Poa annua L. is a highly variable plant from its evolutionary origin as an allotetraploid hybrid between Poa infirma and Poa supina (Figure 1).
It survives as a weed because of its high genotypical and phenotypical variability, rapid germination, survival when uprooted, tolerance to compacted soils, and, most recently, development of resistance to herbicides.
As part of its genetic variability, resistance to all herbicidal modes of action occurs. Fortunately, resistance is mostly local in nature, but it is expanding rapidly. With this rapid increase in resistance and lessons learned from agronomic row crops, we believe a fundamental paradigm shift in control strategies is needed, in addition to cultural practices that help minimize the presence of P. annua to begin with. Although P. annua is a global problem in all turfgrass situations, this article will focus on control strategies in non-overseeded bermudagrass and zoysiagrass.
Most pesticides in turf (and almost all herbicides) are also registered in field crops. The turfgrass industry benefits from this, as many of the basic environmental and toxicology tests required by the U.S. EPA have already been completed by the time these products have reached the turfgrass market.
Figure 2. Global increase in unique cases of herbicide resistance. Data includes herbicide resistance in all of agriculture, including turfgrasses. New cases of herbicide resistance are discovered every year. Illustration by Ian Heap, www.weedscience.org, 2018
Figure 3. Top 10 weed families that have evolved resistance to herbicides. Poaceae has nearly twice as many species as any other plant family, likely because of the wide genetic variability in this family. Illustration by Ian Heap, www.weedscience.org, 2018
The Weed Science Society of America (both authors are active members) tracks worldwide herbicide resistance. Figures 2 and 3 (above) indicate two very disturbing trends. Figure 2 tracks the increase in cases of unique herbicide resistance. Unique herbicide resistance is defined as resistance of a particular weed to a specific mode of action. For instance, if 1,000 cases of simazine-resistant P. annua exist, that counts as only one unique case of resistance. Figure 3 indicates members of the Poa grass family (Poaceae) account for nearly twice as many cases of resistance as any other family of weeds. We believe this is due to wide genetic variation in the Poa family of plants. Many agricultural scientists (including these authors) think herbicide resistance is the most worrisome trend in worldwide agriculture.
The incidence of resistance in turfgrasses is abundant in many areas of the country. In some areas, herbicide resistance on putting greens is so extensive that the only course of action is to shut down and renovate using dazomet (Basamid, AMVAC). Everyone would agree this is not a good position for most golf courses. Therefore, it is time for everyone (turfgrass managers, industry personnel, university researchers, distributors, etc.) to get serious about herbicide resistance.
Herbicide resistance in plants
Given that no new herbicidal mode of action has been introduced to the turfgrass market in more than 25 years, and that no new ones are on the horizon, being best stewards of existing products is the primary way to delay resistance while maximizing the usefulness of these products.
Contrary to what many think, herbicides do not cause resistance to occur or develop in plants. Resistance follows Darwin’s theory of evolution by natural selection, where resistant populations naturally exist, and susceptible biotypes are removed with repetitive use of herbicides. This leaves little competition for the normally low populations of resistant biotypes, and allows them to spread rapidly (Figure 4, below).
Figure 4. A classic example of a mixed Poa annua population, in which green, healthy plants are resistant to herbicides, while adjacent yellow, dying ones are susceptible to herbicides.
Predictive models and history suggest continued use of a product with a single mode of action usually results in resistance developing in approximately 10 years, although as few as seven years has been observed. However, by rotating or mixing products with different modes of action, one can dramatically extend the time before resistance occurs.
For example, it is predicted that it takes 45 years to produce resistance if a herbicide with a particular mode of action is used for two consecutive years and then rotated to another product for the third year. Sixty years are predicted for resistance to develop if herbicides are rotated annually, and resistance can be delayed for up to 90 years if a product with a particular mode of action is used in only one of three years, and products with other modes of action are used in rotation in the other two years.
Resistance most often occurs when an amino acid associated with the plant’s DNA strand is replaced by other amino acids. This substitution then causes plants to overproduce enzymes that the herbicide normally blocks. Alternatively, extrachromosomal DNA molecules are produced that overproduce various enzymes that are normally blocked by certain herbicides. From a scientific point of view, it is important to know the specific avenues by which resistance occurs, but end users are more interested in managing its occurrence.
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