Distribution, biology, and management of glyphosate-resistant Palmer amaranth in North Carolina

The introduction of glyphosate-resistant (GR) crops allowed for topical applications of the herbicide glyphosate. This herbicide revolutionized weed control and crop management. Widespread adoption of this technology and extensive use of glyphosate led to intense selection pressure for evolution of GR weeds.;In 2005, GR Palmer amaranth was suspected in North Carolina. A survey detected GR populations in 49 of 290 fields sampled. Resistance to herbicides that inhibit acetolactate synthase (ALS) was also detected in 52 fields. Five fields had populations exhibiting multiple resistance to both glyphosate and ALS-inhibitors.;Experiments were conducted to determine the resistance mechanism of GR Palmer amaranth. A GR biotype exhibited a 20-fold level of resistance compared to a glyphosate-susceptible (GS) biotype. Maximum absorption of 14 C-glyphosate was observed by 12 hours after treatment (HAT), and was similar among biotypes except at 6 HAT, where GS plants absorbed 67% more than GR plants. Distribution of 14C was similar among biotypes in, above, and below the 14C-glyphosate treated leaf and in roots. This work did not lead to discovery of a resistance mechanism.;Field experiments were conducted to develop management strategies for GR Palmer amaranth in cotton. One evaluated residual control of Palmer amaranth by various herbicides. Of herbicides typically applied PRE or pre-plant, fomesafen, flumioxazin, and pyrithiobac were most effective. Pyrithiobac and S-metolachlor were the most effective postemergence (POST) herbicides. Flumioxazin and prometryn plus trifloxysulfuron were the most effective options for postemergence-directed applications. Integration of these herbicides into glyphosate-based systems would increase Palmer amaranth control.;Another experiment evaluated PRE herbicides in a season-long system. All PRE herbicides increased late-season control. Among individual herbicides, fomesafen and pyrithiobac were most effective. Combinations of fomesafen plus pyrithiobac or diuron and diuron plus pyrithiobac were the most effective PRE applications. Another experiment investigated herbicide systems with residual herbicides applied pre-plant, PRE and POST. Pre-plant applications of flumioxazin and PRE applications of fomesafen increased late-season control, but applications of both were more effective than either herbicide alone. Applications of glyphosate plus pyrithiobac POST were more effective than glyphosate alone. Glyphosate plus S-metolachlor was more effective than glyphosate alone when activated by rainfall or irrigation and when Palmer amaranth had not emerged. These data suggest early-season control of GR Palmer amaranth is critical for successful management in cotton.;Glufosinate is another herbicide effective on Palmer amaranth. However, growers were reluctant to plant glufosinate-tolerant cotton cultivars. Widestrike cotton is GR and also contains a glufosinate tolerance gene used as a selectable marker, however glufosinate tolerance in production situations had not been investigated. Experiments were conducted to evaluate Widestrike cotton tolerance to glufosinate. Yield was reduced by glufosinate in only one of 11 trials by 4%, suggesting acceptable tolerance.;Another experiment evaluated weed control with glufosinate and glyphosate in Widestrike cotton. Control of GR Palmer amaranth by glufosinate-based systems was higher than glyphosate-based systems, which demonstrated that glufosinate-based systems could be used to control GR Palmer amaranth in Widestrike cotton.;In soybean, several glyphosate alternative herbicides could be used to control Palmer amaranth. An experiment was conducted to evaluate control of GS and GR Palmer amaranth from a glyphosate-only system compared to several alternative systems. Glyphosate alone applied once POST was very effective on GS Palmer amaranth and alternative systems with two PREs followed by fomesafen POST provided similar control compared to glyphosate. In fields with GR Palmer amaranth, greater than 80% late-season control was obtained only with systems of two PREs followed by fomesafen POST.