Confirmation of hair fescue (Festuca filiformis) suppression by clethodim in lowbush blueberry (Vaccinium angustifolium)

Hair fescue (Festuca filiformis) is a tuft-forming perennial grass that reduces yields in lowbush blueberry fields. As a fine-leaf fescue, hair fescue is generally considered tolerant to acetyl-coenzyme-A-carboxylase (ACCase)-inhibiting herbicides. Recent research in lowbush blueberry, however, suggests that hair fescue is suppressed by the ACCase-inhibiting herbicide clethodim. This research was, however, limited to a small number of hair fescue populations, and additional research is required to determine the extent to which clethodim can contribute to hair fescue management in lowbush blueberry. The objectives of this research were to 1) evaluate clethodim efficacy on a range of hair fescue populations under field and greenhouse conditions, 2) determine if bearing year clethodim applications suppress hair fescue, and 3) utilize dose response methodology to determine the response of hair fescue to a range of clethodim application rates. Clethodim (91 g a.i. ha-1) reduced hair fescue flower tuft density and tuft inflorescence number in 6 of 9 and 7 of 9 non-bearing year hair fescue populations, respectively, and reduced leaf number and biomass of 15 hair fescue populations in a greenhouse experiment. Clethodim also reduced hair fescue flower tuft density and tuft inflorescence number in five bearing year hair fescue populations. A dose response experiment indicated that the anticipated label registration rate of 91 g clethodim ha-1 will provide adequate hair fescue suppression and that pursuit of higher application rates is not required. Based on these results, clethodim is recommended for hair fescue suppression in lowbush blueberry in Canada.