Issue link: http://read.dmtmag.com/i/61297
Soils & Nutrients Glyphosate resistance M Some orchard and vineyard weeds are resistant. by Geraldine Warner any scientists said weeds could never develop resistance to glyphosate, but in the late 1990s they were proven wrong. "As weed scientists, we were flabbergasted," Dr. Bradley Hanson, exten- sion weed specialist with the University of California, Davis, recalled during a weed management seminar in Wenatchee, Washington, this winter. Resistance to glyphosate was thought unlikely because of the herbicide's unique mode of action and behavior in plants. But there are now at least 13 weed species in the United States that have evolved resistance to glyphosate. Horseweed, also known as marestail (Conyza canadensis), is one orchard and vineyard weed that has been showing resistance to glyphosate in California, Oregon, and now Washington. Some California populations of a related weed, hairy fleabane (Conyza bonariensis), are resistant to both glyphosate and paraquat. What happened? Two things, Hanson says. Roundup-Ready soybeans, introduced in 1996, soon accounted for 90 percent of the country's 60 mil- lion acres of soybean plantings. Then came other Roundup-Ready crops, such as corn, cotton, alfalfa, and sugar beets, which are also grown on millions of acres. Roundup-Ready crops are genetically modified so that the herbicide's target site in the crop plant is unaffected while the weeds are vulnerable. While the resistant crops do not directly cause resistance in weeds, they create an opportunity for in-crop use of a formerly nonselective herbicide, which dramatically increases selection pressure for resistant biotypes. The other factor was that glyphosate became much cheaper after the Roundup patent expired in 2000, and many generic formulations came onto the market. That led to a tremendous increase in use of the product. Glyphosate cost $100 a gallon in the 1970s, compared with $50 in 2008. Today, growers can buy it for $15 a gallon or even less, Hanson said. About 16 million pounds of glyphosate are used annually in California, and glyphosate accounts for 40 percent of all herbicide active ingredients used. The situation is probably similar in Washington and Oregon. Mutations Resistance develops as a result of slight genetic mutations in weeds that can make "That's trouble them unaffected by the herbicide. These mutations occur naturally and are not caused by herbicides, Hanson said. Occasionally, one of these mutations enables a weed to sur- vive exposure to the herbicide and continue to reproduce while susceptible weeds die. When the herbicide continues to be applied, populations of these resist- ant plants increase. These are weeds that used to be controlled but no longer are, even at higher herbicide rates. There are two types of resistance: target-site and nontarget-site. brewing." —Bradley Hanson Herbicides usually affect plants by disrupting the activity of an enzyme that plays a key role in some biochemical process in the plants. Target-site resistance occurs when the enzyme becomes less sensitive to the herbi- cide, usually because of a mutation in the gene coding for the protein. Nontarget-site resistance develops without involving the active site of the herbicide in the plant. There are several ways this can happen. A common type of nontarget-site resistance develops when the plant becomes better able to metabolically degrade the herbicide or move it away from the target site. In the United States, about 125 weeds have developed resistance to 15 herbicide families. Some types of herbicides are more prone to resistance than others. Resistance has been reported to triazine herbicides, which are Photosystem II inhibitors, Hanson said. These were introduced in the late 1960s and were widely used in the early 1970s. Growers switched to ALS inhibitors, which were introduced in the 1980s, Horseweed, also known as marestail, has been showing resistance to glyphosate in California, Oregon, and Washington. Pictured top to bottom: in bloom, as a young stalk, and as a rosette. 30 APRIL 15, 2012 GOOD FRUIT GROWER www.goodfruit.com photos Courtesy of tim miller, Washington state university