Herbicide resistance occurs when a weed demonstrates the ability to survive and reproduce following exposure to a dose of herbicide that is usually lethal (Vencill et al. 2012). For example, a prodiamine-resistant annual bluegrass plant (left) can produce new, healthy root tissue following exposure to doses of prodiamine that are lethal to susceptible plants (right).
What Causes Herbicide Resistance?
Weeds developing herbicide resistance is a natural consequence of failing to implement diversified weed management practices. Natural selection will facilitate those plants best adapted to a particular process increase in a population. Most cases of herbicide resistance have evolved following use of the same herbicide for consecutive years without implementing a different weed management practice (other than herbicide) or applying a different herbicide that employs a different mechanism of action.
Why is Herbicide Resistance Important?
In agronomic cropping systems, herbicide resistant weeds can result is mass economic loss to growers (Norsworthy et al. 2012). In managed turfgrass systems such as golf courses, athletic fields, sod production outfits, and residential and commercial lawns the main implication of resistance is an increased cost to control problematic weeds. This cost can be associated with implementing non-chemical weed management techniques or rotating to more expensive herbicide options.
The greatest concern associated with herbicide resistance is the loss of effective herbicide options. Discovery of new herbicidal mechanisms of action is rare; the last new mechanism of action, inhibition of hydroxyphenylpyruvate dioxygenase (HPPD) was discovered in the early 1980s and commercialized for use in turfgrass with the introduction of mesotrione (Tenacity) and topramezone (Pylex). It is critical that turfgrass managers steward the technology available for weed management currently available.
Report a Potential Herbicide Resistance Case
It is critically important that individuals report any suspected cases of herbicide resistance to University of Tennessee Extension specialists focused on weed management. Reporting allows for specialists in this area to not only track spread across the state of Tennessee but also to conduct tests to diagnose resistance in weeds of interest and provide alternative recommendations for control.
Current Status of Annual Bluegrass Resistance in Tennessee
The University of Tennessee Turf & Ornamental Weed Science team actively works to diagnose cases of herbicide resistant weeds evolving in managed turfgrass systems and track spread of these weeds across the state of Tennessee. The map below highlights counties in Tennessee where herbicide resistant biotypes of annual bluegrass have been confirmed. Resistance in these samples has been to mitotic inhibiting herbicides (e.g., prodiamine), inhibitors of acetolactate synthase (e.g., foramsulfuron, trifloxysulfuron, etc.), photosystem II inhibitors (e.g., simazine), and inhibitors of enolpyruvylshikimate-3-phosphate (EPSP synthase (e.g., glyphosate). This map will be updated as new cases of resistance are confirmed.