Common ragweed (Ambrosia artemisiifolia L.) accession in the Mid-Atlantic region resistant to ALS-, PPO-, and EPSPS-inhibiting herbicides

Common ragweed is a troublesome weed in many crops. Farmers and crop advisors in the coastal Mid-Atlantic region have reported inadequate control of common ragweed in soybean with glyphosate and other herbicide modes of action. To determine if herbicide resistance was one of the causes of poor herbicide performance, twenty-nine accessions from four states (DE, MD, NJ, VA) where common ragweed plants survived herbicide applications and produced viable seeds were used for greenhouse screening. Common ragweed seedlings from those accessions were treated with multiple rates of cloransulam, fomesafen, or glyphosate, applied as a single postemergence (POST) herbicide application. All accessions except one demonstrated resistance to at least one of the herbicides applied at twice the effective rate (2X), seventeen accessions were two-way resistant (glyphosate- and cloransulam-resistant; glyphosate- and fomesafen-resistant) and three-way resistance was present in eight accessions collected from three different states. Based on the POST study, five accessions were treated preemergence (PRE) with acetolactate synthase (ALS)-inhibiting PRE herbicides, and two accessions were treated with protoporphyrinogen oxidase (PPO)-inhibiting herbicides. All accessions treated PRE with the ALS-inhibitors chlorimuron or cloransulam demonstrated resistance at the 2X rates. Both accessions treated PRE with the PPO-inhibitor sulfentrazone, had survivors at the 2X rate. When the same accessions were treated PRE with fomesafen, one had survivors at the 2X rate, and one had survivors at the 1X rate. Results from these tests confirmed common ragweed with three-way resistance to POST herbicides is widespread in the region. In addition, this is the first confirmation that common ragweed accessions in the region are also resistant to ALS- or PPO-inhibiting herbicides when applied PRE.