacted when annual defoliations did not exceed 25% of the total canopy leaf area in `Valencia' and `Hamlin' orange trees for two consecutive years. Overall, fruit weight increased linearly with increasing ratio of leaf area to fruit number, suggesting that fruit enlargement can be limited by leaf area.

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The California table olive (Olea europaea L.) industry relies exclusively on hand harvesting of its primary Manzanillo cultivar. Increased harvesting costs have intensified industry interest in identifying an abscission agent that can be used with developing mechanical harvesting technologies to increase removal rates. Table olives are harvested immature green at horticultural maturity but before physiological maturity. The goal of this research was to reevaluate the potential of ethylene-releasing compounds (ERCs) as olive-loosening agents and to screen additional candidates previously shown to accelerate citrus fruit abscission. Eleven compounds were screened at two separate table olive-growing sites (Fresno and Tehama counties) in California in September until Nov. 2006. Compounds were applied at various concentrations alone or in combination. Fruit detachment force (FDF) and percent fruit drop were measured and leaf loss assessed. Of the compounds evaluated, the ERC ethephon (2-chloroethyl phosphonic acid) and 1-aminocyclopropane-1-carboxylic acid were the most efficacious. In whole tree applications, concentrations of ethephon or 1-aminocyclopropane-1-carboxylic acid above 1000 mg·L−1 reduced FDF to less than 50% of the untreated control within 17 days, but leaf drop increased with increasing concentrations. Addition of 1-methylcyclopropene reduced efficacy of ethephon and delayed leaf drop. Monopotassium phosphate + ethephon (4% and 1000 mg·L−1, respectively) reduced FDF and leaf loss was equivalent to the ethephon alone treatment. Compounds such as methyl jasmonate, coronatine, dikegulac, MAXCEL, traumatic acid, and 5-chloro-3-methyl-4-nitro-1H-pyrazole were not efficacious.

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The application of methyl jasmonate (MeJA) to grapes (Vitis vinifera L.) may decrease fruit detachment force (FDF) and promote the development of dry stem scars on the berries, both of which could improve the quality of machine-harvested raisin grapes. However, treatment with MeJA also promotes preharvest fruit drop, which is undesirable. Thus, experiments were conducted to determine how the concentration of MeJA applied and time after treatment affect FDF and abscission of grapes. Mature ‘Thompson Seedless’ grapevines were treated with one of five different solutions containing 0, 0.2, 2, 10, or 20 mm MeJA, and FDF and fruit abscission were monitored for ≈2 weeks. Treatment with 2 mm or less MeJA had inconsistent effects on FDF and did not promote abscission, whereas treatment with 10 to 20 mm MeJA reduced FDF within 2 to 3 days after treatment (DAT) and promoted abscission, which began on ≈3 DAT and persisted for ≈8 DAT. Thus, to optimize the use of MeJA as a harvest aid for ‘Thompson Seedless’ may require application of between 2 and 10 mm MeJA followed by harvest within 3 DAT.

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In Florida, the combined use of mechanical harvesters and the abscission agent 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) for late-season harvesting (May to June) of fruit of ‘Valencia’ orange is effective at removing mature fruit with minimal adverse effects on the subsequent season's crop. However, CMNP can cause fruit peel scarring, and no data were available on how this affects peel integrity and potential losses resulting from fruit crushing and/or decay before processing. In this study, two late-season harvest dates were tested in commercial orchards during 2009 and 2010. Harvesting treatments consisted of combinations of two mechanical harvester ground speeds (0.8 and 1.6 km·h−1), two harvester shaker head frequencies (185 and 220 cycles/min), and CMNP foliar applications (4 days before harvesting) at 250 and 300 mg·L−1 in a spray volume of 2810 L·ha−1 plus mechanically-harvested and hand-picked controls. After harvesting, fruit samples were randomly collected from each block for peel resistance and postharvest decay evaluations. Peel resistance was determined by measuring both peel puncture force and fruit crush force. Fruit used to study postharvest decay were stored at 27 °C and 50% relative humidity or ambient conditions and evaluated daily for 8 days. Peel resistance was unaffected by mechanical harvesting combinations or CMNP application. No significant effects on postharvest decay were found among treatments for at least 3 days after harvest. However, a significant increase in postharvest decay between CMNP-treated and untreated fruit began between 4 and 6 days after harvest such that by 8 days after harvest, decay was as high as 25% in CMNP-treated fruit. The results indicate that CMNP can be safely used in combination with late-season mechanical harvesting under the conditions described in this study without losses resulting from fruit crushing or decay for at least 3 days, a time period well within the normal commercial harvest-to-processing time of ≈36 h.

