Laboratory Evaluation of Threshold Fluridone Concentrations Under Static Conditions for Controlling Hydrilla and Eurasian Watermilfoil

Fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} was evaluated against Eurasian watermilfoil ( Myriophyllum spicatum L.) and hydrilla ( Hydrilla verticillata (L.f.) Royle) under laboratory conditions at initial treatment rates of 0.0, 0.25, 0.5, 0.75, 1.0, 2.0, 3.0, 4.0, and 25.0 µ g/L for 90 days of exposure. Treatment concentrations < 1 µ g/L had no effect on either hydrilla or Eurasian watermilfoil biomass and resulted in only a small reduction (13 to 33%) in hydrilla net photosynthesis (PTS) and chlorophyll at 0.75 µ g/L. Both hydrilla and Eurasian watermilfoil growth were inhibited by fluridone concentrations between 1.0 to 3.0 µ g/L. Rates of 1.0 and 2.0 µ g/L did not reduce hydrilla biomass below pretreatment levels, but did inhibit growth. Biomass remained static from 30 through 90 days after treatment, while chlorophyll and PTS continued to decrease. Eurasian watermilfoil biomass and physiological variables were inhibited as fluridone rates reached 3.0 µ g/L. With fluridone concentrations of 4.0, and 25.0 µ g/L, hydrilla and Eurasian watermilfoil biomass and physiological variables were reduced (42 to 88%) below pretreatment levels by 30 days after treatment. If PTS readings at shoot apices remained positive following fluridone treatment, shoot biomass increased; whereas negative PTS readings were associated with biomass decreases below pretreatment levels. In general, the appearance of treatment symptoms was delayed as treatment concentrations decreased.