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  • Author or Editor: Jacqueline K. Burns x
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`Valencia' oranges were-treated with an experimental polysaccharide-based coating, a commercial shellac-based water wax, or were left uncoated. The fruit were stored at 16 or 21C with 95% RH. Samples were periodically analyzed for internal gases, flavor volatiles, water loss, and `Brix. Coated fruit had lower internal O2 and higher CO2 and ethylene levels as well as higher levels of many flavor volatiles (including ethanol) compared to uncoated. The differences were greatest for shellac-coated fruit at the higher storage temperature. No differences were found for °Brix. The shellac-coating gave the best weight-loss control and the most restricted gas exchange. The low gas permeability characteristic of this type of shellac coating may result in altered flavor for fruit held at 21C.

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The abscission action of two sulfonylureas and one imidazolinone was evaluated in laboratory studies with harvested orange (Citrus sinensis L. cv. Valencia) fruit and greenhouse studies with orange (cv. Hamlin) and grapefruit (Citrus paradisi Macf. cv. Marsh) trees. Dipping harvested fruit in 90 mg·L–1 imazameth, 2 mg·L–1 metsulfuronmethyl, or 30 mg·L–1 prosulfuron solutions increased levels of internal ethylene. Internal ethylene concentration was higher when fruit were dipped in 2 mg·L–1 metsulfuron-methyl solutions at low pH. Fruit retained on trees and dipped in 2 mg·L–1 metsulfuron-methyl solutions produced more ethylene than control fruit. Drop of treated fruit began when ethylene production was at a maximum. High temperatures (average 33 °C) suppressed ethylene production and fruit drop of metsulfuron-methyl–treated fruit. The results indicate the importance of environmental conditions in evaluating the potential of sulfonylureas and imidazolinones as abscission agents for citrus. Chemical names used: ±-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid (imazameth); methyl 2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2yl) amino] carbonyl] amino] sulfonyl] benzoate (metsulfuron-methyl); 1-(4-methoxy-6-methyl-triazin-2-yl)-3-[2-(3,3,3-trifluoropropyl) phenylsulfonyl] urea (prosulfuron); N-(phosphonomethyl) glycine (glyphosate); 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1 H-imidazol-2-yl]-3-quinolinecarboxylic acid (imazaquin).

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A study was initiated in `Hamlin' orange to determine if a selective citrus abscission material, 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP), could improve mature fruit removal and recovery when used with mechanical harvesters. A trunk shaker and continuous-moving canopy shaker equipped with catch-frames were used at the flatwoods and ridge growing regions in Florida. Both trials were conducted during the first 2 weeks of Dec. 2003. Plots were constructed as randomized complete blocks containing four replicates for each treatment. Each replicate contained five trees at the ridge site or four trees at the flatwoods site. CMNP was applied using a commercial airblast sprayer at 0, 125, 250 and 500 mg·L-1 at a rate of 2,800 L·ha-1 4 days before scheduled harvest. The trunk shaker was operated for either 7 seconds or 2 seconds/tree, whereas the canopy shaker was operated at either 260 cpm or 140 cpm/tree. The data show that the correct time was selected for harvest. Over a 50% reduction in FDF was achieved with the 250 and 500 mg·L-1 treatments, while post-application fruit drop was less than 1.5%. The greatest benefit of abscission agent use was seen when the mechanical harvesters were operated at the least aggressive setting (2 seconds or 140 cpm), where increases in % fruit removal and recovery were over 20% higher than the controls. At the most aggressive settings, numeric increases in % fruit removal and recoveries were measured, but these changes were not statistically significant. The results demonstrated that statistically similar % fruit removal and recoveries could be achieved using less aggressive harvester settings with abscission agent use when compared with most aggressive settings using no abscission agent.

