Assessing a programmatic approach for the control of quinclorac‐resistant smooth crabgrass (Digitaria ischaemum) in established bermudagrass turf

The auxin‐mimicking herbicide quinclorac controls crabgrass (Digitaria spp.) after emergence in cool‐ and warm‐season turfgrass species. Quinclorac resistance has been confirmed in smooth [Digitaria ischaemum (Schreb.) Muhl.] and large crabgrass [Digitaria sanguinalis (L.) Scop.]. Two Mississippi smooth crabgrass populations have been confirmed as quinclorac‐resistant. Field experiments were conducted to evaluate programmatic approaches to control one of these populations. Treatments included the sequential application of pre‐emergence herbicides on 17 Feb. and 10 Apr. 2020, and 25 Feb. and 6 Apr. 2021, alone or with 0.84 kg quinclorac ha–1 applied postemergence on 10 April 2020 and 6 April 2021, then a second application of quinclorac alone on 18 June 2020 and 16 June 2021. Pre‐emergence treatments included 0.05 kg indaziflam ha–1, 0.84 kg prodiamine ha–1, 0.56 kg dithiopyr ha–1, and 0.47 kg prodiamine ha–1 + 0.21 kg imazaquin ha–1 + 0.73 kg simazine ha–1. Commercial standards applied on 10 Apr. and 18 June 2020, and 6 Apr. and 16 June 2021 included 0.16 kg pinoxaden ha–1 and 2.27 kg monosodium methanearsonate ha–1. Four weeks after the last application, all treatments except quinclorac (2020 and 2021), dithiopyr (2020), and pinoxaden (2020) controlled quinclorac‐resistant smooth crabgrass by >90%. Various standard pre‐emergence herbicides adequately controlled the resistant population. Quinclorac combined with pre‐emergence strategies provided greater control of smooth crabgrass than quinclorac alone. Ongoing research is investigating the mechanism of quinclorac resistance. Future research should screen a wider distribution of populations and investigate cross‐ and multiple resistance. Core Ideas Two Mississippi smooth crabgrass were suspected of quinclorac‐resistance. Pre‐emergence control strategy is fundamental to the chemical control of smooth crabgrass. Future research investigates the mechanism of quinclorac resistance in resistant populations.