International Survey of Herbicide-Resistant Weeds

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Status of Herbicide Resistance in Arizona - Bill Mccloskey : January, 2014

To the best of my knowledge, Palmer amaranth is the only herbicide resistant weed in Arizona as of December 2013. In 2013 we saw a big increase in preemergence herbicide use (dinitroanilines) in cotton in the areas where resistance is a problem. Greenhouse studies conducted in the first half of 2013 confirmed resistance to Staple in the Buckeye, AZ population (actually in the field and Palmer population where we discovered glyphosate resistance). In irrigated cotton field where tillage is practiced in Arizona, the dinitroanilines are very effective in suppressing Palmer amaranth emergence and our old chemistries (triazines and ureas) still work well. Thus, I don’t think cotton producers will have much difficulty with this weed in the next few years. In a couple of years, dicamba resistant cotton will available to those with a problem. I am most concerned about the ALS resistant Palmer amaranth populations that seem to be spreading in alfalfa fields. I visited several alfalfa fields this summer in Maricopa County after planting and application of Raptor (imazamox) and Pursuit (imazethapyr) for Palmer amaranth control. I saw a differential Palmer amaranth response to these herbicide applications with some plants remaining a heathy green while others died. There are no other postemergence herbicides available for Palmer amaranth control if these populations are also resistant to glyphosate and it is difficult to time the application of dinitroaniline herbicides in alfalfa to suppress all Palmer amaranth emergence. I collected seed late this fall from two of these populations to test for glyphosate, staple and imidazoline resistance. These experiments will start after the first of the year.

Herbicide Resistant Weeds in Arizona, United States.
#YearSpeciesSite of ActionActivesContacts
1 5699Palmer Amaranth2012Amaranthus palmeri
Palmer Amaranth
United States
(Arizona)
Arizona  ALS inhibitors (B/2), EPSP synthase inhibitors (G/9) Multiple Resistance: 2 Sites of Action
ALS inhibitors (B/2)
EPSP synthase inhibitors (G/9)
glyphosate, pyrithiobac-sodium Bill Mccloskey
Contributing Weed Scientists
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