PART II: Managing resistance: Herbicides

Turfgrass managers may be more familiar with pest resistance to insecticides and fungicides, but resistance to herbicides is becoming more common. However, the phenomenon of herbicide resistance is not a recent discovery. As early as 1968, researchers identified common groundsel resistant to the triazine herbicides (in agronomic crops). To date, we know of 183 cases worldwide of weed biotypes resistant to various herbicides. The United States has 49 of these cases-the most of any country. Turfgrass managers may not be familiar with herbicide resistance because most of these cases have occurred in agronomic crops. With the exception of triazine-resistant annual bluegrass in France (documented in 1975), researchers have confirmed most cases of herbicide resistance in turfgrass situations only recently or are just now doing so.

The mechanisms for the development of herbicide resistance may be different than for the development of resistance to insecticides and fungicides. Therefore, the management practices to prevent resistance development for herbicides may also be different. It may sound simplistic, but it is important to keep in mind that insects respond differently to pesticides than plant pathogens do to fungicides and weeds to herbicides. The point is that insects, plant pathogens and weeds are all fundamentally different organisms. Therefore, we might expect them to respond differently to pesticides.

Herbicide-resistant biotypes have been, and are currently, the subject of extensive study by weed scientists and plant physiologists. While the actual biochemical mechanisms of resistance are complicated, the general process of evolution of resistance to herbicides is fairly simple. Herbicide resistance is the naturally occurring, inheritable ability of some weed biotypes within a weed population to survive a herbicide treatment that would, under normal conditions of use, effectively control that weed population. The terms "naturally occurring" and "inheritable" are key terms in understanding the development of resistance. "Naturally occurring" refers to weed biotypes with the genetic makeup to be resistant to a particular herbicide already existing in the weed population. In other words, the herbicide does not genetically change (mutate) weeds to make them resistant to a herbicide; the weeds resistant to the herbicide occur naturally in the wild population. "Inheritable" means that these naturally occurring resistant biotypes reproduce and pass the genes conferring resistance from one generation of weeds to the next. It is necessary to understand these points for you to develop effective management strategies to delay or eliminate the development of herbicide-resistant weed problems.

The following is an example of how herbicide resistance becomes a problem in turfgrass management. Consider a scenario where you manage 25 acres of newly seeded or sodded turf. For the purposes of this example, let's assume you have only one weed species present. Let's also assume that 3 million individual plants of this one weed species are growing in these 25 acres. You begin to use herbicide X to control this weed because it is the most highly effective and economical to use. Suppose that out of these 3 million plants, just two plants (resistant biotypes) happen to have the genetic makeup to be resistant to herbicide X. For the next few years, in an attempt to get this weed population under control, you continue to use herbicide X. You obtain good control of the susceptible biotypes, and you seem to be getting the weed population under control. However, the resistant biotypes (originating from the two original plants in the population) continue to produce seed and increase in numbers. You probably would not notice a problem because, in general, the weed control is good and few weeds are escaping. Suppose after 5 years you feel you have done a good job in managing this weed population because you have reduced the weed population to almost nothing with herbicide X. Because you are pleased with its performance, you continue to use herbicide X, but after about the eighth year you see a noticeable increase in the weed population. So you increase your rate of herbicide X the next year and the weed population still increases. You use it again in year 10, and the weed population is as high as when you started 10 years ago.

What you have just experienced in this scenario is selection pressure caused by the herbicide that selects for the resistant biotype by eliminating the susceptible biotypes. However, this does not always occur. Pest-control applicators often use the same herbicide every year for 10 or 15 years with good results and no indication of a problem. Why? Many possible explanations exist, but the most likely reason is that biotypes resistant to that specific herbicide do not naturally exist in the wild population in the first place. Resistance is a process of selection, but if no resistant types exist, nothing exists to select. For resistance to occur, the resistant biotypes must already exist naturally in the population, and they must be able to pass this genetic trait to their offspring.

This is also why some types of herbicides are more prone to resistance problems. For instance, of the 183 cases of documented resistant weed biotypes in the world, 61 involve the triazine herbicides (such as atrazine and simazine). Investigators have reported only one case of resistance for glyphosate (Roundup). Considering that all of these herbicides have been extensively used worldwide for many years, this is a curious difference. It is most likely due to the higher frequency of naturally occurring resistant biotypes to the triazines compared to Roundup.

Another important factor in understanding the evolution of resistance development is that resistance develops to a particular mode of action of a herbicide. Therefore, if two herbicides have the same mode of action, resistance to one most likely results in resistance to the other with the same mode of action (this is referred to as cross-resistance). For instance, simazine-resistant annual bluegrass also is resistant to atrazine and other members of the triazine family of herbicides. Therefore, perhaps the most important management practice to prevent the development of herbicide-resistant weeds is to rotate herbicides with different modes of action. For instance, if you use triazines for controlling annual bluegrass in non-overseeded warm-season turf, rotating every 2 to 3 years with another effective herbicide (see table, above) for annual bluegrass will help delay the development of resistance to triazines.

In addition, if you use a herbicide and you begin to see a significant number of weed escapes, applying a post-emergence herbicide will help prevent resistance because the escapes may be a resistant biotype. However, you still should switch to a different pre-emergent.

Researchers have documented several cases of herbicide resistance in turfgrasses. Here are a few notable examples: * Triazine-resistant annual bluegrass. As I previously mentioned, the first case of this was noticed in France in 1975 (on a highway roadside). More recently, triazine resistance has occurred on golf courses in North Carolina and Mississippi. * Dinitroaniline-resistant goosegrass. Goosegrass resistance to the dinitroaniline (DNA) herbicides has occurred on one golf course in Georgia and one in North Carolina. In the case in Georgia, Dr. Tim Murphy (University of Georgia) found that the DNA-resistant goosegrass was also resistant to Dimension (dithiopyr). (While dithiopyr is not a DNA, it has a similar mode of action, so this may be a case of cross-resistance.) *Fenoxaprop-resistant smooth crabgrass. Dr. Jeffrey Derr at Virginia Polytechnic Institute and State University has recently documented a case of Acclaim-resistant crabgrass from a golf-course tee in New Jersey. This is the first reported case of crabgrass resistance to fenoxaprop. However, we know of four documented cases of other grasses resistant to fenoxaprop (junglerice in Costa Rica, hooded canarygrass in Mexico and two cases of littleseed canarygrass: one in Mexico and one in Israel).

Although researchers undoubtedly will uncover more cases of resistance, it remains to be seen how much of a problem this will be for turfgrass managers in future years. However, a thorough understanding of how resistance develops and the management practices that prevent the evolution of resistance can help you prevent this from becoming a problem on your turf. Resistant weeds spring up as localized problems, so your individual efforts at preventing resistance can definitely have an impact.

Dr. Fred Yelverton is professor of weed science at North Carolina State University (Raleigh).

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