Three more weeds refuse to obey the new chemical order
Southern farmers can now add tropical spiderwort, mare's tail and morning glory to their list of glyphosate resistant weeds

Glyphosate-resistant mare's tail infests Southeast Missouri fields

PORTAGEVILLE, Missouri, August 4, 2004, University of Missouri: Mare's tail, a familiar nemesis for Missouri farmers, has reappeared with a new resistance to glyphosate-based herbicides, a University of Missouri weed scientist said.

Mare's tail, also known as horseweed in the Delta region, "isn't a new problem, but until recently, glyphosate controlled it," said Andy Kendig, weed science specialist at MU Delta Research Center in Portageville. "Now, we see fields where everything is burnt down except horseweed. It's really erupted over the past two years."

Glyphosate is the active ingredient in Roundup and other herbicides, upon which some farmers rely almost exclusively. "Some of the mare's tail problem is due to glyphosate-only burndown treatments," Kendig said. "The good news is that we have a couple of very good treatment options. The bad news is that it doesn't always get done."

MU researchers recommend a March application of an herbicide such as 2,4-D or Clarity, he said. "These two herbicides are essential to control mare's tail - or primrose or several other troublesome weeds. Even if there are a few late-germinating horseweeds that escape, this application is still needed."

Without the pre-plant burndown application, he said, "you have to go with mediocre cleanup options. There are only a few choices," including "old-fashioned tillage."

[Also reported at http://deltafarmpress.com/news
/080504-marestail-missouri/]

See also additional story: "Weed control could be circle of truths"

Morning glories creeping their way around popular herbicide, new UGA research reports

August 24, 2004, University of Georgia: Morning glories are beloved mailbox flowers all over rural America, but to farmers, they are something else: a noxious weed that can lower yields and choke harvesting combines. For some 30 years, however, the herbicide glyphosate has kept morning glories quite effectively out of farm fields.

Now, for the first time, however, researchers at the University of Georgia have identified morning glory families that are tolerant to glyphosate – noxious vines that could cause problems for the country's farmers.

"Our study suggests that serious and immediate consideration should be given to developing regional strategies for managing the evolution of tolerance in morning glories," said Regina Baucom, a doctoral student at UGA who directed the research.

Baucom and UGA assistant professor of genetics Rodney Mauricio co-authored the study, which is being published this week in the Proceedings of the National Academy of Sciences. The research was funded by the National Science Foundation and a research grant from Sigma Xi.

"Our interviews with farmers in the Southeast suggest that morning glories can tolerate applications of glyphosate," said Baucom, "and, in some cases, increasing concentrations of the herbicide have been required to control it."

Such an increase in tolerance to the chemical gives researchers a unique opportunity to study the evolutionary genetics of a novel trait and may help them and others slow the rate of evolution of tolerance in morning glories.

What Baucom and Mauricio found was that, in at least one natural population of morning glories they studied, there is a substantial genetic variation for tolerance, meaning that the "evolutionary door" is wide open. For evolution by natural selection to succeed, there must be genetic variation with a population and a significant selective force. This study is a case-in-point of evolution by selection – human-mediated evolution, similar to the evolution of bacteria resistant to antibiotics.

"Given the continued presence of glyphosate, the number of tolerant individuals should increase within the population over time," the scientists reported, "as might the overall level of tolerance of the population." The fact that glyphosate is a relatively recent tool in the fight against weeds led the scientists to conclude that the tolerance trait in this wild population was naturally occurring – not caused by use of the herbicide.

The presence of genetic variation, however, does not in itself guarantee that tolerance to glyphosate will evolve. The requirement also exists of "net selection" for tolerance, and it is acted upon by two components: fitness costs and benefits. The "benefit" of being tolerant must outweigh any sort of "cost" of being tolerant, much akin to the theory of economic cost/benefit models.

In fact, this research has shown that there is positive directional selection for tolerance to glyphosate, meaning that by applying glyphosate, those that are tolerant to the herbicide produce more seeds than those that are susceptible (given that susceptible individuals either die or produce almost no seed). Perhaps more key for the farmer, however, is the finding that in an environment devoid of glyphosate, tolerant families produce many fewer seeds or offspring than susceptible families. This is evidence of a fitness cost of tolerance, and this information can be used in managing or controlling the further evolution of tolerance in morning glories by arguing for not spraying RoundUp® in certain years. Since the issues are so complex, new strategies will have to be considered to control increasing numbers of glyphosate-tolerant varieties.

"For glyphosate, such strategies could involve something as simple as periodically spraying with alternate herbicides, as long as there is little cross-tolerance with glyphosate," said the authors. "If, however, there is cross-tolerance with other causes of plant damage, such as hail, herbivores or pathogens, alternative spraying regimes may not be a viable mechanism for controlling the evolution of glyphosate tolerance."


Little-known weed causing big trouble in Southeast

August 24, 2004, ARS News Service: Like the plant in "Little Shop of Horrors" a little-known weed is growing fast. Tropical spiderwort, inconsequential for seven decades, has recently spread in alarming proportions in fields in Georgia, Florida and North Carolina.

First detected in the United States in the 1930s, the weed has made major gains in Georgia, according to Agricultural Research Service agronomist Theodore Webster of the Crop Protection and Management Research Unit in Tifton, Ga. Webster and his colleagues--Michael Burton and Alan York of North Carolina State University, and Stanley Culpepper and Eric Prostko of the University of Georgia--are monitoring the weed's advances.

In 1999, it was found in five counties in southern Georgia. By 2002, 41 Georgian counties reported tropical spiderwort was present, and 17 listed it as moderate to severe.

A 2003 survey revealed that tropical spiderwort was entrenched in Georgia, affecting 52 counties, with 29 counties listing the weed as moderate to severe. More than 195,000 acres in Georgia are infested. It's now widespread in Florida, and has been discovered on about 100 acres in Goldsboro, N.C.

Tropical spiderwort, Commelina benghalensis, is now the most troublesome weed in Georgia cotton and the second most problematic weed in peanut. The weed competes with crops for water and nutrients, and smothers the crops at the same time. One reason for the surge in the weed's growth is its resistance to the commonly used herbicide glyphosate. Conservation tillage [undertaken in conjunction with the use of GM glyphosate-resistant crops] and reduced use of soil-applied herbicides may also be contributing to the problem.

According to Webster and his colleagues, tropical spiderwort spread has coincided with resurgent cotton production in Georgia. Cotton acreage in the state increased from about 260,000 acres in 1989 to nearly 1.5 million acres in 1995, in part due to the success of the boll weevil eradication program.

Most cotton grown in Georgia is tolerant to glyphosate, allowing growers to spray the chemical on cotton crops to control weeds. Webster and his colleagues are studying the biology and management of tropical spiderwort and will continue to monitor its presence in the Southeast.

 


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