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9-Misc: Glyphosate-resistant weeds may invade Pennsylvania fields

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SOURCE: Pennsylvania State University, USA, News Release
DATE:   May 30, 2003

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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. "This weed gets a foothold
in no-till continuous soybean crops -- where they grow soybeans in the
same field for several years in a row. In Pennsylvania, that's not the
norm. Most of the time, our more diverse crop rotations could keep this
problem at bay.

"In places where it's a problem -- such as Kentucky and Tennessee --
they're rotating no-till cotton with no-till soybeans, so they're using
the same herbicide on different crops. The key to preventing these kinds
of problems is to rotate crops and herbicide families, change modes of
action by not using the same herbicide annually, and use nonchemical
methods to manage weeds."

Curran says if you suspect a resistance problem, notify your county Penn
State Cooperative Extension agent immediately, so that he or she can
confirm that a problem exists.

"Investigate the causes and factors," he says. "How big was it at
application? What rate or applications were applied? Is there a spray
pattern associated with the surviving plant? Are the plants displaying
different levels of herbicide injury? Just because you didn't kill the
weed doesn't mean it's resistant. In fact, frequently it's not."

Each horseweed plant produces hundreds of thousands of seeds that
disburse on the wind like dandelions. Swift action with alternative
herbicides and other physical control methods can eliminate the plants,
Curran says. Once they go to seed, it's too late.

"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."

EDITORS: Contact William Curran at (+1-814) 863-1014 (phone) or (e-mail).


Gary Abdullah
(+1-814) 863-2708
(+1-814) 863-9877 fax #167


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