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ARS Home » Southeast Area » Tifton, Georgia » Crop Protection and Management Research » Research » Publications at this Location » Publication #230457
Title: Establishing the Geographical Distribution and Level of Acetolactate Synthase Resistance of Palmer Amaranth (Amaranthus palmeri) Accessions in Georgia

Author
item WISE, A - UNIV OF GA
item GREY, T - UNIV OF GA
item PROSTKO, E - UNIV OF GA
item VENCILL, W - UNIV OF GA
item Webster, Theodore

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2009
Publication Date: 4/1/2009
Citation: Wise, A.M., Grey, T.L., Prostko E.P., Vencill, W.K., Webster, T.M. 2009. Establishing the geographical distribution and level of acetolactate synthase resistance of Palmer amaranth accessions in Georgia. Weed Science. 23(2): 214-220.

Interpretive Summary: Many pigweeds (Amaranthus spp.) are native to the US, but only a few have become significant weed problems. In Georgia, pigweeds rank as the third most troublesome weed across all agronomic crops, second most troublesome in cotton and vegetables, and among the top five in corn, and soybean. Palmer amaranth was the most troublesome weed in peanut, likely due to the occurrence of herbicide resistance. The ALS-herbicides are an effective group of herbicides used in peanut. Initial ALS-screening in GA indicated there was ALS resistance in Palmer amaranth populations throughout the peanut growing region. However, data indicated heterogeneity of the levels of resistance both among and within the screened accessions. In addition to imazapic resistance, accessions exhibited cross resistance to diclosulam, pyrithiobac, and chlorimuron. Glyphosate resistance was not detected for any accession. While the level of resistance varied by accession, repeated applications of ALS herbicides without alternative modes of action will increase ALS resistant Palmer amaranth populations across the state. Application of ALS herbicides in Georgia peanut increased from 63% treated ha in 1999 to 93% by 2003. Alternatives to managing ALS-resistant Palmer amaranth are necessary for peanut producers to maintain profitability. Research from North Carolina indicated season-long interference of Palmer amaranth at a density of 1 plant/m crop row reduced peanut yields 28%, a density that readily occurs in Georgia. The current University of Georgia recommendation for ALS-resistant Palmer amaranth management for peanut include herbicides from multiple groups, including dinitroanilines (K), chloroacetamides (K3), bipyridiliums (D), and PPO inhibitors (E). Proper stewardship of these herbicides and management of resistant weeds species should include rotations of herbicide mode of action, both within a cropping season, and across crop rotations in multiple years.

Technical Abstract: Palmer amaranth resisitance to acetolactate synthase inhibiting (ALS) herbicides was first identified in Georgia in 2000. Since then, complaints from farmers have increased concerning failure of ALS-herbicides in controlling Palmer amaranth in peanut. Efficacy of ALS-herbicides can be compromised under adverse conditions; therefore seed from Palmer amaranth plants that escaped weed control was collected across the peanut growing region in Georgia. Greenhouse and growth chamber studies were conducted using these seed to evaluate whether weed escapes were a result of Palmer amaranth resistance to ALS herbicides. Each of the 61 accessions collected across Georgia exhibited varying levels of resistance to imazapic applied POST (<55% control, relative to the ALS-susceptible Palmer amaranth). Subsamples of the accessions were evaluated for their response to imazapic rates and indicated variable levels of resistance across Palmer amaranth accessions. The rate of imazapic that provided 50% reduction in Palmer amaranth plant biomass (I50) for the known susceptible biotype was 0.9 g/ha of imazapic. Of the 10 accessions evaluated, eight of them had I50 values that ranged from 3 to 297 g/ha of imazapic. The other two accessions could not be fit to the log-logistic dose response curve and had undeterminable I50 values due to high levels of ALS resistance (>1400 g/ha of imazapic). Herbicide cross resistance experiments indicated that 30 accessions were resistant to the ALS-herbicides imazapic, chlorimuron, pyrithiobac, and diclosulam at the recommended field use rates. However, each of these 30 accessions were susceptible to glyphosate. These data demonstrate that ALS-resistant Palmer amaranth occurs throughout the peanut growing region of Georgia. Growers in Georgia will need to alter their weed control programs in peanut to include herbicides with multiple modes of action that do not rely on ALS herbicides for effective Palmer amaranth control.