Improved Understanding of Synthetic Auxin Herbicide Injury to St. Augustinegrass.

Synthetic auxin herbicides are generally safe when applied to monocotyledonous plants. However, the synthetic auxin herbicides aminocyclopyrachlor and triclopyr are known to be injurious to St. Augustinegrass (Stenotaphrum secondatum) and other warm-season turfgrasses. The reason for this injury is not well understood. Turfgrass tolerance to aminocyclopyrachlor and triclopyr research was conducted at Auburn University. This study concluded aminocyclopyrachlor and triclopyr treatment caused unacceptable St. Augustinegrass injury when applied at ≥0.112 kg ai ha^-1 and 1.19 kg ai ha^-1 respectively. Morphological observations made during this study included necrosis at the leaf apex, swelling of the nodes, and stimulation of adventitious root formation from the nodes. Based upon these observations and an understanding of the mode-of-action of auxinic herbicides, the hypothesis for the current research is that plant death occurs from vascular inhibition at the growing points. Microscopic analysis using a paraffin technique was conducted to further investigate St. Augustinegrass morphological changes due to auxinic herbicide treatment. Sample analysis was conducted on tissue treated with increasing herbicide rates and timing after application compared to a non-treated check.