The sweetpotato cultivar, Covington, was released by the North Carolina State University Sweetpotato Breeding and Genetics Program (Yencho et al., 2008). North Carolina is the leading sweetpotato producer in the United States, with 63,000 acres planted in 2012 and comprises 47% of the national production (North Carolina Department of Agriculture and Consumer Services, 2012). ‘Covington’ has excellent internal and external quality and is grown in over 80% of the commercial acreage across the state (S. Johnson-Langdon, personal communication). When added to the small amount of production of this cultivar in other states, nearly 40% of sweetpotato production across the United States consists of ‘Covington’.
Internal necrosis in ‘Covington’ was first noted in Fall 2006 by a North Carolina grower. Once the incidence was discovered, ‘Covington’ storage root stocks from all of the Certified Seed Growers were evaluated in Feb. 2007. One farm, in which the original report of IN was made, had a high incidence of symptoms, some being severe. A few other farms had less than 2% incidence, whereas most farms had storage roots exhibiting no symptoms.
Internal necrosis symptoms are not evident on the exterior of storage roots, which precludes early detection of affected roots. Symptoms are visible as the cortex of the storage root is exposed through cutting. Symptoms usually start at the proximal end and progress toward the distal end, but rarely extend more than halfway down the length of the storage root. Necrosis symptoms vary from small (a few millimeters) black or brown spots to large areas (3–5 cm) of breakdown (Fig. 1).
Since the first discovery of IN, a number of reports and investigations have addressed the problem. One grower in 2009 reported that 1600 tons of sweetpotato in storage were affected by IN. Another grower reported that ‘Covington’ storage roots from the same harvest but placed in different storage rooms had no IN in one storage room, whereas roots in the other room had IN after ≈3 months storage.
The lack of IN was confirmed through repropagation of storage roots that had IN symptoms. As a vegetatively propagated crop, in the United States, sweetpotato storage roots are bedded to obtain plants, and plants are transplanted to produce the next season’s root crop. When storage roots having IN were bedded for plant propagation, root crop harvested subsequently did not exhibit IN symptoms following full growth and harvest. This was confirmed through controlled studies on the research station and through reports from growers who commercially bedded storage roots with IN symptoms (Schultheis and Thornton, 2007). The presence of pathogens was tested on storage roots with IN symptoms. Small pieces of affected tissue were placed in petri plates with acidified potato dextrose agar and incubated at room temperature. No pathogens were consistently isolated other than occasional secondary infections (Schultheis et al., 2009), suggesting a stress-induced physiological disorder.
Previous research indicated that cultivars differ in degree and severity of incidence (Clark et al., 2013; Dittmar et al., 2010). Both preharvest and postharvest factors may affect or initiate IN. Previous work has provided evidence that ethephon might be a causal factor of IN. Ethephon is a compound that releases ethylene gas, which may accelerate the development of the symptom. Several ethephon-based products are available; they contain ethephon as the active ingredient at concentrations of 22% to 55%, which are designed for different agricultural commodities.
Sweetpotato is sensitive to ethylene in storage (Buescher et al., 1975; Kitinoja, 1987). Buescher et al. (1975) found that sweetpotato exposed to 10 ppm ethylene during or after curing had enhanced levels of respiration and polyphenol oxidase activities and loss of color and flavor after baking. However, the symptoms of IN were not similar to those seen with ethylene damage (R.W. Buescher, personal communication). Storage roots cured or stored with 1–2 ppm ethylene had increased respiration rate compared with roots cured or stored in ambient air conditions with negligible ethylene (Kitinoja, 1987). Elevated respiration differed among cultivars, from low to high in the tested cultivars, Jersey, Garnet, and Jewel. Total phenolic content also differed with ethylene exposure during curing or storing. Sweetpotato storage roots that were freshly or previously air-cured then stored in an ethylene (0.1 ppm) environment for 1 month had a higher total phenolic rating than roots previously cured in the presence of ethylene and held for 1 month with 0.1 to 10 ppm ethylene (Kitinoja, 1987).
The ethephon product PrepTM (Bayer CropScience, Monheim am Rhein, Germany) is currently registered and used to enhance defoliation of cotton (Gossypium hirsutum) and tobacco (Nicotiana tabacum) before harvest. Although not labeled for application on sweetpotato, it has been evaluated as a spray application by the sweetpotato industry to reduce storage root damage at harvest by tightening the epidermis (Main et al., 2009; Wang et al., 2013). Initial research with ethephon was conducted on ‘Beauregard’ sweetpotato storage roots (Schultheis et al., 2000), and more cultivars, including ‘Covington’, were used in later studies (Dittmar et al., 2010). At 60 d after harvest, an incidence of 26.5% of IN was found in ‘Covington’ storage roots without ethephon application; but ethephon greatly increased incidence (74%) in ‘Covington’ compared with 12% of incidence in ‘Beauregard’.
The sweetpotato disorder hardcore has some characteristics in common with IN. This disorder is chill-induced with more incidence and severity in 92 to 100 ppm ethylene than in ambient air (Timbie and Haard, 1977). Exposing sweetpotato storage roots to elevated carbon dioxide [CO2 (5%)] reduced hardcore severity, whereas hyperbaric conditions (1.7 atm) increased the severity compared with 1 atm ambient air.
Although sweetpotato storage roots normally produce very low levels of ethylene [≈0.1 µL·kg−1·h−1 (Cantwell and Suslow, 2013)], internal ethylene production can be increased in response to black rot fungus (Ceratocystis fimbriata) and cut injury (Imaseki et al., 1968a, 1968b). Rapid production of ethylene from sweetpotato storage roots was observed within 6 h after roots were sliced, as it stimulated cell metabolic activities, particularly peroxidase and polyphenol oxidase, enzymes involved in the phenylpropanoid and chlorogenic acid pathways (Imaseki et al., 1968c). Ethylene is also reported to play a passive role in the incidence of hardcore (Timbie and Haard, 1977). The diffusion of ethylene from internal tissue of the storage root decreases as root size increases. As a result, more incidence of hardcore is seen in bigger roots, which have smaller surface to volume ratio.
Use of the cultivar Covington is the one consistent factor in the appearance of significant IN in sweetpotato. To better understand and identify possible factors associated with IN, there were studies conducted in commercial storage rooms and at research stations on a range of insecticides and timing of ethephon application to sweetpotatoes. These studies addressed the following four objectives: 1) determine the incidence and severity of IN in the commercial sweetpotato industry of North Carolina; 2) investigate preharvest factors that are related to IN; 3) investigate postharvest factors that are related to IN; 4) determine when initiation of IN symptoms occur.
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