White oak and northern red oak leaf injury from exposure to chloroacetanilide herbicides

Previous research by the authors found simulated acetochlor (with atrazine) and s-metolachlor drift to white oak at the leaf unfolding stage caused loss of interveinal tissues (leaf tatters). Reports of leaf tatters in the landscape and nursery settings are more common on white oak (Quercus alba L.) than on northern red oak (Quercus rubra L.). Our objectives were to determine if white and northern red oak differed in susceptibility to chloroacetanilide herbicides, if injury varied between chloroacetanilide herbicides, and if adding atrazine increased leaf injury. Two-year-old seedlings at the leaf unfolding stage were treated with acetochlor, s-metolachlor, and dimethenamid-P alone or combined with atrazine at 1%, 10%, and 25% of the standard field use rate. Within 6 days, all chloroacetanilides at 10% and 25% field use rates, alone or combined with atrazine, caused leaf tatter injury in both species. Acetochlor, s-metolachlor, and dimethenamid-P caused a similar type of leaf injury. Atrazine did not cause loss of leaf tissues or increase injury from chloroacetanilides. At 1% field use rate, only acetochlor, acetochlor + atrazine, and dimethenamid-P caused leaf injury to northern red oaks. The white oaks were not injured by all of the chloroacetanilide treatments at 1% field use rate. The northern red oaks were slightly more susceptible to chloroacetanilides compared with the white oaks. A second study found acetochlor only injured northern red oak when applied at the leaf unfolding stage and only at 25% of field use rate. Acetochlor at 1% field use rate did not injure red oak. Research is needed to explain the greater frequency of leaf tatters on white oaks than on northern red oaks in the landscape and to develop strategies to avoid tree injury.