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The P&PDL Picture of the Week
for 14 July 2003

Glyphosate Resistant Horseweed/Marestail

Glenn Nice, Department of Botany & Plant Pathology, Purdue University

Horseweed (Conyza canadensis), also known as marestail, or fleabane, is no stranger to the producers of Indiana. A native of North America, it is an annual that usually flowers from June to September. It starts out as a rosette then can grow up to 1.5 to 3 ft, but some plants have been recorded as tall as 6 feet (Figure 1). I have not seen these in Indiana yet. The leaves are alternate, linear and have simple blades that are entire or have slightly toothed margins. Stems are erect and tend to be unbranched; however, branching can occur at the upper portions. This branching often appears after a herbicide application. Flowers are arranged in a panicle of numerous white ray flowers (1 to 2/16 of an inch long) and 20 to 40 yellow disk flowers (Figure 2).

Horseweed seed spreads by wind. The seed are small achenes (0.5 to 1/16 of an inch long), with a pappus of tan to white bristles. A pappus is a structure that allow the seed to be taken by the wind (Figure 3). Pappus are also found on dandelion seed. Work done by Penn State University, PA, and Universidad de Buenos Aires, Argentina reported horseweed dispersal of seed up to 1192.7 ft with a 10 mph wind speed when the seed was released 2.5 ft above the ground*.

In the past the use of tillage restricted the growth of horseweed, leaving it to only to live on the boarders of crop land in waste areas, pastures, and ditches. However, the adoption of the soil conservation practice, no-till, horseweed has become more troublesome.

Glyphosate is one of the most commonly used herbicides available for the control of weeds in soybean. Because of its safety and efficacy, it is almost ubiquitous. It is found in many different herbicides labeled for agronomic uses, vegetation control, aquatic plant control, and for use in and around our homes. Recently, horseweed has also become a weed of interest due to reports of it being resistant to glyphosate. Weed resistance is referred to as a weed that was once controlled by a herbicide or class of herbicides, that is no longer controlled by that same herbicide or class of herbicides. This change in control usually is due to some natural genetic modification changing the plants physiology, metabolism, structure, or habits. A large criterion of this is that the ability to survive or escape this herbicide application can appear in the next generation of the weed. The resistance would be inherited from one generation to the next like blue eyes from parent to child.

The first confirmed reports of glyphosate resistant horseweed in the United States came from the state of Delaware. However, now glyphosate resistant horseweed populations have also been found in Arkansas, Indiana, Maryland, Michigan, Mississippi, New Jersey, Ohio, and Tennessee. Preliminary work done on resistant horseweed in Indiana was done on two populations from Jackson county. Now five counties are reported to have resistant populations. However, it is probable that resistance is more wide spread than those five counties. Ohio State University and Purdue University have been conducting greenhouse studies on these populations to determine glyphosate resistance and investigate activity from other herbicides.

Seed from Delaware and the two populations in Jackson county, Indiana, raised in the Purdue University Botany & Plant Pathology green house and sprayed when they were in the rosette stage. Some of the use rates in this research are not labeled rates and should NOT be used in practice. They are rates selected for research purposes only. Roundup WeatherMax, FirstRate, Gramoxone Max, and 2,4-D were applied at 1x and 4x rates.

All three populations were controlled by the 1 pt/A rate of 2,4-D (Figure 4). Firstrate stopped growth but was variable between the two Jackson counties and Delaware. Horseweed plant material was reduced between 50 and 90% (Figure 5). In both the Roundup WeatherMax and Gramoxone Max treatments some plants were controlled yet others were not. The seed samples were separated out by plant. Likely the seed collected were collected from several plants and mixed together, representing a population and not a plant. Observations in the field are that susceptible horseweed are generally controlled well, but are next to other plants that are not. Horseweed plants were often stunted but continued to grow after an application of Roundup WeatherMax (Figure 6). In the case of Gramoxone Extra, plants appeared to be controlled but within a few days regrowth occurred from the center (figure 7). The response was variable with Gramoxone Max. One Jackson population was controlled better than the other.

Weed resistance is not a new concept. Weeds have developed resistance to several of the herbicides that we have used to control them. Some examples are ALS resistance by giant ragweed, some pigweeds, horseweed, kochia, foxtail, and others. Other examples are triazine (atrazine and simazine) resistance by pigweeds, common lambsquarters, horseweed, and others. It is important to understand that these resistant biotypes are out there and that resistance can develop with continuous use of herbicides with the same mode of action (how a herbicide controls a weed). However, it is also important to understand that because resistant biotypes are appearing, it does not necessarily mean that you have these biotypes in your field. In most cases with proper rotation of herbicides mode of action the herbicides that have resistant weeds are still effective valuable herbicides. Resistance is not the norm, but the exception to the susceptible weeds.

For an extensive list of resistant weeds in the US and the rest of the world, go to http://www.weedscience.org/in.asp.

*Predicting long-distance dispersal of horseweed (Conyza canadensis) using wind tunnel experiments. Accessed 7/12/03. http://www.agronomy.psu.edu/WeedEcology/research/MeetingAbstracts/horseweed.pdf

Click on the small image to view a larger image.

Figure 1. Horseweed growing in
an untreated field.
Figure 2. Horseweed flowering.
Figure 3. Horseweed seed.
Figure 4. Horseweed from
Jackson county Indiana sprayed
with 1x and 4x rate of 2,4-D.
Figure 5. Horseweed from
Jackson county Indiana sprayed
with 1x and 4x rate of FirstRate.
Figure 6. Horseweed from
Jackson county Indiana sprayed
with 1x and 4x rate of Roundup
Figure 7. Horseweed from Jackson county Indiana sprayed with 1x and 4x rate of Gramoxone Extra.

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Last updated: 14 July 2003/amd
The Plant & Pest Diagnostic Laboratory at Purdue University