United States-exported lettuce faces phytosanitary barriers in overseas markets due to the presence of quarantined pests including western flower thrips (Frankliniella occidentalis) for Taiwan, and california pea leafminer (Liriomyza langei) and lettuce aphid (Nasonovia ribisnigri) for Japan (Liu, 2008, 2012). Although methyl bromide fumigation is still exempted for quarantine treatment usage while its production is being phased out globally as mandated by the Montreal Protocol, it may not be available in the future. Methyl bromide fumigation also injures lettuce and shortens lettuce shelf life. Therefore, it is critical to develop a safer alternative to meet pest quarantine requirement for exported lettuce.
Phosphine has been used extensively for stored product pest control for over 80 years, and now has emerged as a main methyl bromide alternative (Chaudhry, 1997; Fluck, 1973; Horn et al., 2003). However, there are disadvantages with phosphine fumigation. Phosphine acts slowly against insects and often requires long treatment times to achieve effective control of target pests. For some tolerant pests, fumigations of over 10 d are needed (Hole et al., 1976). Phosphine is more active at higher temperatures and therefore low-temperature phosphine fumigation also takes longer time to control pests (Bond, 1984; Hole et al., 1976; Liu, 2008). Some insects also developed resistance to phosphine (Schlipalius et al., 2002).
Traditionally, phosphine generated from metal phosphides has not been used for pest control on fresh commodities due to phytotoxicity caused by ammonia, which is cogenerated to modulate the phosphine generating chemical reaction. In recent years, cylindered phosphine gas free of ammonia has been used successfully in Chile to control pests on fresh fruit and vegetables (Horn and Horn, 2004; Horn et al., 2005, 2010). However, information on potential impact on postharvest quality of fresh products is still very limited (Jamieson et al., 2012; Klementz et al., 2005; Liu, 2008, 2011, 2012). One-day fumigation with 1000 ppm phosphine at 2 °C for control of western flower thrips was found to be safe for fruit and vegetables including lettuce (Liu, 2008). Western flower thrips was also controlled effectively without negative impact on lettuce quality in a pallet scale 18-h fumigation of prechilled lettuce under an insulation cover (Liu, 2011). However, a 3-d fumigation with phosphine at a high concentration of 2200 ppm at 2 °C for controlling lettuce aphid resulted in significant injuries to both romaine and head lettuce in the form of brown stains identical to CO2 injuries (Lipton et al., 1972; Liu, 2012). Because cylindered phosphine is free of ammonia, it is possible that factors other than phosphine may be responsible for the injuries to fumigated lettuce.
In the 3-d low-temperature phosphine fumigation as reported by Liu (2012), the lettuce was sealed in fumigation chambers for 3 d without air exchange. This might have resulted in significant changes in atmosphere composition as lettuce consumes oxygen and produces CO2 and other volatiles such as ethylene. After harvest, head lettuce produces CO2 at 17 mg·kg−1·h−1 at 5 °C (Saltveit, 2004) and releases ethylene at <0.1 μL·kg−1·h−1 at 20 °C (Kader and Kasmire, 1984). Lettuce is also very sensitive to both CO2 (Kader and Morris, 1977) and ethylene (Kim and Wills, 1995; Schouten, 1985). Exposure to 1% CO2 or 0.1 ppm ethylene may cause injuries to sensitive lettuce cultivars under unfavorable atmospheric conditions (Lipton et al., 1972; Morris et al., 1974). For lettuce, CO2 causes brown stains (Lipton et al., 1972), and ethylene causes russet spotting (Morris et al., 1974). Cultivars vary significantly in their sensitivities to CO2 and ethylene (Brecht et al., 1973; Cantwell and Suslow, 2001). In this study, we conducted low-temperature phosphine fumigation treatments of head and romaine lettuce with and without CO2 and ethylene absorbents to determine whether scrubbing CO2 and ethylene can reduce or prevent fumigation associated injuries to lettuce. Lettuce injuries were also analyzed for associations with lettuce weight and quality.
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