Tiafenacil (Gamma, Helm Agro) is now labeled in wine grapes in Oregon, Washington, and Idaho. Tiafenacil is a non-selective postemergence herbicide controlling broadleaves and grasses. It is a new group 14 herbicide, PPO inhibitor, in the N-phenyl-imides chemical family. It is a contact herbicide that provides fast-acting control of weeds and degrades rapidly in the soil. Tiafenacil was developed as a paraquat alternative in Korea.
Tiafenacil (Gamma) is labeled for basal-direct applications in grapes. Recommended rates are 0.5 to 1.5 oz/A of Gamma. For best performance, the adjuvants crop oil or methylated seed oil must be used and ammonium sulfate.
Weeds controlled
Most annual broadleaves were controlled when treated at less than 5 inches tall. Suppression of Canada thistle was also noted, but plants recovered after treatment. Grasses controlled include Italian ryegrass (<5 inches), annual bluegrass, and other annual grasses. In our studies, higher rates are needed for grass control. Tiafeancil can be tank-mixed with a variety of herbicides. We noticed the best performance when mixing with glufosinate or glyphosate.
Figure 1. Weed response to tiafeancil (Gamma) in research vineyard in Oregon. Nontreated control plots left, and tiafenacil (Gamma 0.5 oz/A) on the right 14 days after treatment.
Application Timing
Tiafenacil can be used during the dormant or non-dormant season. The pre-harvest interval is 7 days. The maximum use rate per season is 4.5 oz of Gamma per acre per season and cannot be retreated within 14 days. Only vineyards established for 2 years or more can be treated with tiafenacil.
Remarks
My research work was done in silt-loam soils of Oregon, and it is wise to test lower rates in coarser soils to avoid crop injury.