Golf Course Management

NOV 2012

Golf Course Management magazine is dedicated to advancing the golf course superintendent profession and helping GCSAA members achieve career success.

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research Herbicide-resistant weeds: A 21st century problem Herbicide-resistant weeds are appearing on golf courses worldwide. The development of herbicide-resistant weeds will be one of the great challenges for golf course superintendents and other turf managers in the 21st century. To this point, herbicide resistance has been discussed in terms of the "potential problem" it could cause in the future. Well, my friends, the future is now. Over the past several years, herbicide-resistant weed populations have been found at an astonish- ing pace. Annual bluegrass (Poa annua) and goose- grass (Eleusine indica) are the two primary species that are developing resistance on golf courses, but others are right behind them. This article provides information on how herbicide-resistant weeds develop and what you can do to manage them. What is herbicide resistance? The following is a basic overview of what her- bicide resistance is and some of the nuances of how plants develop resistance. Be forewarned: Much of the language used to discuss herbicide resistance has changed drastically over the past five to 10 years. Types of resistance Herbicide resistance is the ability of a plant to survive and reproduce following a herbicide appli- cation. There are two types of resistance: evolved and innate. Evolved herbicide resistance is a change in a specific weed species that was once suscep- tible to or killed by a herbicide, but now can sur- vive and reproduce following treatment with that herbicide. In other words, in the past, the herbi- cide killed the weed, but the herbicide is no longer effective. Many weeds were never controlled by a given Scott McElroy, Ph.D. 84 GCM November 2012 herbicide. For example, foramsulfuron (Revolver, Bayer) has never been able to kill bermudagrass (Cynodon species). This is innate herbicide resis- tance, which is the ability of a plant to survive and reproduce following a herbicide application from the very first exposure to that herbicide; that is, no Resistance will initially appear when a herbicide application controls most of the plants, but one or two plants survive. In this photo, most of the Poa annua plants at Holiday GC in Panama City, Fla., were controlled by the herbicide, but a few were not affected. Photo by Richie Edwards one has ever killed that species with that herbicide. Two other relevant terms are tolerance and susceptibility. Tolerance is basically a synonym of resistance, but the term has fallen out of favor in the scientific literature. Susceptible or herbicide susceptibility means a plant species is injured and cannot complete its life cycle following treatment. Susceptibility is not always limited to the ability of a plant to reproduce and complete its life cycle. Desirable turfgrass or other plants are said to be susceptible if they are injured by a herbicide to the point at which the injury is considered intolerable. Evolved herbicide resistance What should concern superintendents most is evolved herbicide resistance, which occurs when weed species that were formerly injured or killed by a herbicide have since changed so that they are no longer harmed by the herbicide. How do these weeds change? There are two answers to this question. The ecological answer involves what superintendents and other turfgrass managers have done to cause the weed species to develop resistance, and the biochemical answer involves how the plant species actually changed its biochemistry to develop resistance. Let's deal with

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