A major concern of sweetpotato producers is the potential negative effects from herbicide drift or sprayer contamination events when dicamba is applied to nearby dicamba-resistant crops. A field study was initiated in 2014 and repeated in 2015 to assess the effects of reduced rates of N,N-Bis-(3-aminopropyl)methylamine (BAPMA) or diglycloamine (DGA) salt of dicamba, glyphosate, or a combination of these individually in separate trials with glyphosate on sweetpotato. Reduced rates of 1/10, 1/100, 1/250, 1/500, 1/750, and 1/1,000 of the 1× use rate of each dicamba formulation at 0.56 kg ha−1, glyphosate at 1.12 kg ha−1, and a combination of the two at aforementioned rates were applied to ‘Beauregard’ sweetpotato at storage root formation (10 d after transplanting) in one trial and storage root development (30 d after transplanting) in a separate trial. Injury with each salt of dicamba (BAPMA or DGA) applied alone or with glyphosate was generally equal to or greater than glyphosate applied alone at equivalent rates, indicating that injury is most attributable to the dicamba in the combination. There was a quadratic increase in crop injury and a quadratic decrease in crop yield (with respect to most yield grades) observed with an increased herbicide rate of dicamba applied alone or in combination with glyphosate applied at storage root development. However, with a few exceptions, neither this relationship nor the significance of herbicide rate was observed on crop injury or sweetpotato yield when herbicide application occurred at the storage root formation stage. In general, crop injury and yield reduction were greatest at the highest rate (1/10×) of either salt of dicamba applied alone or in combination with glyphosate, although injury observed at lower rates would be cause for concern after initial observation by sweetpotato producers. However, in some cases yield reduction of No.1 and marketable grades was observed following 1/250×, 1/100×, or 1/10× application rates of dicamba alone or with glyphosate when applied at storage root development.

Management options are needed to limit sweetpotato yield loss due to weeds. Greenhouse studies were conducted in 2018 in Greensboro, NC, and in the field from 2016 to 2018 in Clinton, NC, to evaluate the effect of bicyclopyrone on sweetpotato and Palmer amaranth (field only). In greenhouse studies, Covington and NC04-531 clones were treated with bicyclopyrone (0, 25, 50, 100, or 150 g ai ha−1) either preplant (PP; i.e., immediately before transplanting) or post-transplant (PT; i.e., on the same day after transplanting). Sweetpotato plant injury and stunting increased, and vine length and shoot dry weight decreased with increasing rate of bicyclopyrone regardless of clone or application timing. In field studies, Beauregard (2016) or Covington (2017 and 2018) sweetpotato clones were treated with bicyclopyrone at 50 g ha−1 PP, flumioxazin at 107 g ai ha−1 PP, bicyclopyrone at 50 or 100 g ha−1 PP followed by (fb) S-metolachlor at 800 g ai ha−1 PT, flumioxazin at 107 g ha−1 PP fb S-metolachlor at 800 g ha−1 PT, flumioxazin at 107 g ha−1 PP fb S-metolachlor at 800 g ha−1 PT fb bicyclopyrone at 50 g ha−1 PT-directed, and clomazone at 420 g ai ha−1 PP fb S-metolachlor at 800 g ha−1 PT. Bicyclopyrone PP at 100 g ha−1 fb S-metolachlor PT caused 33% or greater crop stunting and 44% or greater marketable yield reduction compared with the weed-free check in 2016 (Beauregard) and 2017 (Covington). Bicyclopyrone PP at 50 g ha−1 alone or fb S-metolachlor PT resulted in 12% or less injury and similar no. 1 and jumbo yields as the weed-free check in 2 of 3 yr. Injury to Covington from bicyclopyrone PT-directed was 4% or less at 4 or 5 wk after transplanting and marketable yield was similar to that of the weed-free check in 2017 and 2018.

