ERDC/TN APCRP-BC-19 December 2010 Effect of Dried Mycoleptodiscus terrestris Inoculums on Fluridone-Susceptible and Fluridone-Resistant Hydrilla Biotypes by Judy F. Shearer and Michael J. Grodowitz PURPOSE: This technical note describes an aquarium study conducted to determine the efficacy of dried Mycoleptodiscus terrestris (Gerd.) Ostazeski against fluridone-susceptible and fluridone- resistant hydrilla biotypes. INTRODUCTION: Hydrilla verticillata (L.f.) Royle, a member of the Hydrocharitaceae, was introduced into Florida in the 1950’s (Schmitz et al. 1991). The introduction was the dioecious biotype with only gynoecious plants making up the original infestation. Today hydrilla has invaded at least 21 states either as dioecious or monoecious biotypes or both (Richardson 2008). Called “the perfect aquatic weed” by Langeland (1996), the plant has several characteristics that make it an excellent competitor in freshwater systems. It has low light and carbon dioxide compensation points (Van et al. 1976) and it reproduces by several means including fragmentation, stolons, and turions that develop in leaf axils or subterraneously from rhizomes (Langeland 1996). Seeds have been reported in monoecious hydrilla populations (Langeland and Smith 1984) but are in all likelihood not a major reproductive mechanism. For years fluridone was the chemical of choice for hydrilla management. It was thought that because the dioecious biotype reproduced only vegetatively, herbicide resistance would not become a problem. However, in the late 1990’s, a differential response to fluridone was noted in some hydrilla-infested lakes in Florida. Studies by Michel et al. (2004) of 200 water bodies in Florida revealed that at least 20 of them had hydrilla phenotypes with two- to six-fold higher fluridone resistance. Further genetic research found three independent herbicide-resistant biotypes that arose from the selection of somatic mutations at the arginine 304 codon of the enzyme phytoene desaturase (pds) (Michel et al. 2004). Fluridone is an inhibitor of the enzyme. Following cloning of the gene for pds from both herbicide-susceptible and herbicide-resistant hydrilla biotypes, it was found that the three pds variants had specific activities similar to the wild type but were two to five times less sensitive to fluridone (Michel et al. 2004). Historically, methods utilized for hydrilla control have included mechanical, chemical, and bio- logical technologies. Mechanical control is expensive, costing in excess of $1000 per acre, and poses some logistical problems in disposing of the harvested biomass (Langeland 1996). Control is often of short duration and several harvests are necessary per year. Alternative chemicals to fluridone for hydrilla management include copper products, diquat, endothall, imazamox, and penoxsulam. Similar to fluridone, the enzyme acetolactate synthase (ALS) inhibitors, imazamox and penoxsulam, require long contact times (up to 90 days) to achieve hydrilla control. All of the other alternative herbicides are fast acting and provide more rapid kill than penoxsulam, imazamox, or fluridone.