Growth chamber studies were conducted using 55-L aquariums to evaluate the efficacy of the herbicide triclopyr and the fungal pathogen Mycoleptodiscus terrestris, applied alone and in combination against Eurasian watermilfoil. Treatments included 0.15, 0.40, and 1.50 mg acid equivalent (ae)/L triclopyr, 0.08, 0.16, and 0.32 ml/L M. terrestris, combinations of both agents at all rates, and an untreated control. Plants were exposed to all treatments for a 24-h contact time and plant biomass (shoot and roots) was recorded 6 wk after application. For both triclopyr and M. terrestris applied alone, plant control increased with treatment concentration. Compared with untreated plants, 1.50 mg/L triclopyr and 0.32 ml/L M. terrestris reduced Eurasian watermilfoil by 100 and 91%, respectively. Lower doses of herbicide or pathogen were less effective and plant recovery was observed from surviving plant tissues (stems and root crowns). Although M. terrestris at 0.08 ml/L did not significantly reduce shoot or root biomass and 0.15 mg/L triclopyr provided only 53% control of plants, combining both agents at these rates reduced Eurasian watermilfoil by 90%. Results demonstrated that integrating low doses of triclopyr with an indigenous pathogen, M. terrestris, can improve control of Eurasian watermilfoil. Lower use rates of triclopyr would minimize impacts to sensitive nontarget vegetation, reduce application costs, and may minimize impacts of label-imposed use restrictions.

Nomenclature: Triclopyr; Eurasian watermilfoil, Myriophyllum spicatum L. MYPSP; Mycoleptodiscus terrestris (Gerd.) Ostazeski