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Effective abscission agents that decrease fruit detachment force (FDF) are sought by the California raisin industry to improve the continuous tray mechanical harvesting method. Such agents might also enable mechanical harvest of table and wine grapes (Vitis vinifera L.), but few agents are known to be effective for grape. Thus, methyl jasmonate (MeJA) and six other compounds known to stimulate abscission of other fruits were screened for their ability to reduce FDF of mature ‘Thompson Seedless’ grapes. Most compounds tested reduced FDF to some extent, but MeJA was particularly effective. Solutions containing between 45 and 4500 ppm MeJA reduced FDF by at least 50% to 85% compared with nontreated fruits. Application of 2250 and 4500 ppm MeJA to ‘Thompson Seedless’ vines caused 25% to 50% fruit drop, respectively, within 10 d after treatment (DAT). The efficacy of MeJA was verified in a second experiment in which solutions of 0, 1125, 2250, or 4500 ppm MeJA were applied to clusters of ‘Crimson Seedless’ grapes; at 14 DAT, FDF declined as a linear function of MeJA applied. The grapes did not abscise, but berries treated with 2250 to 4500 ppm MeJA had slightly lower soluble solids than nontreated fruits. Solutions of 0 or 4500 ppm MeJA applied to clusters of ‘Cabernet Sauvignon’ and ‘Merlot’ grapevines reduced FDF by 66% and 75%, respectively. Fruit drop was estimated to be less than 10%. Thus, a solution containing up to 4500 ppm MeJA may be an effective abscission agent to facilitate mechanical harvest of ‘Cabernet Sauvignon’ or ‘Merlot’.

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The abscission compound CMNP (5-chloro-3-methyl-4-nitro-1H-pyrazole) was applied to fully mature sweet orange trees at different spray volumes using a vertical, multiple-fan air-blast sprayer to determine distribution of fruit loosening throughout the canopy and subsequent effects on mechanical harvester efficiency. CMNP was applied at 0, 935, 1871, and 2806 L·ha−1 in three ‘Valencia’ and one ‘Hamlin’ grove. Spray coverage was measured using water-sensitive paper and fruit loosening was measured by fruit detachment force (FDF). Spray coverage and FDF were measured at 1-, 2-, and 4-m height within the canopy and inside the canopy near the trunk and on the periphery of the canopy. Spray coverage increased with volume of CMNP applied. Spray coverage was higher at 4 m than 1 and 2 m, which were similar. Spray coverage within the canopy was decreased almost half compared with that of the periphery. FDF was unaffected by spray volume at the different heights except in one trial where fruit had higher FDF at 4 m. Fruit inside the canopy did not loosen as much as fruit outside the canopy in three of the four trials. FDF inside the canopy averaged 52 to 84 N, whereas fruit on the periphery of the canopy averaged 50 to 74 N. CMNP promoted fruit drop, but only in two trials was the amount over 5% of the total yield for the 2806-L·ha−1 treatment. The fruit were harvested by canopy shakers that captured fruit on catch frames, except one of the ‘Valencia’ trials in which the canopy shaker did not have a catch frame. The percent of the total crop removed by the harvesters increased when CMNP was applied at higher spray volumes except in the ‘Hamlin’ trial in which there was no difference among volume treatments. The percent of the total crop removed by the harvester but not captured by the catch frame increased at higher volumes of CMNP applied for two of the three trials in which catch frames were used. Fruit loss with greater volume of CMNP applied was promoted by peripheral canopy contact with the front shield of the harvester that knocked fruit down before the catch frame moved under that portion of the canopy. Recovery percentage, or the percentage of total yield that was caught and conveyed to bulk collection by the harvester catch frame, averaged 78.1% to 87.8% of total yield. Higher CMNP volume with increased removal rate compensated for higher catch frame loss, providing overall higher recovery percentage. Based on the goals of minimizing fruit drop and maximizing fruit recovery, the range of FDF that should be reached by harvest is 40 N to 65 N for canopy shakers equipped with catch frames. These trials underscore the importance of adequate CMNP coverage for reducing in-canopy variation of fruit loosening and maximizing fruit removal.

