Sweetpotato is an important specialty crop in the United States (Wadl et al., 2018). After many years of decline beginning in the 1950s, per-capita U.S. consumption of sweetpotatoes has shown a dramatic increase during the past 10 years, and production levels have increased to a level not seen since the 1940s. Nationally, sweetpotato production increased by 37.6% from 2012 to 2017—by far the biggest jump of any vegetable crop (U.S. Department of Agriculture, 2019; Washington Post, 2019). These gains have been the result of new cultivars, better management practices, and increased industry/consumer demand for more nutritious foods, including a wide range of value-added products. The economic viability of sweetpotato production can be undermined as a result of the susceptibility of commercial varieties to biotic (e.g., diseases, insect pests, weeds) and abiotic (e.g., environment variability, low fertility) factors that can dramatically influence yield, quality, and marketability of the harvested storage roots. Effectively managing weeds is a top priority for sweetpotato growers.
Three weeds that are of primary concern to sweetpotato producers in the Carolinas are yellow nutsedge (Cyperus esculentus L.), purple nutsedge (Cyperus rotundus L.), and Palmer amaranth (Amaranthus palmeri S. Watts). Yellow and purple nutsedge affect sweetpotato yield and quality negatively, and are the most difficult-to-control weeds in sweetpotato across the Southeast because of the high rainfall experienced during the growing season. In Mississippi, Meyers and Shankle (2015) reported that yellow nutsedge densities of 5 to 90 shoots/m2 of planted sweetpotato row resulted in yield losses of 18% to 96% depending on grade. A single nutsedge shoot at 2 weeks after transplanting can increase 7.6 times by the end of the season, resulting in exponential asexual reproduction of this troublesome weed (Meyers and Shankle, 2015). In addition to reducing yield, nutsedge roots, rhizomes, shoots, and tubers grow into and through sweetpotato storage roots, resulting in storage roots being marketed for processing instead of fresh consumption, at an 80% decrease in value to the producer. Palmer amaranth is the most competitive weed and is the driving force for weed management programs in North Carolina and South Carolina. It can easily exceed sweetpotato height within 10 d after transplanting and ultimately reaches heights greater than 6 ft and spreads to nearly 5 ft wide (Meyers et al., 2010). As a result, the weed can block up to 82% of available light from reaching the sweetpotato crop and decreases marketable yield by 85% (Meyers et al., 2010). Simply leaving palmer amaranth in the field for 2 weeks can result in a 5% reduction of marketable sweetpotato yield (Smith et al., 2020).
To manage weeds, sweetpotato growers use herbicides, between-row cultivation, mowing, and hand removal. Currently, 10 herbicides are registered for managing weeds in season (Kemble, 2020). Those commonly used are flumioxazin, S-metolachlor, clomazone, and two graminicides (sethoxydim and clethodim). Although napropamide and dimethyl tetrachloroterephthalate are registered for sweetpotato, they provide inconsistent and often inadequate weed control (Weir, 2001). S-metolachlor can provide excellent pigweed control if applied and activated before weed emergence. Also, it is the only herbicide registered in sweetpotato that can provide yellow nutsedge suppression. However, producers are reluctant to apply S-metolachlor at transplanting, citing concerns that it results in misshapen storage roots of some cultivars (Meyers et al., 2012; Personal communication with multiple growers in South Carolina). It has been well documented that delaying application of S-metolachlor until 10 d after transplanting can reduce some of the injury (Meyers et al., 2012). Optimizing application timing of S-metolachlor, as well as identification of cultivars that are tolerant to the effects of S-metolachlor, are critical to weed suppression in Southeast sweetpotato production.
The identification and selection of S-metolachlor-tolerant plants is a priority for sweetpotato breeders. S-metolachlor is the only herbicide registered in sweetpotato for suppression of yellow nutsedge and it provides excellent control of pigweed species; therefore, it is important for sweetpotato growers in the Southeast to plant sweetpotato cultivars that have tolerance to S-metolachlor (Meyers and Shankle, 2017). In vitro methods are efficient for screening stress tolerance in different plants, demanding lower resources, materials, and time than field trials (Sakhanokho and Kelley, 2009). Hydroponics has been used to access herbicide tolerance or resistance in weeds (Brosnan et al., 2014; Cutulle et al., 2009). In our study, research was conducted to determine whether a hydroponics screening with S-metolachlor could discriminate between a known S-metolachlor-sensitive cultivar and a cultivar that is known to be tolerant to the herbicide.
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