In apples, development of physiological disorders is a function of many components, including cultural management, growing environment, fruit maturity, and conditions during storage. Disorders related to dysfunctions or aberrations in the development of the epidermal tissue (peel) are often linked to climatic conditions during the growing season and are initiated when a particular metabolic system(s) exhibits stress-induced hysteresis. These include russet, staining, cracking, splitting, flecking, bitter pit, blotch, lenticel marking, radiation injury, delayed sunscald, superficial scald, and soft scald (Meheriuk et al., 1994; Pierson et al., 1971; Porritt et al., 1982). Together, these disorders may render unmarketable as much as 20% of total production. Considering that the value of apples in Washington state alone in 2006 was $1.4 billion (National Agricultural Statistical Service, 2007), reducing the loss due to physiological disorders is of significant economic importance.
Since 2000, lenticel breakdown disorder (LB) has been a high priority area for research investigations in the arid apple growing regions of the United States. LB symptoms are not visible at harvest nor are they usually apparent on unprocessed fruit after storage. It is usually after typical fruit processing and packing that symptoms are fully expressed (Fig. 1). Particularly frustrating for the warehouse is that it may take up to 48 h for LB to appear after fruit have been packed (E.A. Curry, personal observation). If symptoms are detected before shipment, there are often significant repacking costs; if undetected, the negative impact on repeat sales can be lasting.
The distinctive features of LB are: 1) it is not visible at harvest, 2) symptoms on unprocessed fruit in storage are not visible, 3) symptoms are expressed mainly after typical processing (dump tank, washing, waxing, and packing), 4) pitting is round and centered on the lenticel, 5) little if any corking of the cortex tissue is evident, and 6) there is a cavity underlying the sunken pit. It is different from jonathan spot and lenticel spot in that pitting is always present and usually progressive with increased time in storage.
Although prevalent on ‘Gala’ apples, especially ‘Royal Gala’, LB has also been observed on ‘Fuji’, and to a lesser degree on ‘Granny Smith’, ‘Golden Delicious’ (Fig. 1B), and ‘Delicious’. Early symptoms on packed fruit from regular atmosphere (RA) storage are visible in angled light as slight indentations in the epidermis about 1 to 2 mm, usually symmetrical and centered on a lenticel, without any darkening. With time, the depth of the dimple increases and the pit often becomes progressively darker. The darkening appears to be largely a function of how many layers of cells have desiccated, thereby compressing the cell walls, and the degree of phenolic browning therein. When a fully developed pit is sliced in half, a cavity is present commencing several cell layers beneath the hypodermis (Fig. 2). In severe cases, pits may overlap and appear coalesced. Generally, there is little corking in the cortex beneath the pit (Fig. 2C).
Initial studies were focused on poststorage processing, and a number of factors were identified that, if modified, could significantly reduce, but not eliminate, symptom expression (Curry, 2003). Importantly, this earlier work established that certain orchards (or blocks within orchards) showed no propensity for LB, whereas others were highly susceptible. Our efforts then focused on preharvest environment.
Previous studies showed that water vapor permeance of apple cuticle in storage was linked to cuticle microcracking (Maguire et al., 1999). This, together with our ensuing observation (E.A. Curry, unpublished data) that fruit subject to conditions of high desiccation potential during the final weeks of fruit enlargement had a greater propensity to develop LB, led to the hypothesis that reducing water vapor permeance of the cuticle preharvest by applying a lipid-based, hydrophobic coating would reduce LB development on fruit in storage. Hydrophilic and lipophilic edible films and coatings have been shown to alter food moisture content (Debeaufort et al., 1998; Hagenmaier and Shaw, 1990; Kester and Fennema, 1989; Morillon et al., 2002; Quezada-Gallo et al., 2000). Our objective in this series of trials was to determine if preharvest topical application(s) of lipophilic coatings would reduce the incidence and/or severity of LB in ‘Gala’ apples in RA storage.
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