Effect of pH on Submersed Aquatic Plant Response to Flumioxazin

Flumioxazin (2-[7-fluoro-3,4-dihydro-3-oxo-4-(2-propynyl)-2H-1,4-benzoxazin-6-yl]-4,5,6,7-tetrahydro-1H-isoindole1,3(2H)-dione) was applied to the submersed aquatic plant species coontail (Ceratophyllum demersum L.), egeria (Egeria densa Planch.), hydrilla (Hydrilla verticillata [L.f.] Royle), and vallisneria (Vallisneria americana Michaux) at concentrations of 0, 100, 200, 400, 800, and 1600 µg active ingredient (a.i.) L -1 under high (9.0) and neutral (7.0) pH. Flumioxazin was more efficacious when applied to plants growing in neutral pH conditions than when applied under high pH conditions. Coontail was the only submersed species to be controlled in high pH conditions at the maximum label Experimental Use Permit (EUP) concentration of 400 µg L -1 . Other species evaluated in this study required concentrations >3194 µg L -1 to reduce biomass by 50% when applied to high pH water. In contrast, plants exposed in neutral pH water conditions, were often severely injured following exposure to flumioxazin. Increasing tolerance of species treated in neutral pH water based on dry-weight calculated effective concentration 50% (EC50) values were (in µg L