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This study evaluated the effects of huanglongbing (HLB) symptom severity on fruit detachment force (FDF) and fruit mechanical properties in sweet oranges (Citrus sinensis) as indicators of potential HLB-influenced preharvest fruit drop and postharvest damage and breakdown. FDF for fruit from HLB-asymptomatic trees (AS) was substantially greater than for fruits from HLB-symptomatic trees or branches. Fruit hardness, resilience, springiness, and cohesiveness values, measured by texture profile analysis (TPA), represent various aspects of fruit firmness and recovery capacity once compressed, and were lower in HLB-symptomatic fruits in comparison with AS fruits. On the other hand, puncture force, toughness, and deformation values, measured by a puncture test and representing peel toughness, were higher in HLB-symptomatic fruits than in the AS fruits. The results indicate that fruits from severely HLB-symptomatic sweet orange trees are more likely to have problems with preharvest fruit drop, and postharvest pressure damage and breakdown, but may have less puncture damage in harvesting, transportation, packing, and juice processing. Citrus HLB or greening is a devastating citrus disease associated with Candidatus Liberibacter asiaticus (CLas). HLB has been reported in 40 countries in Asia, Africa, Australia, South America, and North America and was originally discovered in southern China in 1919 (Bov e, 2006). After being infected with CLas, citrus trees develop HLB symptoms including decline with typical symptoms such as reduced plant height, yellow shoots, blotchy mottled leaves, canopy thinning, premature abscission of fruits, and some out-of-season flushing and flowering (Bov e, 2006; da Grac xa, 1991). HLB severely affected fruit are small, lopsided, and poorly colored, with the juice having a bitter or sour taste and off-flavors (Baldwin et al., 2010; Plotto et al., 2010). As HLB severity increases, preharvest fruit drop becomes an increasing problem, which has contributed to declining yields in Florida, especially in the last few years (Bassanezi et al., 2011; Gottwald, 2010; USDA NASS, 2015; Zhao et al., 2015). Abscission is a naturally occurring process that enables plants to shed organs (Kostenyuk and Burns, 2004). Abscission is characterized by a series of physiological and biochemical events in the abscission zone that lead to cell wall breakdown in the few rows of cells on either side of the fracture line and ultimately to detachment of the organ (Burns et al., 2003; Roberts et al., 2002). However, very little data are available on the effect of HLB on FDF and abscission of citrus fruit. One of the objectives of this study was to determine the effect of HLB severity on FDF of ‘Hamlin’ and ‘Valencia’ sweet oranges (C. sinensis). Another objective of this research was to determine how HLB severity affects orange peel and fruit physical and biomechanical properties, which may contribute to mechanical injury of fruit during harvest, and postharvest handling and processing, and may also influence juice extraction and resulting juice quality. Tissue morphology (cell size, cell arrangement, and cell wall structure etc.) and material composition of fruits have an effect on their mechanical properties. Puncture, compression, cutting, tensile, shear, and TPA tests are usually used to estimate the maturity or resistance of fruit to force (Singh and Reddy, 2006). It has been reported that the maximum force in a puncture test represents the resistance of the fruit peel, and the force-deformation ratio in a whole fruit compression test represents whole fruit resistance (Ortiz and Torregrosa, 2014). McDonald and Shaw (1987) and Juste et al. (1988) found that puncture resistance of citrus peel decreased with fruit age. The force required to puncture or burst a fruit is directly related to the contact area. This is a function of two variables: the punch diameter used and the radius of curvature of the fruit. The fruit can withstand higher contact forces when subjected to larger punch diameter (Flood et al., 2006). ‘Dancy’ tangerines and ‘Navel’ and ‘Temple’ oranges showed lowest resistances to rupture (7.8, 20.6, and 92.1 N, respectively) and puncture force (0.5, 1.1, and 6.5 N, respectively); ‘Duncan’ grapefruit and ‘Valencia’ orange had medium resistance with 283.2 and 199.9 N in rupture force, respectively, and 8.5 and 11.8 N in puncture force, respectively; however, lemon and lime showed the highest resistance to rupture with 356.7 and 371.4 N, respectively, and to puncture with 30.4 and 39.2 N, respectively (Ahmed et al., 1973). Miller (1986) also showed significant differences in compression force and puncture parameters among varieties of citrus. No significant differences for harvest time (after reaching harvest maturity), or abscission chemical application was detected for burst, puncture, and peel tensile strength properties in three orange varieties (Churchill et al., 1980). Singh