Sweetpotato production has increased dramatically in the state of Hawai’i from a farm gate value of $0.99 million in 2002 to $7.3 million in 2011 (Hawaii Department of Agriculture, 2013). The primary reason for the increased production was due to approval of a quarantine treatment to control insect pests, allowing export of sweetpotatoes out of the state. In 2011, sweetpotatoes were grown on 1100 acres with an average yield of 15,200 lb/acre. Most sweetpotato production is on the island of Hawai’i where the purple-fleshed cultivar Okinawan is grown and exported to the United States mainland.
There is an increased demand for sweetpotatoes by consumers in the United States, due to greater awareness of its healthful benefits (Wang et al., 2016). Purple-fleshed cultivars are rich in anthocyanins, which are popular dietary antioxidants (Teow et al., 2007). Cooked, purple-fleshed sweetpotatoes contain anthocyanins at 21.7 mg/100 g fresh weight and rank fourth in total anthocyanin levels among 20 fruit and vegetables consumed in Hawai’i (Franke et al., 2004). Antioxidants are thought to reduce the harmful effects of oxidative stress and to prevent development of chronic diseases (e.g., heart disease or cancer).
Sweetpotatoes are grown vegetatively from one cropping cycle to the next, and cultivar decline over time (reduced yields and quality) has been observed. For example, ‘Centennial’ has been grown since 1958 at the Sweet potato Research Station in Chase, LA, and its yield has decreased by 46% during a 35-year period (Villordon and LaBonte, 1995). This decline is thought to be due to an accumulation of viruses, other pathogens, and deleterious mutations (Clark et al., 2002).
In Hawai’i and elsewhere in the tropics, farmers obtain cuttings from extant plantings, and their “seed” stock has not been subjected to virus testing or any form of visual screening to eliminate off-types. This lack of virus-tested “seed” stock could lead to extreme off-grade shapes and sizes and inconsistencies in flesh color. Little is known about sweetpotato viruses present in Hawai’i. During 2013–14, 192 sweetpotato samples were collected on the islands of Kauai, Oahu, Molokai, Maui, and Hawai’i, and evaluations for 27 known sweetpotato viruses were made using polymerase chain reaction (PCR)-based detection assays. None of the targeted viruses were detected during this survey, either due to their absence in these samples or limitations of the detection assays employed (Melzer, 2014).
Another threat to the sustainability of sweetpotato production in Hawai’i is reduced root quality because of insect pests. The two major insect pests of sweetpotato in Hawai’i are the sweetpotato weevil (Follett, 2006) and the rough sweetpotato weevil (Heu et al., 2014). The sweetpotato weevil is a major pest of sweetpotato worldwide, feeding and multiplying within the storage root, resulting in inedible roots (Nottingham and Kays, 2002). The rough sweetpotato weevil is a new, invasive pest in Hawai’i, first detected on the island of Oahu in 2008, with subsequent detection on the island of Hawai’i in 2014 (Heu et al., 2014). This insect pest has not been found yet in the mainland United States. Grubs of rough sweetpotato weevil feed on storage root surfaces, severely damaging their appearances and reducing marketability.
The objectives of the field trials conducted at the Kula Agricultural Park on the island of Maui were to compare yield and pest resistance of 1) ‘Okinawan’ obtained from a C field with TC ‘Okinawan’ and 2) TC sweetpotato cultivars Okinawan, LA 08-21p, and Murasaki-29.
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