LABORATORY EVALUATION OF THRESHOLD FLURIDONE CONCENTRATIONS UNDER STATIC CONDITIONS FOR CONTROLLING HYDRILLA AND EURASIAN WATERMILFOIL

Fluridone yl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone} was evalu ated against Eurasian watermilfoil (Myriophyllum spicatum L.) and hydr illa (Hydrilla verticillate (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 mu g/L for 90 days of exposure. Treatment concentrations < 1 mu g/L had no effect on either hydrilla or Eurasian water-milfoil bi omass and resulted in only a small reduction (13 to 33%) in hydrilla n et photosynthesis (PTS) and chlorophyll at 0.75 mu g/L. Both hydrilla and Eurasian watermilfoil growth were inhibited by fluridone concentra tions between 1.0 to 3.0 mu g/L. Rates of 1.0 and 2.0 mu g/L did not r educe hydrilla biomass below pretreatment levels, but did inhibit grow th. Biomass remained static from 30 through 90 days after treatment, w hile chlorophyll and PTS continued to decrease. Eurasian watermilfoil biomass and physiological variables were inhibited as fluridone rates reached 3.0 mu g/L. With fluridone concentrations of 4.0, and 25.0 mu g/L, hydrilla and Eurasian watermilfoil biomass and physiological vari ables were reduced (42 to 88%) below pretreatment levels by 30 days af ter treatment. If PTS readings at shoot apices remained positive follo wing fluridone treatment, shoot biomass increased; whereas negative PT S readings were associated with biomass decreases below pretreatment l evels. In general, the appearance of treatment symptoms was delayed as treatment concentrations decreased.