Survey of Glyphosate- and Imazapic-Resistant Palmer Amaranth (Amaranthus palmeri) in Florida

Herbicide-resistant Palmer amaranth (Amaranthus palmeri S. Wats.) is the most troublesome weed in Florida cotton (Gossypium hirsutum L.) and peanut (Arachis hypogaea L.) production. In 2012, Palmer amaranth populations were surveyed to document the extent and level of resistance to glyphosate and imazapic. Mature seedheads were sampled at 31 locations in the northwest region of Florida where cotton and peanut are predominantly produced, and in the north-central region of Florida that mainly consists of peanut production. Palmer amaranth plants were grown from field-collected seed in the greenhouse and treated with glyphosate and imazapic to establish a dose response for each population. Glyphosate-resistant Palmer amaranth was found in 11 populations that were located primarily in the northwest region, with some populations having up to 30-fold resistance. Imazapic resistance was much more widespread, being found in 29 of the 30 populations tested. Since peanut production in Florida has historically relied on successive imazapic applications, it is not surprising that widespread resistance to imazapic has developed. Resistance to both herbicides was found in 10 populations. Results from this survey reveal the magnitude of herbicide resistance in Florida and the need for growers to diversify management strategies in the future.