Occurrence and Management of Herbicide Resistant Palmer Amaranth in Arizona

Herbicide resistant Palmer amaranth was first discovered in Buckeye, AZ (western Maricopa Co.) in July of 2012 and greenhouse studies in 2012/2013 confirmed this population was resistant to glyphosate and pyrithobac-sodium. Additional field collections, field assays and greenhouse studies in 2013 and 2014 confirmed the occurrence and spread of resistance in Palmer amaranth populations in Arizona. A population in San Tan Valley (eastern Maricopa Co.) is resistant to ALS inhibitors but not glyphosate while a population in Marana, AZ (Pima Co.) and suspected resistant populations in Red Rock and Pearce, AZ appear to be resistant to glyphosate. Sampling is underway to determine spread of resistance in Palmer amaranth populations within the state. Herbicide resistance genes in Palmer amaranth do appear to be moving across the state despite Extension education efforts promoting sanitation and prevention. Field research conducted on managing herbicide resistant Palmer amaranth in cotton along with a discussion of management in alfalfa and corn grown adjacent to cotton will be presented.