Hexanal is a naturally occurring, volatile C-6 aldehyde formed via the lipoxygenase pathway in plants from linoleic acid (Hildebrand, 1989). Volatiles formed by this pathway in wounded plants have antifungal properties, as shown by early research with wounded tomato leaves in which hyphae of both Alternaria alternata and Botrytis cinerea were inhibited (Hamilton-Kemp et al., 1992). In trials conducted on B. cinerea, the related compound (E)-2-hexenal, which differs by having a double bond, was more effective against spores than mycelium of the fungus and stimulated its growth at low concentrations (Fallik et al., 1998). Thus, it was concluded that maintenance of a high vapor-phase level of (E)-2-hexenal was necessary to inhibit mycelial growth and to avoid enhancing postharvest mold problems.
Botrytis cinerea causes the postharvest disease known as gray mold and is considered one of the most important diseases of stored pears. Wounds or injuries are the primary infection courts for the initiation of gray mold in apples and pears (Rosenberger, 1990). The fungus forms a nest rot by growing from infected to healthy fruit at storage temperatures as low as –0.6 ºC (Ogawa and English, 1991). Lennox et al. (2004) reported that, on ‘Anjou’ pears, incidence of stem-end gray mold was higher than calyx-end and puncture gray mold. Archbold et al. (1997) showed that hexanal, among other lipoxygenase pathway products, exhibited potential as postharvest fumigants for control of B. cinerea. Subsequent studies showed that when ‘Crimson Seedless’ table grapes were fumigated with (E)-2-hexenal, gray mold was suppressed but not completely controlled (Archbold et al., 1999).
Penicillium expansum Link causes the postharvest disease known as blue mold and is considered the most important disease of stored apples (Turechek, 2004). Hexanal vapor was successfully used to control P. expansum on apple slices after 48 h of continuous exposure to 100 ppm of the product (Song et al., 1996). When the fungus was allowed to grow for 48 h in air, a subsequent treatment with 450 ppm stopped growth completely (Song et al., 1998). Fan et al. (2006) found that under similar conditions of constant exposure to hexanal, P. expansum lesion development on whole apples was reduced. These antimicrobial results are supported by Lanciotti et al. (1999), who found that hexanal strongly inhibited molds, yeasts, and mesophilic and psychrophilic bacteria, and extended the shelf life of apple slices. Hexanal has also been used to control postharvest decay in stone fruit incited by Monilinia laxa and Rhizopus stolonifer (Caccioni et al., 1995).
Hexanal has other properties in addition to its antimicrobial activity. Aroma volatiles are important sensory attributes of ripe fruit, but may also play functional roles in plant–pathogen interactions (Archbold et al., 2000). Hexanal treatment stimulated aroma production in ‘Jonagold’ and ‘Golden Delicious’ apple slices (Song et al., 1996, 1998). When added to modified atmosphere (MA) (70% N2 and 30% CO2), hexanal also prevented a browning reaction for 16 d at 15 ºC (Lanciotti et al., 1999).
The goal of this study was to determine whether hexanal vapor could be used as a fumigant to prevent postharvest decay incited by either B. cinerea or P. expansum on stored apples and pears. To accomplish this objective, small quantities of inoculated fruit were fumigated to establish optimum rate, temperature, and duration for fumigation before and after fruit were wounded. Preharvest treatment of fruit in the orchard with cyprodinil was also evaluated because it was thought unlikely that hexanal would control pathogen conidia in wounds, and cyprodinil had previously been shown to be effective in preventing B. cinerea decay in wounds of fruit stored for 3 months (Sholberg et al., 2003a). Larger volumes of fruit were used in subsequent experiments to determine whether hexanal alone or a combination of cyprodinil and hexanal could be used for control of postharvest decay in stored fruit. Characteristics of hexanal such as concentration during fumigation, effect on fruit quality and aroma, and possibility of causing fruit damage were also studied. A preliminary report on the use of hexanal fumigation for control of postharvest decay of pome fruit has been published (Sholberg and Randall, 2005).
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