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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), two harvester shaker head frequencies (185 and 220 cycles/min), and CMNP foliar applications (4 days before harvesting) at 250 and 300 mg L in a spray volume of 2810 L ha 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 8C 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. Florida produced 6.5 million t of sweet oranges 1⁄2Citrus ·sinensis (L.) Osbeck during the 2008–2009 harvest season from 200,000 ha (Florida Agricultural Statistics Service, 2010). More than 95% of this fruit is used to produce orange juice, making Florida the second largest orange juice producer in the world. Harvesting is the single largest cost associated with citrus production in Florida, accounting for up to 50% of the total cost of production (Muraro, 2009). This is in contrast with Brazil, the largest producer of orange juice in the world, where harvesting costs are much lower such that during the 2000–2001 harvest season, Brazilian citrus growers were able to deliver frozen concentrated orange juice to the port of Tampa, FL, for only $0.0757 more per pound soluble solids than Florida growers. This discrepancy in production costs has been a major driving force in Florida’s efforts to develop mechanical harvesting technology (Florida Department of Citrus, 2010; Whitney, 1995). Adoption of mechanical harvesting systems has been slow. During the 2008–2009 harvest season, less than 15,000 ha (7% of total acreage) of citrus was mechanically harvested in Florida (Florida Department of Citrus, 2009). A number of issues have contributed to the slow adoption of mechanical harvesting technology, including processor concerns about the quantity of debris mixed with mechanically harvested fruit (Spann and Danyluk, 2010), production manager concerns about the effects of mechanical harvesting on tree health (Li et al., 2005), and issues related to the late-season harvesting of ‘Valencia’ oranges (Melgar et al., 2010). ‘Valencia’ orange trees typically bloom in late February through March in Florida and the mature crop is harvested beginning in March and extending into mid-June. Thus, there is an overlap between the young growing fruit and mature fruit. Studies have shown a significant reduction in the subsequent year’s yield when ‘Valencia’ orange trees are mechanically harvested after 1 May (Hedden et al., 1984; Roka et al., 2005; Whitney, 1975). This is approximately the point when the young developing fruit have obtained a large enough size ( 2.5 cm diameter) and mass that they will be removed by the canopy shaker mechanism along with the mature fruit. Because of the ever-growing demand for not-fromconcentrate (NFC) orange juice, processors require that growers hold fruit on the trees later into the season to provide fresh fruit as long as possible. Thus, efforts have been underway to find a viable way to harvest ‘Valencia’ orange fruit after 1 May without impacting subsequent year yields. The abscission agent CMNP effectively loosens mature citrus fruit without affecting immature fruit, allowing mechanical harvest of ‘Valencia’ orange fruit after 1 May without impacting subsequent year yields (Burns et al., 2006). However, CMNP can sometimes cause peel injury consisting of a brown ring of depressed flavedo tissue around the stylar end of fruit where the CMNP spray solution accumulates (Alferez et al., 2006). This injury, commonly referred to as peel scarring, is associated with phospholipase A2 (PLA2) activity and can be prevented by inhibiting PLA2 with aristolochic acid (Alferez et al., 2006). No further characterization of this scarring has been conducted and processors are concerned that it may reduce peel integrity and/or increase fruit decay before processing. In Florida, harvested oranges for processing are transported to the processing plant in open-topped semitrailers that hold 27 t of fruit. Fruit are stored until processing in these trailers under ambient conditions. Generally, fruit are transported from the orchard to the processing plant the day of harvest and are processed within 24 h. However, processors are concerned that fruit treated with CMNP, because of the scarring that can occur, may have weaker peels and be more susceptible to damage during