Blueberries and grapes are major export crops for the United States. In 2014, the United States exported 19,524 mt of fresh blueberries to destinations such as Canada, Mexico, Japan, South Korea, and India (UC AIC, 2015; USDA ERS, 2015) and over 400,000 mt of grapes mostly to Canada, southeast Asia, Mexico, China, and Australia (USDA FAS, 2015). For interstate transport of blueberries to California (CDFA, 2015) and export to certain countries, blueberries and grapes are treated to mitigate insect pests and thus allow the export of regulated produce out of quarantine areas (USDA APHIS PPQ, 2015). Methyl bromide is the most common phytosanitary treatment used for these fruit in the United States. It is a low cost, rapidly dispersing gas that targets a wide range of insect pests but it is in the process of being phased out because of its depleting effect on the ozone layer (US, EPA, 2013). Fumigation requires breaking the cold chain as the fruit must be fumigated at ambient temperatures, which negatively impacts product quality and shelf life. For these reasons, sulfur dioxide (SO2) and carbon dioxide (CO2) followed by cold treatment are now used for grapes exported to Australia (Chervin et al., 2012).
Ionizing irradiation at low doses is a promising phytosanitary treatment that is increasing in use worldwide for various fruits such as guavas, dragon fruit, rambutan, and mangoes (Kume and Todoriki, 2013). The U.S. Department of Agriculture has approved a generic dose of 400 Gy for the sterilization of all insects excluding the pupae and adults of the insect order Lepidoptera (USDA APHIS PPQ, 2015). Irradiation is effective against pests common to blueberries, such as plum curculio, Conotrachelus nenuphar (Herbst) and the blueberry maggot, Rhagoletis mendax, (Hallman and Miller, 1994) as well as grape pests such as omnivorous leaf roller (Platynota stultana), western flower thrips (Frankliniella occidentalis), spider mites, and grape mealybug (Psuedococcus maritimus).
On blueberries, irradiation appears to mostly impact firmness, but the extent is dependent on variety. ‘Climax’, ‘Sharpblue’, and ‘Brightwell’ blueberry varieties softened at 500 and 1000 Gy, whereas ‘Tifblue’ and ‘Brigatta’ blueberries retained firmness (Golding et al., 2014; Miller et al., 1994, 1995; Miller and McDonald, 1996). Similar to these findings, Trigo et al. (2006) observed a dose dependent loss of firmness in ‘Bonita Blue’ blueberries irradiated at doses up to 3000 Gy. Overall acceptance of the blueberries, however, was unaffected below 750 Gy. Similarly, grapes have a high tolerance to low-dose irradiation. Kim et al. (2014) found that ‘Crimson Seedless’ and ‘Sugraone’ varieties tolerated irradiation up to 800 Gy although texture of the ‘Sugraone’ grapes was impacted at 800 Gy. The shelf life of two cultivars of Syrian grapes, Helwani and Baladi were improved by 50% following irradiation at 500–1000 Gy and 1500–2000 Gy, respectively (Al-Bachir, 1998).
These studies show that for both fruit, blueberries and grapes, irradiation at 400 Gy has little impact on quality under ideal storage temperatures. What is unknown is the effect of subsequent distribution and retail display under ambient conditions on fruit quality. Under ideal storage temperatures, the fruit may be able to tolerate the stress of irradiation and phytotoxic impacts may not be obvious, especially at the low dose of 400 Gy. However, the added stress of warm temperatures may allow irradiation-induced phytotoxic effects to manifest and have negative consequences on shelf life. For example, Sea et al. (2015) found that irradiation at 400 Gy delayed ripening in pears but that Mountain pears exhibited symptoms of irregular ripening. McDonald et al. (2012) observed development of pitting on ‘Lane Late Navel’ oranges treated at 200 Gy and higher during 3 weeks of refrigerated storage, which was exacerbated during 1 week at ambient temperature. Thus, this study was designed to explore the impact of storage and subsequent retail display on the quality of three varieties of blueberries and two varieties of grapes irradiated at a target dose of 400 Gy for phytosanitary purposes.
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