Fomesafen Herbicide Programs for Palmer Amaranth Control in Sweetpotato

The objective of this study was to determine the effect of fomesafen-preplant on Palmer amaranth control and sweetpotato tolerance, yield and quality. Studies were conducted at the Horticultural Crops Research Station, Clinton, NC on a sandy loam soil. The experimental design was a randomized complete block with three (2012) or four (2013) replications. Herbicide treatments included fomesafen-preplant alone at 0.20, 0.28, 0.36, 0.42, 0.56, and 0.84 kg ai/ha (kg/ha) or followed by (fb) S-metolachlor at 1.12 kg/ha 7 days after transplanting (DAP), and fomesafen 0.28 kg/ha followed by S-metolachlor at 1.12 kg/ha 14 DAP. Additional treatments included flumioxazin preplant 0.105 kg/ha alone, S-metolachlor 1.12 kg/ha 7 DAP and a sequential treatment of flumioxazin fb S-metolachlor (standard treatment), flumioxazin preplant fb clomazone plus S-metolachlor 7 DAP, clomazone 0.63 kg/ha 7 DAP, and napropamide 2.24 kg/ha 7 DAP. A weedy and weed-free check was included for comparison. ‘Covington’ and ‘Evangeline’ sweetpotato slips were transplanted with a mechanical transplanter. In 2012, Palmer amaranth control 28 DAP was at least 90% from all treatments except clomazone and fomesafen alone at 0.20 kg/ha.  At 50 DAP, all sequential treatments of fomesafen fb S-metolachlor provided at least 95% control except the fomesafen at 0.20 and 0.36 kg/ha which provided approximately 80% control. Palmer amaranth control by fomesafen fb S-metolachlor at 14 DAP was 66% which was less than control when fomesafen fb S-metolachlor 7 DAP (96%). In 2013, at 28 DAP all the treatments provided at least 90% Palmer amaranth control except clomazone (51%). The trend was similar at 50 DAP. In both years, Palmer amaranth control from fomesafen at 0.28, 0.36, 0.42, 0.56, and 0.84 kg/ha was similar to treatments including fomesafen fb S-metolachlor 7DAP and flumioxazin alone or fb S-metolachlor (standard treatment) or fb clomazone plus S-metolachlor. Maximum injury and stunting from fomesafen at 0.36 and 0.20 kg/ha fb S-metolachlor was 16 and 8%, respectively. Marketable yield (Jumbo and No.1 grades) was lower in plots treated with S-metolachlor alone or 0.20 or 0.84 kg/ha fomesafen or clomazone in 2012. In 2013, marketable yield was lower than weed-free treatment from fomesafen fb S-metolachlor 14 DAP or fomesafen at 0.20 or 0.36 kg/ha or clomazone. In both years, all other treatments had similar yield as the weed-free. Overall, fomesafen rates from 0.28 to 0.56 kg/ha alone or in sequential with S-metolachlor provide adequate control of palmer amaranth without causing significant injury and yield reduction in sweetpotato.