Studies were conducted to determine the tolerance of sweetpotato and Palmer amaranth control to a premix of flumioxazin and pyroxasulfone pretransplant (PREtr) followed by (fb) irrigation. Greenhouse studies were conducted in a factorial arrangement of four herbicide rates (flumioxazin/pyroxasulfone PREtr at 105/133 and 57/72 g ai ha–1, S-metolachlor PREtr 803 g ai ha–1, nontreated) by three irrigation timings [2, 5, and 14 d after transplanting (DAP)]. Field studies were conducted in a factorial arrangement of seven herbicide treatments (flumioxazin/pyroxasulfone PREtr at 40/51, 57/72, 63/80, and 105/133 g ha–1, 107 g ha–1 flumioxazin PREtr fb 803 g ha–1S-metolachlor 7 to 10 DAP, and season-long weedy and weed-free checks) by three 1.9-cm irrigation timings (0 to 2, 3 to 5, or 14 DAP). In greenhouse studies, flumioxazin/pyroxasulfone reduced sweetpotato vine length and shoot and storage root fresh biomass compared to the nontreated check and S-metolachlor. Irrigation timing had no influence on vine length and root fresh biomass. In field studies, Palmer amaranth control was≥91% season-long regardless of flumioxazin/pyroxasulfone rate or irrigation timing. At 38 DAP, sweetpotato injury was≤37 and≤9% at locations 1 and 2, respectively. Visual estimates of sweetpotato injury from flumioxazin/pyroxasulfone were greater when irrigation timing was delayed 3 to 5 or 14 DAP (22 and 20%, respectively) compared to 0 to 2 DAP (7%) at location 1 but similar at location 2. Irrigation timing did not influence no.1, jumbo, or marketable yields or root length-to-width ratio. With the exception of 105/133 g ha–1, all rates of flumioxazin/pyroxasulfone resulted in marketable sweetpotato yield and root length-to-width ratio similar to flumioxazin fb S-metolachlor or the weed-free checks. In conclusion, flumioxazin/pyroxasulfone PREtr at 40/51, 57/72, and 63/80 g ha–1 has potential for use in sweetpotato for Palmer amaranth control without causing significant crop injury and yield reduction.

An experiment was conducted at two locations in 2003 and 2004 to determine the timing and rate of thifensulfuron that is safe to use on sweetpotato. Thifensulfuron was applied 1, 2, and 4 wk after transplanting (WAP) in 2003 and 4, 6, and 8 WAP in 2004. Within each timing, thifensulfuron treatments were 1.1, 2.1, 3.2, 4.3, and 8.5 g ai/ha plus a weed-free control. The 1 and 2 WAP timings of thifensulfuron reduced the yield of number 1 roots greater than 25%. The 4, 6, and 8 WAP timings had less than 15% reduction in yield, with the 6 WAP timing reducing number 1 roots and total yield 10% or less. When 4.3 g/ha of thifensulfuron was applied 4 WAP, total yield was reduced 13%. The 6 and 8 WAP timings had little yield reduction, with no rate response observed. Application of 4.3 g/ha of thifensulfuron at 6 WAP would allow for control of problematic weed species while limiting potential yield loss. Yield loss from a 4 WAP application of thifensulfuron may in fact be a delay in crop maturity that could be recovered if the sweetpotato harvest was delayed to allow for the optimal amount of number 1 grade roots to be produced.

Differences in clomazone tolerance among sweetpotato cultivars were first observed in April 2007 after use of the herbicide for weed control in fields containing the sweetpotato breeding project at the U.S. Vegetable Laboratory. Susceptible cultivars and experimental clones exhibited severe foliar chlorosis and reduced growth, whereas the most tolerant were not injured. Twelve cultivars and experimental clones were included in a greenhouse experiment to quantify the differences in clomazone tolerance. In the greenhouse, the clomazone concentration that caused moderate injury or reduction in shoot growth to the most tolerant clones was approximately 10 times the concentration that caused a similar response in the most susceptible clones. Two older cultivars, Excel and Sumor, that were developed before the registration of clomazone for use in sweetpotato were susceptible. Clomazone is an important component in sweetpotato weed management, and susceptibility is an undesirable trait that should be avoided in new sweetpotato cultivars.

Studies were conducted in 2003 and 2004 to determine the effect of application timing and halosulfuron rate on sweetpotato yield and quality. Halosulfuron was applied 1, 2, and 4 wks after transplanting (WAP) sweetpotato in 2003, and 2, 3, and 4 WAP in 2004. Treatments within each timing included halosulfuron at 13, 26, 39, 52, and 65 g ai/ha plus a weed-free control. Combined over year, site, cultivar and rate, halosulfuron applied at 1, 2, 3, and 4 WAP stunted sweetpotato 32, 15, 11, and 14%, respectively, rated 2 wks after treatment. The stunting observed with the 1 and 2 WAP timings caused a 17 and 10% reduction in yield of No. 1 roots, respectively, compared with the weed-free control. The 3 and 4 WAP timings of halosulfuron did not reduce yield of No. 1 roots. Total yield was reduced approximately 11% at the 1, 2, and 3 WAP application timings. Halosulfuron at 4 WAP did not reduce total yield. Combined over year, site, and cultivar, halosulfuron applied at 39 g/ha did not reduce the weight of No. 1 roots or total crop yield and thus could be an effective POST option for weed control in sweetpotato.