Interference and Control of Palmer Amaranth (Amaranthus palmeri) in Sweetpotato

Abstract

ABSTRACT

MEYERS, STEPHEN LAWRENCE. Interference and Control of Palmer Amaranth (Amaranthus palmeri) in Sweetpotato. (Under the direction of Katie Jennings and Jonathan Schultheis.)

The most common and troublesome weed in North Carolina sweetpotato is Palmer amaranth, an upright, branching, annual weed with rapid growth and high fecundity. Field studies were conducted in 2007 and 2008 to develop a Palmer amaranth management program in sweetpotato using density models to establish thresholds, and herbicides for control. Palmer amaranth was established at 0, 0.5, 1.1, 1.6, 3.3, and 6.5 plants/m within the sweetpotato row and densities maintained season-long. Jumbo, no. 1, and marketable sweetpotato yield loss ranged from 56 to 94%, 30 to 85%, and 36 to 81%, respectively for 0.5 to 6.5 Palmer amaranth/m. Yield loss displayed a positive linear relationship with Palmer amaranth light interception. Light intercepted by the Palmer amaranth canopy increased linearly from 0.5 to 6.5 plants/m and was greater than 42% regardless of density. Palmer amaranth height was greater than 2 m for all treatments and plant canopy width (66 to 136 cm) and shoot dry biomass/plant (0.3 to 1.1 kg) decreased linearly as density increased. Volumetric soil water content differed by treatment at one location. Soil moisture 8 weeks after transplanting (WAP) was greatest at a Palmer amaranth density of 3.3 plants/m. Preemergence herbicide treatments consisted of flumioxazin applied 2 days before transplanting at 91 or 109 g ai/ha alone or followed by (fb) S-metolachlor at 0.8, 1.1, or 1.3 kg ai/ha applied immediately after transplanting or 2 WAP. Palmer amaranth control was similar for all rates of S-metolachlor. In 2008, flumioxazin at 109 g/ha provided greater control than 91 g/ha. Flumioxazin fb S-metolachlor immediately after transplanting provided over 90% season long Palmer amaranth control. Flumioxazin fb S-metolahclor 2 WAP provided over 90% control in 2007 but 38 to 79% control in 2008. S-metolachlor applied alone immediately after transplanting provided 80 to 93% and 92 to 96% control in 2007 and 2008, respectively. S-metolachlor applied alone 2 WAP did not provide acceptable Palmer amaranth control. Visual crop injury due to treatment was less than 3%. Sweetpotato yield corresponded to Palmer amaranth control. Sweetpotato root shape was unaffected by all treatments. Glyphosate applied through a Dixie wick applicator was evaluated for Palmer amaranth control and safety to sweetpotato. In 2007, treatments consisted of glyphosate wicked 6 and 8 WAP and glyphosate wicked 6 and 8 WAP fb rotary mowing 9 WAP. In 2008, treatments consisted of glylphosate wicked once 4 or 7 WAP, wicked sequentially 4 and 7 WAP, mowed once 4 WAP, and mowed 4 WAP fb wicking 7 WAP. Palmer amaranth contacted by the wicking apparatus was controlled, but plants shorter than the wicking height escaped treatment. Interference prior to and between glyphosate treatment applications contributed to large sweetpotato yield losses. Treatments of glyphosate applied 7 or 8 WAP (in 2007 and 2008, respectively) frequently had greater no. 1 and marketable yields compared to the weedy control. However, jumbo, no. 1, and marketable yields for all glyphosate and mowing treatments were generally less than half the weed-free control. No additional control was provided by mowing. Cracked sweetpotato roots were observed in glyphosate treatments and percent cracking (by weight) ranged from 0 to 12 for no. 1 grade roots, and 0 to 9 for marketable roots. No cracked roots were observed in weedy, weed-free, or mowing once 4 WAP treatments. Relatively low Palmer amaranth densities contribute to large sweetpotato yield losses. Palmer amaranth should be managed below the sweetpotato canopy with a preemergence program consisting of flumioxazin pretransplant and S-metolachlor after transplanting.