Sweetpotato is an important crop in the United States and was worth over $716 million in gross farm value in 2015 [U.S. Department of Agriculture (USDA), 2016a] with North Carolina, California, Mississippi, and Louisiana farmers planting over 89% of the production area (USDA, 2016b). North Carolina ranks number one in sweetpotato production in the United States with 87,000 acres planted ($332 million in gross farm value) in 2015, which accounts for more than half of the total acreage planted nationwide (USDA, 2016b). Sweetpotato transplant size and planting depth in production fields are important considerations because sweetpotato is vegetatively propagated by using nonrooted stem cuttings (also called transplants or slips) for commercial production. The size of sweetpotato transplants can vary considerably in sweetpotato propagation beds (A. Thornton, personal communication). When transplanted into production fields, transplant sizes can affect the yield and quality of sweetpotato storage roots. Most growers use 7- to 14-inch-long transplants (considered optimal size transplants) to plant production fields (Barkley et al., 2017; Smith et al., 2009). The sweetpotato cultivar Covington is unlike most other commercial cultivars as it has a growth habit in which the top canopy leaves are mostly at the same height, but the location of the apical meristem can vary from being close to the top of the leaf canopy to a few inches below. Because the apical meristem location can vary with respect to the top of the canopy, transplant sizes for this variety often vary when cut on the propagation bed (Barkley et al., 2017). ‘Covington’ is grown on 88% of the sweetpotato acreage in North Carolina (North Carolina Crop Improvement Association, 2014).
Transplants are cut from plant beds by hand using a knife or by using mechanical plant cutters and then boxed by hand for planting in production fields (Smith et al., 2009). Thus, ‘Covington’ transplants often vary in plant size, which can lead to planting difficulties in the field. Extreme variation in plant size can lead to skips as laborers have challenges in handling and placing transplants into the fingers of a mechanical transplanter (Fig. 1). Variation in transplant size makes achieving a uniform planting depth for sweetpotato transplants difficult because planting depth is contingent on position of the transplant within the fingers of the mechanical transplanter. Laborers follow the mechanical transplanter to replant transplants by hand because laborers on the mechanical transplanter periodically fail to place good quality plants in the mechanical transplanter fingers. The planting depth can also affect transplant survival and overall yields because of the number of nodes located beneath the soil surface (Meyers et al., 2017). Sweetpotato transplants should be set deep, with a portion of the plant above the soil surface, but with at least three nodes below the soil surface (Granberry et al., 1986). Other reports have shown that sweetpotato transplants are typically planted at a depth of 7.6–10.2 cm (3–4 inches) and several nodes are placed underground to maximize potential root set (Boudreaux et al., 2005).
Environmental conditions such as precipitation and initial soil moisture have also been shown to be key components to transplant survival and root set in sweetpotato (Gajanayake et al., 2013; Meyers et al., 2017). Soil moisture stress during plant establishment can limit the initial growth and development of sweetpotato roots and eventually affect storage root yield (Indira and Kabeerathumma, 1988; Pardales and Esquibel, 1997). Sweetpotato are often grown on nonirrigated lands and have been considered drought-tolerant (Constantin et al., 1974), although the response to moisture stress varies with variety (Villareal et al., 1979). According to Nair (2000), storage root yield decreases if drought conditions exist in the initial 10–30 d after planting (DAP), which is the time period of storage root initiation. Edmond and Ammerman (1971) also reported reduced storage root yields if drought occurred within the first 6 weeks after planting.
Sweetpotato growers have some knowledge about the transplant size that should be used to maximize plant stands, but documentation as to the interaction of preplant irrigation, transplant size, and planting depth and their resulting effect on plant survival and storage root quality and yield is limited. Thus, studies were conducted to determine the effect of transplant size, planting depth, and preplant irrigation on ‘Covington’ sweetpotato transplant survivability and storage root yield and quality.
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