Sweetpotato is an important crop worldwide, valued at over $9.4 billion (Food and Agriculture Organization of the United Nations, 2012). In the United States, sweetpotato production is valued at $698 million in gross farm value [U.S. Department of Agriculture (USDA), 2015a]. North Carolina ranks first in sweetpotato production in the United States with 73,000 acres planted in 2014, which accounts for more than half of the total acreage planted nationwide (USDA, 2015b). ‘Covington’ sweetpotato, released by the North Carolina Agricultural Research Service in 2005, accounts for 88% of certified seed root acreage in North Carolina, followed by ‘Beauregard’ and ‘Evangeline’ at 5% and 3%, respectively (North Carolina Crop Improvement Association, 2014; Yencho et al., 2008). ‘Evangeline’ sweetpotato was released by the Louisiana Agricultural Experiment Station in 2007 and was reported to have southern root-knot nematode (Meloidogyne incognita) resistance, yields that are comparable to ‘Beauregard’, and an excellent flavor profile (La Bonte et al., 2008). In addition to ‘Covington’, ‘Evangeline’ under North Carolina growing conditions had been minimally investigated in terms of production practices and was, therefore, considered an important component in the study.
A sweetpotato crop is established with nonrooted cuttings (slips), which are vegetatively produced in field propagation beds. North Carolina growers use a wide (24 to 73 bu/1000 ft2) range of seed root densities to produce sweetpotato slips (D. Godwin, D. Scott, and J. Jones, personal communication). However, a lack of knowledge exists on what is the optimum seed root density and size for production of sweetpotato slips. Depending on grower’s equipment, the propagation beds are elevated about 10 inches and are 36 inches wide (Wilson et al., 1977). Mainly canner size roots (1 to 1.75 inches diameter) are used as seed source for propagation beds (Smith et al., 2009). These seed roots are cured [85 °F, 85% to 90% relative humidity (RH)] for ≈1 week after harvest to heal wounds to the skin that occurs during harvest and then stored at 55 to 60 °F and 80% to 85% RH (Edmunds et al., 2008; Kemble, 2013; Steinbauer and Kushman, 1971). Before bedding, the seed roots are presprouted in storage (85 °F, 85% RH) for 20 to 28 d (Kemble, 2013). Sweetpotato seed roots are placed into the propagation beds and then covered with 2 inches of soil (Wilson et al., 1977). Clear polyethylene mulch is then placed over the beds to increase the soil temperature and facilitate sprouting. Mulch is vented after 7 d to prevent an accumulation of carbon dioxide and then removed completely once shoot emergence begins and before bed temperatures become too hot. Once slips reach optimal size (7 to 14 inches), they are cut either by hand or with mechanical cutters, and packed in boxes (1000 slips/box) to transport and plant directly into the production field (Smith et al., 2009). Many commercial growers in North Carolina adopt a once-over harvest strategy, meaning that only one cutting is obtained from slip propagation beds. However, there are some growers that cut slips multiple times, coming back when slips have regrown to optimal size. Adopting a multiple harvest slip production strategy is common in geographical locations that have longer sweetpotato-growing seasons. Australian growers can harvest seedbed cuttings at least four to five times with 4 weeks between harvests (Northern Territory Government, 2005).
At the end of the growing season, when storage roots are harvested, a portion of the roots are saved to use as seed roots for the following year. This process is repeated for a number of years, each of which is called a generation (G). However, clones can slowly decline over each generation due to the accumulation of viruses, pathogens, and mutations (Clark et al., 2002; Villordon and LaBonte, 1996). Therefore, it is recommended to use no older than G-5 (the number indicates number of years in field production) seed root stock (Bryan et al., 2003).
Research is lacking with respect to documenting the effects of sweetpotato seed root density and size on transplant yield and quality. Thus, the primary objective of this research was to identify optimal seed root density and size of ‘Covington’ and ‘Evangeline’ that maximize production and quality of slips and return on investment. This project was conducted in direct response to grower interest and with sweetpotato industry funding.
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