Oleocellosis Injury of Fruitlets from Late-season Mechanical Harvesting of ‘Valencia’ Orange Trees after Different Irrigation Treatments Does Not Affect Internal Fruit Quality

Oleocellosis or oil spotting on the peel of citrus fruit is a common post-harvest injury caused by improper handling. Mechanical injury allows phytotoxic oil to leak out of oil glands and cause injury to surrounding flavedo cells, resulting in oleocellosis. Mechanical harvesting (MH) of ‘Valencia’ sweet orange is conducted in late spring, when the next season’s fruitlets are in their early stages of development. There is a concern that mechanical injury from harvesting machines can cause oleocellosis and fruit drop of young, green ‘Valencia’ sweet orange fruitlets, especially late in the harvest season when fruitlets are relatively large. We evaluated the effects of winter drought stress and subsequent late-season MH with a canopy shaker on oleocellosis of ‘Valencia’ sweet orange fruitlets. In April, mature fruit size, juice content, total soluble solids, and acidity were unaffected by previous winter drought stress treatments. Mechanical harvesting removed ’90% to 95% of mature fruit and 20% to 50% of fruitlets depending on previous drought stress treatments and harvesting date. Beginning 1 week after the late harvest (13 June), attached fruitlets were tagged and visually evaluated approximately every other month to determine oleocellosis injury until the late-season harvest 12 months later. Only 12% of the fruitlets had oleocellosis on more than 30% of their surface area. Up to 75% of the fruitlets from the previously drought-stressed trees had less than 10% of their surface area injured after MH and 11% of these fruitlets dropped before harvest. Nonetheless, there was no significant increase in fruit drop with increased surface area injured nor was juice quality affected at harvest. Overall, fruit surface oleocellosis decreased and healed as fruit expanded, but surface blemishes did not completely disappear. Thus, fruitlet oleocellosis in late-season mechanically harvested trees was cosmetic and did not increase fruit drop nor alter internal fruit quality. Mechanical harvesting of citrus fruit for juice processing from large-scale commercial plantings in Florida has economic advantages (Roka et al., 2008) over hand-harvesting, especially when adequate seasonal labor is not available (Brown, 2005; Roka, 2004). The shaking and visible injuries that citrus trees may experience during MH with trunk or canopy shakers are considered major impediments to the widespread adoption of MH (Li and Syvertsen, 2005; Zekri and Syvertsen, 2008). However, bark and branch injuries by canopy shakers are often not worse than injuries from ladders during hand picking operations (Buker et al., 2004), and there tends to be less leaf and twig loss from MH after the initial year of MH. The most common MH injuries in citrus and other fruit trees include leaf loss, twig loss, bark scuffing, flower loss, green immature fruit (fruitlet) drop, and exposure of roots at the soil surface (Halderson, 1966). Nonetheless, there is no evidence that such injuries in healthy well-managed citrus trees lead to any tree loss or decrease in yield from either long-term MH (7 to 8 years; Hedden et al., 1988; Whitney, 1995) or any other physiological stress associated with MH when compared with hand-harvested trees (Li and Syvertsen, 2005). There is a continuing problem in late-season MH of ‘Valencia’ sweet orange, however, because fruitlets from the next season’s crop may be large enough to be unavoidably removed by the harvesting machine (Burns et al., 2006). Although winter drought stress of ‘Valencia’ sweet orange trees can effectively delay flowering so that fruitlets are not large enough to be susceptible to MH (Melgar et al., 2010), there is concern that mechanical injury of fruitlets could lead to subsequent premature fruit drop or lower fruit quality. The effects of late-season MH injury to fruitlets of ‘Valencia’ sweet orange exposed to previous winter drought stress have not been described. Oleocellosis, rindspot, or oil spotting of citrus fruit is a common injury of the flavedo (Fawcett, 1916) caused by mechanical injury from hail, wind, and improper handling (Brodrick, 1970; Cahoon et al., 1964; Dodson, 1966). Mechanical injury allows phytotoxic essential oils (and terpenes) to leak out of oil glands and cause injury to surrounding flavedo cells resulting in oleocellosis. The injured superficial tissues dry out and form a thin, cracked scab on the surface of the fruit (Fawcett, 1916) and the injured rind may not continue to develop mature color, leaving a green or pale area where the injury occurred. Oleocellosis can appear on fruit as small as 2 to 3 cm diameter (Jamieson et al., 2006) and appears to be greater on fruitlets than on fully colored or mature fruit. This may be a perception only because green damaged areas do not degreen and are more noticeable on fruitlets. We determined how MH affected green fruitlets from the next season’s crop in late-harvested ‘Valencia’ sweet orange trees. We tested the hypothesis that different irrigation treatments from the previous winter would affect oleocellosis injury, subsequent fruitlet drop, and fruit quality at maturity. Materials and Methods Plant material and growth conditions. The study was conducted at the University of Florida/IFAS Citrus Research and Education Center, Lake Alfred, FL (long. 28.09 N, lat. 81.73 W; elevation 51 m). Uniform 14-yearold ‘Valencia’ sweet orange (Citrus sinensis L. Osbeck) trees on ‘Swingle’ citrumelo (C. paradisi Macfad. · Poncirus trifoliata L. Raf.) were used. There were 12 experimental plots of 10 trees each that were grown in pairs such that spacing between adjacent trees was 2 m and 5 m between pairs of trees within the row; between-row spacing was 7 m. There were three wintertime irrigation treatments in a randomized block design with three replicate blocks of 10 trees each. Treatments began on 13 Dec. 2007 and consisted of 1) no irrigation with rain shelter-covered soil (= drought); 2) no irrigation, no cover (= rain only); and 3) regular microsprinkler irrigation as needed with no cover (rain + irrigation = wellirrigated control). The rain-out shelter groundcover was Tyvek (Dupont , Wilmington, DE), which covered the entire soil surface under tree canopies. Treatments were maintained until 21 Mar. 2008 (100 d) after which the Tyvek covers were removed and all trees were well-irrigated. Flowering intensity. Flowering intensity was estimated by counting the number of flowers after petal fall within a 0.3 · 0.6-m square frame placed against the branches of the tree 1.5 m from the ground (Ribeiro et al., 2008). Flowers were counted every 2 weeks from the beginning of March to the end of May. Counts were made in three pairs of trees per treatment on 30 shoots per tree, 15 shoots on the east and 15 on the west side of the canopy, within the limits of the frame. The Received for publication 28 Oct. 2010. Accepted for publication 28 Jan. 2011. Current address: Citrus Center, Texas A&M University-Kingsville, 312 N International Boulevard, Weslaco, TX 78596. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 46(3) MARCH 2011 1 | POSTHARVEST BIOLOGY AND TECHNOLOGY