North Carolina accounts for 56% of national sweetpotato production in the United States, with 95,000 acres of the crop harvested in 2016 (U.S. Department of Agriculture, 2017). Among all the sweetpotato cultivars, Covington is especially adapted to North Carolina growing conditions, producing high yielding and high quality storage roots (Yencho et al., 2008), and because of its adaptability is grown on more than 88% of the commercial acreage across the state (Schultheis, 2016). ‘Beauregard’ is the predominant orange flesh cultivar grown in Louisiana and Mississippi (81% and 70% of acreage, respectively) because of its high yield and excellent adaptation to the environmental conditions encountered in those states (Meyers, 2016; Rolston et al., 1987; Sistrunk and Smith, 2016). ‘Carolina Ruby’ is a specialty cultivar released by the North Carolina Agricultural Research Service (Collins et al., 1999). It is sold by many home gardening vendors but is also sold by certified seed growers in North Carolina for production in smaller commercial acreages for its attractive dark ruby skin, orange flesh, and outstanding taste (S. Scott, personal communication).
Internal necrosis occurs in the sweetpotato flesh near the proximal end as small light brown/black areas, but severe symptoms can extend half way longitudinally down the storage root with large brown/black areas across the entire cross section (Clark et al., 2013). Although precise statistics on the economic impact of this issue are not available, more than $1 million was lost by one grower who had suffered from this problem during one season (J.R. Schultheis, unpublished data). A commercial storage survey of ‘Covington’ has revealed that internal necrosis is widespread in the North Carolina industry but high incidence and severity only happens in a few farms each year (Jiang et al., 2015). Incidence is erratic across years and among storage rooms, and in a survey, patterns of storage temperature and relative humidity after curing were not associated with the incidence (Jiang, 2013; Jiang et al., 2015). Incidence varies among cultivars, with Covington and Hatteras having significantly greater incidence than other commercially important cultivars, such as Beauregard (Clark et al., 2013). The symptoms of internal necrosis are not transferred over growing seasons/generations (Schultheis and Thornton, 2007) and no microorganisms have been consistently isolated from infected roots suggesting that it might be a stress-induced physiological disorder (Schultheis et al., 2009). No relationship between internal necrosis and registered insecticides or herbicides in sweetpotato production systems have been found (Beam et al., 2017; Jiang et al., 2015).
Preharvest ethephon spray and postharvest curing are two reported factors, and when used alone or interactively, which have caused elevated incidence of internal necrosis (Clark et al., 2013; Jiang et al., 2015). Although ethephon is not registered for use in sweetpotato, an interest existed in pursuing a registration for this use because of the potential to reduce storage root damage at harvest through tightening the epidermis (Wang et al., 2013). In ‘Covington’ storage roots, ethephon induced–internal necrosis was first seen 6 DAH during curing, and incidence progressively increased to 30 DAH (Jiang et al., 2015). In the study, removing either ethephon or curing reduced necrosis incidence. However, curing is a standard healing process to maintain shelf life and improve quality; thus, growers continue to use this process (Edmunds et al., 2008). In many commercial cases, where roots were cured but had not been sprayed with ethephon (Jiang et al., 2015), necrosis incidence was still detected indicating that additional unknown factors in the field or storage might have contributed to the development of internal necrosis.
In years with heavy rainfall, excessive flooding may occur in sweetpotato fields in North Carolina. For example, in the major sweetpotato growing regions, an accumulated 9.1, 10.1, and 6.9 inches of rainfall was observed between 26 Sept. and 1 Oct. 2010 for Sampson, Nash, and Johnston counties, respectively (State Climate Office of North Carolina, unpublished data). In Sept. 2014, Johnston County experienced an 8.6-inch rainfall within a 1-week period. Depending on the soil texture, these levels of heavy and continuous precipitation could cause flooding in the field. Farmers have suggested that high internal necrosis occurs in storage roots harvested from fields with heavy rainfall, which led to our hypothesis that excessive moisture may contribute to internal necrosis. With flooding, roots suffer oxygen deficiency (Jackson, 1985), which can increase the concentration of 1-aminocyclopropane-1-carboxylic acid (ACC), the precursor of ethylene, in flooded roots of tomato plants [Solanum lycopersicum (Else and Jackson, 1998)]. Delivery of ACC from roots to shoots in flooded plants also increased 6 h after flooding and coincided with epinastic leaf curvature. This increased ethylene synthesis in response to flooding has also been observed in sweetpotato plants (Patterson et al., 1979). Because ethephon, an ethylene-generating compound, has a known effect on the induction of internal necrosis, we hypothesized that a similar reaction to ethephon might be induced by flooding, which could elevate ethylene levels in roots and shoots of sweetpotato and thus cause elevated levels of internal necrosis.
The objective of our studies was to determine the effect of ethephon application rates, simulated flooding, and storage duration on the development of internal necrosis (incidence and severity) in storage roots of three sweetpotato cultivars. Results from these studies test our hypothesis that excessive moisture may contribute to internal necrosis in sweetpotato storage roots and will provide new information on the development of internal necrosis in longer term (9–150 d) storage after curing.
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