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Technician J’Lynn Howell (right) homogenizes hydrilla samples while plant physiologist Franck Dayan observes the presence of carotenoids (the yellow layer in the test tube) that indicate the plant is herbicide resistant. Click the image for more information about it.

Hydrilla's Resistance to Herbicide Gives Scientists a New Challenge

By Luis Pons
November 1, 2005

Scientists with the Agricultural Research Service (ARS) and a private firm have encountered a troubling turn of events in the fight against an invasive weed that's choking many waterways in the southeastern United States.

The researchers--at ARS' Natural Products Utilization Research Unit in Oxford, Miss., and with SePRO Corp., a Carmel, Ind.-based plant-protection management firm--found that a form of hydrilla (Hydrilla verticillata) has developed resistance to fluridone, the most effective herbicide against it.

They've pegged the resistance to a gene mutation in the dioecious, female form of hydrilla. So far, this mutation has been found only in hydrilla inhabiting several Florida lakes. A monoecious hydrilla--a form that has both male and female flowers on the same plant--that first appeared in the middle Atlantic states has, to date, not shown resistance to the herbicide.

The ARS studies in Oxford were conducted by plant physiologist Franck Dayan and molecular biologists Brian Scheffler and Albrecht Michel. Scheffler now leads ARS' Mid-South Area Genomics Laboratory in Stoneville, Miss., while Michel is no longer with ARS.

Hydrilla's ability to thrive even in adverse conditions has led researchers to dub it "the perfect aquatic weed." Rooted in bottom sediments, it grows long, thin stems that rapidly reach the water's surface and form a thick mat. It was introduced to the United States from Southeast Asia in the 1950s near Tampa, Fla.

According to Dayan, the resistant hydrilla has increased treatment costs for affected lakes.

The new discoveries have spurred government and SePRO scientists and aquatic systems managers to seek additional environmentally friendly ways to combat the weed. Biological agents being studied for controlling hydrilla include some insects and the fungal pathogen Mycoleptodiscus terrestris, which when used with fluridone seems to increase hydrilla's susceptibility to the herbicide.

Read more about this research in the November 2005 issue of Agricultural Research magazine.

ARS is the U.S. Department of Agriculture's chief in-house scientific research agency.