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Colletotrichum acutatum J. H. Simmonds infects citrus flower petals, causing brownish lesions, young fruit drop, production of persistent calyces, and leaf distortion. This suggests that hormones may be involved in symptom development. To identify the types of hormones, cDNA clones encoding proteins related to ethylene and jasmonate (JA) biosynthesis, indole-3-acetic acid (IAA) regulation, cell-wall modification, signal transduction, or fruit ripening were used to examine differential gene expressions in calamondin (Citrus madurensis Lour) and/or `Valencia' sweet orange (Citrus sinensis Osbeck) after C. acutatum infection. Northern-blot analyses revealed that the genes encoding 1-aminocyclopropane-1-carboxylate (ACC) oxidase and 12-oxophytodienoate required for ethylene and JA biosynthesis, respectively, were highly up-regulated in both citrus species. Both gene transcripts increased markedly in petals, young fruit and stigmas, but not in calyces. The transcripts of the genes encoding IAA glucose transferase and auxin-responsive GH3-like protein, but not IAA amino acid hydrolyase, also markedly increased in both species 5 days after inoculation. The expansin and chitinase genes were slightly up-regulated, whereas the senescence-induced nuclease and ß-galactosidase genes were down-regulated in calamondin. No differential expression of transcripts was detected for the genes encoding expansin, polygalacturonase, and serine-threonine kinase in sweet orange. As compared to the water controls, infection of C. acutatum increased ethylene and IAA levels by 3- and 140-fold. In contrast, abscisic acid (ABA) levels were not significantly changed. Collectively, the results indicate that infection by C. acutatum of citrus flowers triggered differential gene expressions, mainly associated with IAA, ethylene, and JA production and regulation, and increased hormone concentrations, consistent with the hypothesis of the involvement of phytohormones in postbloom fruit drop.

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Methyl jasmonate (MJ) was tested as a potential abscission chemical to enhance mechanical harvest of `Hamlin' and `Valenica' orange [Citrus sinensis (L.) Osb.]. In field experiments, a solution of 1, 5, 10, 20, or 100 mm MJ was applied either as a stem wrap to individual fruit or as a spray to entire trees or canopy sectors. Solutions of 10, 20, and 100 mm MJ resulted in significant and consistent reduction of fruit detachment force and caused fruit drop within 7 to 10 days. Fruit loosening was preceded by an increase in the internal ethylene concentration of fruit similar to that of other experimental abscission compounds. While concentrations of 10 mm and less caused no or negligible phytotoxicity, solutions exceeding 10 mm MJ induced unacceptable levels of leaf abscission.

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1-MCP is a gaseous ethylene binding inhibitor that controls or delays ethylene-related postharvest problems in a range of horticultural commodities. Our previous work demonstrated that exposure of calamondin to 1-MCP 16 hours before canopy sprays of ethephon greatly reduced unwanted leaf drop while only partially inhibiting the ability of ethephon to cause fruit loosening. The objective of this work was to determine whether formulated 1-MCP (SmartFresh) could be used in the field to stop defoliation caused by abscission agent applications without significantly altering abscission agent-induced fruit loosening. Spray solutions containing 400 mg·L-1 ethephon with 0, 1, 2.5, and 5 mm 1-MCP were applied to canopies of `Hamlin' and `Valencia' (Citrus sinensis). Timing of 1-MCP applications was a) 24 hours before, b) in combination with, or c) 24 hours after ethephon. Ethephon at 400 mg·L-1 significantly reduced fruit detachment force (FDF) but caused >70% leaf drop within 15 days after application in both cultivars. Applications of 1-MCP reduced ethephon-associated leaf abscission but had little effect on the ability of ethephon to reduce FDF. Timing of 1-MCP applications did not affect the ability of ethephon to cause fruit loosening; however, the best consistent treatment for control of leaf drop was achieved with the combined application of 5 mm 1-MCP and 400 mg·L-1 ethephon. 1-MCP was used in combination with the abscission agents coronatine, methyl jasmonate (MeJa) and 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) to determine its effect on leaf drop and fruit loosening. Leaf drop in trees treated with ethephon, coronatine, and MeJa was reduced by addition of 1-MCP. However, fruit loosening was largely prevented when 1-MCP was used in combination with coronatine or MeJa. Like ethephon, CMNP-induced fruit loosening was not affected by 1-MCP. The results demonstrate the ability to control ethephon-induced leaf abscission without affecting mature fruit loosening by targeting ethylene binding in citrus.