and Reddy (2006) studied postharvest physico–mechanical property changes of ‘Nagpur’ mandarins by performing peel tensile tests, peel cutting, fruit compression, puncture resistance, and fruit cutting tests under ambient and refrigerated storage conditions, and found that the peel tensile strength, modulus of elasticity, and cutting force decreased with storage period in both ambient and refrigerated conditions. In contrast to these studies, little is known about how a disease, such as HLB, affects peel and fruit mechanical properties in citrus, thus this study explores these effects in two major Florida juice orange varieties, ‘Hamlin’ and ‘Valencia’. The aim of this study was to assess the effect of HLB symptom severity on FDF and fruit mechanical properties in sweet oranges as indicators of potential HLBinfluenced preharvest fruit drop and postharvest fruit damage and breakdown. Materials and Methods Plant materials. Fifteen-year-old ‘Hamlin’ and ‘Valencia’ orange trees [C. sinensis (L.) Osbeck] on ‘Swingle’ citrumelo [Citrus paradisi Macf. · Poncirus trifoliata (L.) Raf.] rootstock, in a commercial grove Received for publication 5 Nov. 2015. Accepted for publication 19 Feb. 2016. We thank the China Scholarship Council (201206765008) and Fundamental Research Funds for the Central Universities of China (2013PY127) for their financial support for this research. Mention of a trademark or proprietary product is for identification only and does not imply a guarantee or warranty of the product by the U.S. Department of Agriculture. The U.S. Department of Agriculture prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or family status. Corresponding author. E-mail: jinhe.bai@ars. usda.gov. 356 HORTSCIENCE VOL. 51(4) APRIL 2016 located in Vero Beach, FL, were selected for this experiment. The trees received common cultural practices and the grower’s standard pest and disease management, which means that enhanced nutritional and pesticide/fungicide spray programs were applied to maintain vigor and productivity of the trees under the severe HLB pressure (Baldwin et al., 2012). For each cultivar, experimental trees were located in a two-acre block, the border trees/rows were excluded from sampling, and trees with average canopy sizes were selected. The experimental trees were tested by real-time polymerase chain reaction in late 2013 by using the method of Li et al. (2006), and all were CLas positive, despite that some trees were not yet showing HLB symptoms. The study was conducted using a randomized complete block design with five replicates. For ‘Hamlin’, three conditions/types of fruit samples were collected. The first type was from severely symptomatic HLB trees (HLB) with twig dieback, short shoots, blotchy mottle, curled leaves, and small, misshapen fruits. The second type was from HLB-symptomatic branches (HLB-B) on trees for which at least 50% of the canopy was HLB asymptomatic. The third type was fruits from HLB-asymptomatic trees (showing no visible HLB symptoms, AS). ‘Hamlin’ fruits were sampled on 18 Nov. and 2 and 16 Dec. 2013 and 6 Jan. 2014. Twelve fruits, of average size, shape, and color, per replicate/ tree were sampled on each date. Fruit with attached stems ( 50 mm) were clipped in the early morning and transferred to the laboratory within 2 h. For ‘Valencia’, only two types of fruit were collected, AS and HLB, because it was not possible to find trees that qualified as HLB-B trees. Samples were taken on 19 Jan., 3 and 19 Feb., and 3 Mar. 2014 in the same manner as for ‘Hamlin’. Fruit detachment force. The FDF was measured using a digital pull force gauge (Force Five; Wagner Instruments, Greenwich, CT). Fruit stems, 50 mm in length, were secured by a clamp, which was connected to the force gauge (Burns et al., 2003; Pozo et al., 2004). Fruit were then twisted and pulled to emulate hand harvesting. Pulling was performed vertically along the longitudinal axis direction of the fruit and then bent to one side until the stem separated from the fruit. The force was expressed in Newton. At each harvest, 60 fruits were used per HLB condition (HLB, HLB-B, and AS). Visual classification of fruit color and abscission zone types. Directly after measurement of FDF, the pattern and color of the detachment zone were evaluated from both fruit and stem (peduncle) ends and designated as abscission zone C (AZ C) detachment and non-AZ C detachment. AZ C detachment indicates fruit detached from abscission AZ C, with a round and smooth detachment scar with dark brown color on the fruit end and no attached calyx. On the stem end, the columella vascular bundles were dark brown with irregular indentations (Fig. 1A and B). All other types of detachment, usually caused by physical breakage of the vascular bundles due to the pulling force, left the calyx remaining on the fruit side. Meanwhile, the broken stem

Impacts of Huanglongbing Symptom Severity on Fruit Detachment Force and Mechanical Properties of Sweet Oranges (Citrus sinensis)