transport and storage or may begin to decay more quickly than untreated fruit. Received for publication 4 Feb. 2011. Accepted for publication 25 Apr. 2011. We thank Robert Ebel and Peter Newman for their technical assistance with the field aspects of this research. We also express our appreciation to OXBO International Corp. for use and technical assistance with the multiple-fan sprayer and mechanical harvester and AgroSource, Inc. for providing the abscission agent. We thank Taw Richardson and Mark Trimmer for their helpful advice and support. To whom reprint requests should be addressed; e-mail spann@ufl.edu. 1006 HORTSCIENCE VOL. 46(7) JULY 2011 Our objective was to determine if fruit treated with CMNP and subjected to mechanical harvesting have reduced peel integrity, measured as peel puncture force and fruit crush force, or are more susceptible to postharvest decay within the commercial timeframe of normal harvest to processing. Materials and Methods Plant material and culture. The trials were conducted in a 21-year-old commercial citrus orchard of ‘Valencia’ sweet orange trees on ‘Carrizo’ citrange rootstock (C. sinensis · Poncirus trifoliata) near Immokalee, FL, managed according to standard practices (Obreza and Morgan, 2008; Parsons and Morgan, 2010). The trees were spaced 3.0 · 6.5 m (500 trees/ha) with an average canopy height of 4.2 m and width of 3.8 m maintained by annual mechanical hedging. The lower limbs of the canopy had been mechanically skirted to 1 m above the soil. Fruit was harvested at two dates during the lateseason time period in two consecutive seasons, 8 and 26 May 2009 and 4 and 29 May 2010. 5-Chloro-3-methyl-4-nitro-1H-pyrazole treatments and harvesting. Four d before each harvest date, the trees were sprayed with the abscission agent CMNP at 250 mg a.i./L (2009) or 300 mg a.i./L (2010) in a spray volume of 2810 L ha using a multifan vertical airblast sprayer (Model 4000L; Oxbo International, Clear Lake, WI) as described by Ebel et al. (2009). Unsprayed trees served as controls. Trees were harvested 4 d after CMNP application using a tractor-powered continuous travel canopy shake harvester (Model 3210; Oxbo International). In 2009, the harvester was operated at a ground speed of 0.8 or 1.6 km h in combination with two shaker head frequencies (185 and 210 cycles/min) for a total of four ground speed · shaker head frequency combinations. In 2010, the harvester was operated at 1.6 km h with a shaker head frequency of 220 cycles/min. In 2009 only, both CMNP-treated and untreated trees were harvested by hand as additional controls. Four eight-tree plots of each treatment combination (ground speed · shaker head frequency · CMNP treatment for 2009; CMNP treated and untreated for 2010) were harvested each year at both harvest times. Fruit puncture and crush force. Sixty fruit were randomly sampled from each plot at the time of harvest and transported to the laboratory. All data collection (fruit crush force, peel puncture force, and peel deflection) was performed the day after harvest. Two 10-fruit subsamples from each plot were used for fruit puncture and crush force measurements, respectively. Fruit puncture force was measured at one point per fruit along the fruit equator (stem axis oriented perpendicular to the punch) using a digital texture analyzer (Model TA-XTZi; Texture Technologies Corp., Scarsdale, NY) equipped with a 0.632-cm diameter punch and operated at a speed of 5 mm s. Puncture force (Newtons) and maximum fruit peel deflection (millimeters) before puncture were recorded. The digital texture analyzer used for fruit puncture force did not have sufficient capacity to crush whole citrus fruit. Thus, to measure fruit crush force, a device was built to consistently squeeze each fruit against a flat surface at a uniform speed while continuously recording the force applied until each fruit ruptured. The device consisted of a box 0.5 · 0.3 · 0.6 m (width · depth · height) constructed from plywood (20 mm thick). Inside the box was a plywood platform that held a

hortscience

Application of the Abscission Agent 5-Chloro-3-methyl-4-nitro-1H-pyrazole Does Not Affect Peel Integrity or Postharvest Decay of Mechanically Harvested Late-season Fruit of ‘Valencia’ Orange during the Normal Commercial Harvest-to-processing Period