Combining ALS-inhibiting Herbicides with the Fungal Pathogen Mycoleptodiscus terrestris for Control of Hydrilla

BACKGROUND: Numerous studies have shown that combining the indigenous fungal pathogen, Mycoleptodiscus terrestris (Gerd.) Ostazeski, hereafter referred to as Mt, with low doses of herbicides has excellent potential as an integrated strategy for long-term management of hydrilla and Eurasian watermilfoil (Myriophyllum spicatum L.) (Netherland and Shearer 1996; Nelson et al. 1998; Shearer and Nelson 2002; Nelson and Shearer 2005). With the advent of resistance to fluridone (1-methyl-3phenyl-5[3-(trifluoromethyl)phenyl]-4 (1H)-pyridinone) in hydrilla populations, it became essential to find new products for hydrilla management. Recently two new herbicides, penoxsulam (2-(2,2-difluoroethoxy)-N-(5,8-dimethoxy[1,2,4]triazolo[1,5-c]pyrimidin-2-yl)-6-(trifluoromethyl)benzenesulfonamide) and imazamox (2-[4,5-dihydro-4 methyl-4-(1-methylethyl)5oxo-1H-imidazol-2-yl]-5-(methoxymethyl)-3-pyridinecarboxylic acid), have undergone registration and a third, bispyribac-sodium (sodium 2,6-bis[(4,6-dimethoxy-2-pyrimidinyl)oxy]benzoate) is currently being evaluated under an Experimental Use Permit (EUP) for aquatic use. The three herbicides are classified in different herbicide families but all have a similar mode of action; inhibition of the enzyme acetolactate synthase (ALS) (Koschnick et al. 2007). Current thought is that inhibition of ALS leads to starvation of a plant to the amino acids, isoleucine, valine, and leucine, and is the primary mechanism by which ALS herbicides cause plant death (Tranel and Wright 2002). Similar to fluridone, ALS inhibitors require extended contact times (> 60 days) at designated use rates (ppb range) to achieve > 90 percent plant control. Reducing contact time requirements by integrating these herbicides with Mt would improve hydrilla control in areas where contact time is influenced by water exchange. Previous studies have shown that herbicide contact time can be reduced by as much as 50 percent when combining herbicides with Mt as a simultaneous treatment (Netherland and Shearer 1996; Nelson et al. 1998; Shearer and Nelson 2002).