Efficacy of Two Dithiopyr Formulations for Control of Smooth Crabgrass [Digitaria ischaemum (Schreb) Schreb. ex Muhl.] at Various Stages of Growth

Abstract

Although dithiopyr has been used for smooth crabgrass [Digitaria ischaemum (Schreb) Schreb. ex Muhl.] control for many years, data describing the efficacy of a new, water-based formulation of dithiopyr for smooth crabgrass control are limited. Research was conducted in Knoxville, TN, and Griffin, GA, evaluating water-based and wettable powder dithiopyr formulations at 0.56 and 0.43 kg·ha−1 for smooth crabgrass control when applied at the pre-emergence (PRE), one- to two-leaf (1LF), one- to two-tiller (1TL), and greater than three-tiller (3TL) stages of growth. These treatments were compared with quinclorac (0.84 kg·ha−1) applied at the same POST timings (i.e., 1LF, 1TL, and 3TL). When applied PRE, all dithiopyr treatments provided greater than 85% smooth crabgrass control at the end of the trial in both locations. At the 1LF stage, both rates and formulations of dithiopyr provided greater than 93% smooth crabgrass control at 4 weeks after application and greater than 77% at the end of the trial. Applied at the 1TL stage in Tennessee, no differences in smooth crabgrass control were detected between quinclorac and any dithiopyr treatment at the end of the trial; when applied in Georgia at the 1TL stage, quinclorac provided greater smooth crabgrass control at the end of the trial than either rate or formulation of dithiopyr. Although no differences were detected between any dithiopyr treatment and quinclorac applied at the 3TL stage in Tennessee, smooth crabgrass control at the end of the trial measured less than 70% for all treatments. At the end of the trial in Georgia, smooth crabgrass control with quinclorac (91%) was greater than both formulations of dithiopyr. These findings suggest that both the wettable powder and water-based formulations of dithiopyr can be used to effectively control smooth crabgrass at the PRE and 1LF stages of growth, but quinclorac should be selected over dithiopyr for control of tillering smooth crabgrass plants. Turfgrass managers should implement smooth crabgrass control measures at PRE and 1LF timings, because erratic responses can be observed with both dithiopyr and quinclorac applications to smooth crabgrass after tillering. Chemical names used: dithiopyr (S,S-dimethyl 2-(difluoromethyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-3,5-pyridinedicarbothioate); quinclorac (3,7-dichloro-8-quinolinecarboxylic acid).