Using Growing Degree Days to Maximize the Efficacy of Applications of Fluazifop and Mesotrione for Control of Dallisgrass (Paspalum dilatatum) in Tall Fescue.

Dallisgrass (Paspalum dilatatum) is a problematic turfgrass weed throughout the southern United States. Forthcoming restrictions on the use of monosodium methanearsonate (MSMA) in turfgrass management will limit the number of herbicides that can be used to provide selective control of this species. In 2008, a two-year study was initiated at the University of Tennessee (Knoxville, TN) evaluating programs for selective dallisgrass control in tall fescue (Festuca arundinacea).  Treatments included fluazifop-p-butyl at 0.105 kg ha^-1, mesotrione at 0.28 kg ha^-1, and fluazifop-p-butyl tank mixed with mesotrione at 0.105 kg ha^-1 and 0.28 kg ha^-1, respectively. All treatments included a non-ionic surfactant at a 0.25% v/v ratio. Treatments were applied singly and sequentially (3 week interval) in early spring [<200 growing degree days (GDD₁₀)] and early summer (>500 GDD₁₀) to a mature stand of tall fescue maintained as a utility turf with respect to irrigation, fertility, and mowing. Accumulated GDD₁₀ values were calculated using a base temperature of 10°C beginning on 1 January each year. In early spring (<200 GDD₁₀) , single and sequential applications of fluazifop-p-butyl at 0.105 kg ha^-1 provided greater than 90% control of dallisgrass 77 days after initial treatment (DAIT) in 2008. When applied in early summer (>500 GDD₁₀), single and sequential applications of fluazifop-p-butyl at 0.105 kg ha^-1 only provided 0.0 and 30.0% control 77 DAIT, respectively. A single application of fluazifop-p-butyl plus mesotrione in early spring (<200 GDD₁₀) provided 93% control of dallisgrass 77 DAIT in 2008. When applied in early summer (>500 GDD₁₀) the same treatment only provided 56.7 % control 21 DAIT and 0% control 77 DAIT. Sequential applications of this treatment yielded similar results. These data suggest that application timing may affect the level of dallisgrass control provided by applications of fluazifop-p-butyl and fluazifop-p-butyl tank mixed with mesotrione.