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Herbicide-resistant weeds may invade Pennsylvania crops

(Tuesday, July 15, 2003 -- CropChoice news) -- Pennsyslvania State University, 05/30/03: UNIVERSITY PARK, Pa. -- Field crop producers and landscapers across Pennsylvania should be vigilant over the next few weeks for a new strain of super-weed threatening to gain a foothold in the state, according to an agronomist in Penn State's College of Agricultural Sciences.

William Curran, professor of weed science and extension specialist for Penn State Cooperative Extension, warns that surrounding states have been wrestling with a variety of the common annual weed known as horseweed or marestail. This variety, he says, is showing resistance to glyphosate, the active ingredient in many popular herbicides.

"Horseweed is very common in the Northeast -- it's actually a native species in Pennsylvania," Curran says. "It's mostly a problem along roadsides and areas that aren't tilled. Glyphosate is the active ingredient in Round-Up, TouchDown, Glypho-Max and more; it's the primary product used to kill emerging weeds at planting time. If a weed is resistant to glyphosate, it's a huge threat not only to soybeans but to all crops grown with no-till planting techniques."

In 2000, glyphosate-resistant strains of the weed were identified in a few isolated fields on Maryland's Eastern Shore. By 2001, it had moved into about 30 fields. By 2002, it was in many no-till soybean fields in Delaware. It since has been identified in New Jersey, Maryland, Virginia, Ohio, Tennessee, southern Kentucky and possibly as far west as Missouri.

"This year, we're really concerned about this weed getting a foothold in Pennsylvania," says. Curran "It could become an annual summer problem not just for corn and soybean growers, but also for the landscaping industry, where glyphosate-based herbicides are used frequently to kill weeds among bedding plants, in shrubs and on roadsides."

Over the next several weeks, Curran urges farmers, landscapers and others to keep an eye out for horseweed that isn't controlled by the standard application of herbicide.

"I think it's inevitable that it'll be here -- it probably already is, and just hasn't been identified," Curran says.

"We're talking about horseweed today, but there are other weeds that can develop resistance," he says. "There have been some glyphosate performance problems with common lambsquarters and other weeds on Maryland's Eastern Shore and in portions of the Midwest. There have been more resistance problems worldwide with the ALS-inhibitor family -- a different family of herbicides -- than with any other family, and we're just starting to see those problems in Pennsylvania the last two or three years. The bottom line is that overreliance on any pest management strategy will eventually produce problems and possibly failure. Herbicide or pest resistance is just one example that's close to home."