Glyphosate-Resistant Palmer Amaranth Can Infect Plants 200 Meters Away
March 2, 2009 Even the most resilient things have their weaknesses. When Superman was exposed to kryptonite, he lost his special powers. In the Bible, Samson was the strongest man ever to live, but he lost his strength when Delilah cut his hair. In agriculture, weeds like Palmer amaranth can grow 2 inches a day and seem unstoppable, until they meet an application of glyphosate.
That was a simple and effective solution, used by farmers across the Cotton Belt to control Palmer amaranth and a host of other weeds. But unlike Superman and Samson, Palmer amaranth in some areas has overcome its glyphosate-induced fate, developing resistance to the panacea farmers rely on most. Glyphosate-resistant Palmer amaranth is a reality, and it will spread like wildfire if farmers don’t take necessary precautions against this possible field-choking weed.
Researchers at the University of Georgia-Tifton are addressing this problem head-on. By studying the weed-resistance phenomenon and how the trait is passed to other plants, researchers are better able to educate producers on solutions to the problem. Some of their findings were presented January 31 at the Tifton campus during a Weed Resistance Management Summit, sponsored by Syngenta.
One major concern of glyphosate-resistant Palmer amaranth is its proliferation. It is not an anomaly cropping up in various areas and remaining isolated. It spreads from a resistant to a non-resistant plant through the transfer of pollen, and this transfer capability was addressed by Dr. Lynn Sosnoskie during the summit.
“One reason why this is a problem is because this species is dioecious, and dioecious means that male and female flowers are located on separate plants. Pollen needs to travel between plants for seed production to occur. Movement of pollen facilitates movement of the genes that the plant carries, and a herbicide resistance gene in one population can be transferred to a susceptible population in a remote field by pollen,” Sosnoskie said.
If It Looks Like A Golf Ball, It Flies Like Golf Ball
Pollen comes in various shapes and sizes that determine how easily the grains are carried through the atmosphere. In the case of Palmer amaranth, its pollen is shaped very efficiently – perfect for maximizing distribution through the air.
“Two of the most important characteristics are that the pollen grains are small and that they are spherical, and this allows for more efficient transport through the air,” Sosnoskie said. “Palmer amaranth possesses both of these traits – they are spherical and small, approximately 20-40 micrometers in length. To get a better idea, it is more than 2,000 times smaller than a golf ball. These are tiny, and smaller particles move further in the air than larger particles do.”
The pollen may be much smaller than a golf ball, but there are some striking similarities. The dimples on a golf ball are not just for looks. Those dimples allow Tiger Woods to hit the ball even further, giving the ball lift and distance. Palmer amaranth pollen has a similar design that produces a similar effect.
“Palmer amaranth grains are also dimpled on the surface, not unlike a golf ball. The dimples on the golf ball give it lift, and that is what increases its flight distance,” Sosnoskie said. “So in addition to being small and spherical so that it doesn’t have a lot of drag on it, there’s an extra advantage for Palmer amaranth pollen.”
How Far Can It Go?
Sosnoskie is in the beginning stages of collecting data on how far and fast pollen can fly, but some modeling with equations suggests that it can travel hundreds of miles. The theoretical maximum flight distance of an object is a function of three variables: mixing height, wind speed and the settling velocity of the particle.
The mixing height is an upward vertical variable that describes how high particles will mix in the atmosphere. According to Sosnoskie, based on data collected from the EPA and Georgia Forestry Commission, “mixing heights for the state of Georgia for small particles like smoke or pollen grains to get moved up into the atmosphere are about 2,000 meters.”
The next component is wind speed, or how far the particles get moved in a lateral direction. In Georgia, wind speed ranges from 0-10 mph at the mixing height of Palmer amaranth pollen.
Settling velocity is the final variable, describing how quickly particles in the air fall to the ground. If mixing height moves a particle upward and wind speed moves the particle laterally, then settling velocity is the rate that the particle falls back to the ground. “Settling velocity is a function of the size of the particle, the density of the particle and the density and viscosity of the medium it is moving through,” Sosnoskie said.
According to Sosnoskie, this theoretical study reveals that Palmer amaranth pollen could travel up to 435 miles if conditions were ideal. This would allow pollen to travel not only between fields, but between counties and states. However, there is another variable to the equation. Pollen may travel 435 miles, but that doesn’t mean it will be able to pollinate another plant when it arrives at its final destination.
Sosnoskie explained, “This just talks about how far pollen can move. This is not synonymous with gene dispersal, necessarily. The pollen grains physically can move (this far), but are they going to be biologically active when they reach a susceptible female? Probably not at the end of that 435 mile flight; they will probably have been killed by atmospheric pollutants, UV radiation and temperature.
“Longer dispersal distances require longer flight times, and that is going to expose them to a lot of environmental conditions. However, we don’t know at this time how far the resistant genes are going to be transferred, so we are also conducting studies to look at the gene flow in addition to the pollen flight.”
In May 2006, researchers at UGA-Tifton conducted an experiment in Macon County, Georgia, to determine gene flow of glyphosate-resistant Palmer amaranth. After planting 179 glyphosate-resistant males in the middle of a 75-acre field, susceptible females were planted at intermittent distances from 1-200 meters from the glyphosate-resistant males.
In October 2006, seeds from 300 mature female plants were harvested, and a study began on the progeny. As of publication time, the full results from the study were not available, but a preliminary study showed that biologically active, glyphosate-resistant pollen was viable at 200 meters.
Targeting Future Efforts
“What does this mean for everyone else?” Sosnoskie asked. “I am not telling you how to control it; I am just telling you how bad the problem’s going to be. Well, glyphosate resistance isn’t everywhere. We do know that we are finding it more often, but once we know where it is and we know how far and how fast it is going to move, then we can target our efforts. We can develop management efforts that are specific to those areas that we know are severely infested or in danger of coming infested.”
Editor’s Note: See the next edition of Cotton Grower April 2007, to learn about management techniques for glyphosate-resistant Palmer amaranth, as well as what producers can do to help prevent it.
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