Field studies were conducted in 2007 and 2008 at Clinton and Faison, NC, to evaluate the influence of Palmer amaranth density on ‘Beauregard’ and ‘Covington’ sweetpotato yield and quality and to quantify the influence of Palmer amaranth on light interception. Palmer amaranth was established at 0, 0.5, 1.1, 1.6, 3.3, and 6.5 plants m⁻¹within the sweetpotato row and densities were maintained season-long. Jumbo, number (no.) 1, and marketable sweetpotato yield losses were fit to a rectangular hyperbola model, and predicted yield loss ranged from 56 to 94%, 30 to 85%, and 36 to 81%, respectively for Palmer amaranth densities of 0.5 to 6.5 plants m⁻¹. Percentage of jumbo, no. 1, and marketable sweetpotato yield loss displayed a positive linear relationship with Palmer amaranth light interception as early as 6 to 7 wk after planting (R²= 0.99, 0.86, and 0.93, respectively). Predicted Palmer amaranth light interception 6 to 7, 10, and 13 to 14 wk after planting ranged from 47 to 68%, 46 to 82%, and 42 to 71%, respectively for Palmer amaranth densities of 0.5 to 6.5 plants m⁻¹. Palmer amaranth height increased from 177 to 197 cm at densities of 0.5 to 4.1 plants m⁻¹and decreased from 197 to 188 cm at densities of 4.1 to 6.5 plants m⁻¹; plant width (69 to 145 cm) and shoot dry biomass plant⁻¹(0.2 to 1.1 kg) decreased linearly as density increased.

Drought, ultraviolet-B (UV-B), and nitrogen stress are significant constraints for sweetpotato productivity. Their impact on plant growth and development can be acute, resulting in low productivity. Identifying phenotypes that govern stress tolerance in sweetpotatoes is highly desirable to develop elite cultivars with better yield. Ten sweetpotato cultivars were grown under nonstress (100% replacement of evapotranspiration (ET)), drought-stress (50% replacement of ET), UV-B (10 kJ), and low-nitrogen (20% LN) conditions. Various shoot and root morphological, physiological, and gas-exchange traits were measured at the early stage of the crop growth to assess its performance and association with the storage root number. All three stress factors caused significant changes in the physiological and root- and shoot-related traits. Drought stress reduced most shoot developmental traits (29%) to maintain root growth. UV-B stress increased the accumulation of plant pigments and decreased the photosynthetic rate. Low-nitrogen treatment decreased shoot growth (11%) and increased the root traits (18%). The highly stable and productive cultivars under all four treatments were identified using multitrait stability index analysis and weighted average of absolute scores (WAASB) analyses. Further, based on the total stress response indices, ‘Evangeline’, ‘O’Henry’, and ‘Beauregard B-14’ were identified as vigorous under drought; ‘Evangeline’, ‘Orleans’, and ‘Covington’ under UV-B; and ‘Bonita’, ‘Orleans’, and ‘Beauregard B-14’ cultivars showed greater tolerance to low nitrogen. The cultivars ‘Vardaman’ and ‘NC05-198’ recorded a low tolerance index across stress treatments. This information could help determine which plant phenotypes are desirable under stress treatment for better productivity. The cultivars identified as tolerant, sensitive, and well-adapted within and across stress treatments can be used as source materials for abiotic stress tolerance breeding programs.

Studies were conducted in 2007 and 2008 to determine the effect of flumioxazin andS-metolachlor on Palmer amaranth control and ‘Beauregard’ and ‘Covington’ sweetpotato. Flumioxazin at 0, 91, or 109 g ai ha⁻¹was applied pretransplant 2 d before transplanting alone or followed by (fb)S-metolachlor at 0, 0.8, 1.1, or 1.3 kg ai ha⁻¹PRE applied immediately after transplanting or 2 wk after transplanting (WAP). Flumioxazin fbS-metolachlor immediately after transplanting provided greater than 90% season-long Palmer amaranth control.S-metolachlor applied alone immediately after transplanting provided 80 to 93% and 92 to 96% control in 2007 and 2008, respectively. Flumioxazin fbS-metolachlor 2 WAP provided greater than 90% control in 2007 but variable control (38 to 79%) in 2008.S-metolachlor applied alone 2 WAP did not provide acceptable Palmer amaranth control. Control was similar for all rates ofS-metolachlor (0.8, 1.1, and 1.3 kg ha⁻¹). In 2008, greater Palmer amaranth control was observed with flumioxazin at 109 g ha⁻¹than with 91 g ha⁻¹. Sweetpotato crop injury due to treatment was minimal (< 3%), and sweetpotato storage root length to width ratio was similar for all treatments in 2007 (2.5 for Beauregard) and 2008 (2.4 and 1.9 for Beauregard and Covington, respectively). Sweetpotato yield was directly related to Palmer amaranth control. Results indicate that flumioxazin pretransplant fbS-metolachlor after transplanting provides an effective herbicide program for control of Palmer amaranth in sweetpotato.

Studies were conducted in 2010 and 2011 to determine the effect of herbicide-based Palmer amaranth management systems in ‘Covington' sweetpotato. Treatments consisted of three herbicide application times. Pretransplant applications were flumioxazin at 107 g ai ha⁻¹, fomesafen at 280 g ai ha⁻¹, flumioxazin at 70 g ha⁻¹plus pyroxasulfone at 89 g ai ha⁻¹, or no herbicide. A second herbicide application was applied within 1 d after transplanting (DAP) and consisted ofS-metolachlor at 800 g ai ha⁻¹, clomazone at 630 g ai ha⁻¹, or no herbicide. Two weeks after planting (WAP) plots receivedS-metolachlor at 800 g ha⁻¹, metribuzin at 140 g ai ha⁻¹, a tank mix ofS-metolachlor at 800 g ha⁻¹plus metribuzin at 140 g ha⁻¹, hand-weeding followed by (fb)S-metolachlor at 800 g ha⁻¹, or no herbicide. Crop tolerance, Palmer amaranth control, and sweetpotato yield in systems containing fomesafen pretransplant were similar to flumioxazin-containing systems. Systems containing flumioxazin plus pyroxasulfone pretransplant resulted in increased crop stunting and decreased sweetpotato yield in 2010, compared with systems containing flumioxazin or fomesafen, but were similar to systems with flumioxazin or fomesafen in 2011. In 2010, systems containingS-metolachlor applied within 1 DAP resulted in increased sweetpotato injury, similar Palmer amaranth control, and reduced no. 1, jumbo, and total sweetpotato yield, compared with systems with clomazone. In 2011, systems containing clomazone were more injurious to sweetpotato than systems receivingS-metolachlor, but Palmer amaranth control and sweetpotato yield were similar. Systems containing metribuzin 2 WAP resulted in increased sweetpotato injury and Palmer amaranth control (in 2010) but similar no. 1 and total sweetpotato yields, compared with systems containingS-metolachlor at 2 WAP. Hand-weeding fbS-metolachlor provided greater Palmer amaranth control and no. 1 sweetpotato yield than did systems ofS-metolachlor without a preceding hand-weeding event in 2010.