Development of Herbicide Resistant Smooth Crabgrass Following Quinclorac Use in Fine Turf.

Crabgrass (Digitaria spp.) is one of the most common and widespread turfgrass weeds in the United States. Quinclorac is an herbicide that provides postemergence control of both smooth (Digitaria ischaemum (Schreb.) Schreb. ex Muhl.) and large crabgrass (Digitaria sanguinalis (L.) Scop.). However, a population of smooth crabgrass located on a golf course tee in Franklin, Indiana was unresponsive to applications of the label rate (0.84 kg ha^-1) and up to 3X the label rate (2.52 kg ha^-1) of quinclorac (Drive 75 DF) following <12 years of use. Dose response experiments were conducted twice in the greenhouse to determine the level of quinclorac resistance in this population. Plants were grown either from smooth crabgrass seed collected from the Franklin, Indiana location or a purchased susceptible population for comparison. Both resistant (R) and susceptible (S) smooth crabgrass plants were treated with a wide range (0.01x to 80x label rate) of quinclorac application rates in order to obtain the effective dose required to affect plant response 50% relative to the upper and lower limit (ED₅₀). Using both fresh plant biomass and visual control ratings 14 days after treatment (DAT), the ED₅₀ was estimated to range from 0.10 to 0.41 kg ha^-1 for the quinclorac susceptible smooth crabgrass population which is consistent with a labeled herbicide rate of 0.84 kg ha^-1. For the suspected quinclorac resistant smooth crabgrass population, ED₅₀ ranged from 3.8 to >30 kg ha^-1. The quinclorac resistant smooth crabgrass biotype was >37-fold resistant to quinclorac as calculated by the R/S ratio. To avoid the develop of quinclorac resistance in smooth crabgrass, alternate the mechanism of action for postemergence herbicide applications, use preemergence herbicides when reseeding of desirable turf is not needed, and enhance cultural controls where possible.