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An abscission agent (5-chloro-3-methyl-4-nitro-1H-pyrazole [CMNP]) at 300 mg·L–1 in a volume of 2810 L·ha–1 was applied to Valencia orange trees [Citrus sinensis (L.) Osb.] on 22 May 2004. At this time, immature and mature fruit were present on the tree simultaneously. Three days after application, fruit were mechanically harvested using a trunk-shake-and-catch system. The power to the shaker head was operated at full- or half-throttle (FT or HT, respectively), and the duration of trunk shaking was 2 seconds at FT or 4 seconds at FT and HT. Mature fruit removal percentage and number of immature fruit removed, and fruitlet weight and diameter were determined. Mature fruit removal percentage with 2 seconds at FT or 4 seconds at FT harvesting ±CMNP, or 4 seconds at HT + CMNP was not significantly different and ranged between 89% to 97%. Harvesting at 4 seconds HT without CMNP removed significantly less mature fruit than any treatment. CMNP did not affect immature fruit removal by the trunk shaker. Harvesting at 4 seconds at HT removed significantly less immature fruit than 2 seconds at FT or 4 seconds at FT. No significant difference in fruitlet weight or diameter was measured between any trunk shaker harvest operation and CMNP treatment. Trunk shaking frequency was estimated to be 4.8 and 8.0 Hz at HT and FT, respectively. Yield in 2005 was determined on the same trees used for harvest treatments in 2004. CMNP did not impact yield. No significant difference in yield was seen between the hand-picked control and 4 seconds at HT, whereas yield in the remaining treatments was lower. The results demonstrate that CMNP application combined with low frequency trunk shaker harvesting can achieve high percentage of mature fruit removal with no significant impact on return yield of the following crop.

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This study was conducted to determine the relationship of 5-chloro-3-methyl-4-nitro-1H-pyrazole (CMNP) concentration and canopy shaker frequency on fruit detachment force, pre-harvest fruit drop, and mechanical harvesting fruit removal of ‘Hamlin’ and ‘Valencia’ sweet orange cultivars. CMNP was applied at 0, 200, and 300 mg·L−1 in a carrier volume of 2806 L·ha−1. Four days after CMNP application, fruit were harvested with a canopy shaker that was operated at 3.0, 3.7, and 4.3 Hz at a tractor speed of 1.6 km·h−1. The experiment was repeated 3× for ‘Hamlin’ (December, early January, and late January) and twice for ‘Valencia’ (March and April) during the 2008–2009 harvest season. Fruit detachment force was reduced by at least 50% for all CMNP-treated trees compared with the untreated controls at the time of harvest and was lower for 300 mg·L−1 than 200 mg·L−1 on three of the five dates tested. Pre-harvest fruit drop evaluated immediately before mechanical harvesting was higher for all CMNP-treated ‘Hamlin’ than untreated controls at all harvest dates, whereas 300 mg·L−1 application resulted in higher pre-harvest fruit drop in ‘Valencia’ when compared with 200 mg·L−1 or the untreated controls on both application dates. CMNP-induced fruit drop was higher in ‘Hamlin’ than ‘Valencia’. CMNP had a greater effect on fruit removal at lower canopy shaker frequencies. The interaction of total fruit weight removed was not significant on any date as a result of variability among trees in the study. These data indicate that the amount of loosening by CMNP was concentration-dependent and facilitated removal, especially with lower canopy shaker frequencies. Development of viable commercial practices should use the percent of the total crop harvested and not the actual weight of fruit removed in determining efficacy of CMNP and harvest efficiency of the mechanical harvesters.

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