Field dodder is an important weed in alfalfa grown for seed, and stringent control is required to keep the alfalfa seed from becoming contaminated with dodder seed. Pendimethalin has been the primary tool used to control dodder in alfalfa seed production for more than 25 yr. Flumioxazin was recently registered in alfalfa seed and forage crops, but its activity on field dodder was unknown. Control of field dodder with flumioxazin and pendimethalin was compared in greenhouse trials in a loamy sand soil. The number of emerged dodder seedlings able to twine on a simulated host were counted weekly for a 4-wk period following herbicide application. Flumioxazin applied at the alfalfa field use rate of 0.14 kg ai ha^-1 completely controlled dodder over the initial 4-wk period. Dodder either failed to emerge or emerged and died without twining on the simulated host in the 4-wk period. When dodder was reseeded after 4 wk in flumioxazin-treated soil, the number of twined dodder seedlings was reduced by 56% in one trial but was unaffected in a second trial compared with nontreated checks. In the initial 4-wk period following herbicide application, flumioxazin controlled field dodder similar to pendimethalin applied at 2.2 and 4.4 kg ai ha^-1 in both trials. However, after additional dodder seed was planted at 4 wk after treatment, dodder was suppressed more by pendimethalin than flumioxazin in the additional 4-wk period. Flumioxazin offers alfalfa seed and forage producers a new mode of action to manage early-emerging field dodder.

Nomenclature: Flumioxazin; pendimethalin; field dodder, Cuscuta pentagona Engelm.; alfalfa, Medicago sativa L.