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Coronatine is a polyketide phytotoxin produced by several plant pathogenic Pseudomonas spp. The effect of coronatine on abscission in Citrus sinensis L. Osbeck `Hamlin' and `Valencia' orange fruit, leaves, fruitlets, and flowers was determined. Coronatine at 200 mg·L-1 significantly reduced fruit detachment force of mature fruit, and did not cause fruitlet or flower loss in `Valencia'. Cumulative leaf loss was 18% with coronatine treatment. Coronafacic acid or coronamic acid, precursors to coronatine in Pseudomonas syringae, did not cause mature fruit abscission. Ethylene production in mature fruit and leaves was stimulated by coronatine treatment, and 1-aminocyclopropane-1-carboxylic acid oxidase (ACO) and 12-oxo-phytodienoate reductase (12-oxo-PDAR) gene expression was upregulated. A slight chlorosis developed in the canopy of whole trees sprayed with coronatine, and chlorophyll content was reduced relative to adjuvant-treated controls. Leaves formed after coronatine application were not chlorotic and had chlorophyll contents similar to controls. Comparison of coronatine to the abscission compounds methyl jasmonate, 5-chloro-3-methyl-4-nitro-pyrazole and ethephon indicated differences in ethylene production and ACO and 12-oxo-PDAR gene expression between treatments. Leaf loss, chlorophyll reduction and low coronatine yield during fermentation must be overcome for coronatine to be seriously considered as an abscission material for citrus.

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Table olives (Olea europaea) traditionally are hand harvested when green in color and before physiological maturity is attained. Hand harvesting accounts for the grower's main production costs. Several mechanical harvesting methods have been previously tested. However, tree configuration and fruit injury are major constraints to the adoption of mechanical harvesting. In prior work with a canopy shaker, promising results were attained after critical machine components were reconfigured. In this study, stereo video analysis based on two high-speed cameras operating during the harvesting process were used to identify the sources of fruit damage due to canopy-harvester interaction. Damage was subjectively evaluated after harvest. Fruit mechanically harvested had 35% more bruising and three times as many fruit with broken skin as that of hand-harvested fruit. The main source of fruit damaged in the canopy was the strike-impact of fruit by harvester rods. Implementation of softer padding materials were effective in mitigating fruit injury caused by the impact of rods and hard surfaces. Canopy acceleration was correlated with fruit damage, thus restricting improvements needed for fruit removal efficiency through increased tine frequency.

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Postbloom fruit drop (PFD) of citrus is incited by the fungus Colletotrichum acutatum J. H. Simmonds and may result in young fruit drop and severe yield losses. Previous studies suggested that imbalance of growth regulators such as auxin, ethylene, and jasmonic acid (JA) plays an essential role in young fruit abscission. In this work, we determined the factors associated with fungal-induced fruit drop by testing compounds inhibitory to hormonal transport or biosynthesis. As assessed on sweet orange (Citrus sinensis Osbeck) and grapefruit (C. paradisi Macf.) for 4 years, we found that many auxin transport and action inhibitors such as 2,3,5-triiodobenzolic acid (TIBA), 2-(4-chlorophenoxy)-2-methyl-propionic acid (clofibrate), or quercetin and JA biosynthesis inhibitors such as salicylic acid (SA) and aspirin (methyl-SA) applied 7 d after C. acutatum infection resulted in higher percentages of young fruit retention compared with the water controls. The commercial products ReZist and Actigard, widely used as systemic acquired resistance (SAR) agents, also improved fruit retention. Furthermore, application of gibberellic acid (GA3) on sweet orange, regardless of C. acutatum infection, significantly increased fruit retention. These commercial products may be very useful in managing this destructive disease of citrus in the field.

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