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Weed control wake up call

What happens when weed control is not as effective as it should be?

November 14, 2007  By Rosalie I. Tennison

What happens when weed control is not as effective as it should be? Perhaps
a switch to another product would solve the problem. But, what if there is not
another product to try? That is the scenario that is facing Canadian potato
growers, according to University of Guelph and Agriculture and Agri-Food Canada
(AAFC) weed researchers.

"The basis of good weed management in potatoes hinges on a few good products
that are very old," says Dr. Clarence Swanton of the University of Guelph's
Department of Plant Agriculture. He reports that reviews of the two main potato
weed control products conducted by the Pest Management Regulatory Agency (PMRA)
should be seen as red flags by the industry. Linuron was reviewed and approved
for continued use by PMRA. But, Sencor is currently under review and there are
no guarantees the results will be positive. For this reason, Swanton suggests
the industry should be on the lookout for new chemistry.

"We have weed species in Ontario resistant to both Linuron and Sencor,"
Swanton comments. "We need to start looking for new chemistries now."
In particular, Linuron (Group 7) resistant pigweed has been identified, as has
pigweed resistant to Group 2 herbicides in Ontario. If Sencor (Group 5) is lost,
those growers who have these bio-types will have no option left to control them
in their potato crop. He suggests that waiting until there are no weed control
options available and then beginning the search for replacements will be an
unsatisfactory game of catch up. While the search is on, a search that could
take years, growers will be faced with uncontrollable weed problems in their


Over the past several years we have lost some effective herbicides, such as
Gesagard, Patoran and others, notes Dr. Jerry Ivany of AAFC in Prince Edward
Island. "Sencor is the backbone of weed control programs in potatoes on
Prince Edward Island, but lamb's quarters resistant to Sencor is being found
in more and more fields and growers have to switch to Linuron to get control
of these species," he says. "We have a good list of herbicides effective
on annual and perennial grasses but fewer and fewer herbicides to control broadleaved
weeds are available."

But, new chemistry is not the only answer. Swanton reports there is no weed
management work being conducted on potatoes in Ontario. Unlike Prince Edward
Island, where Ivany has been evaluating alternate methods of weed control (see
Organic options for weed control, page 19), the Ontario industry has
not promoted or sought this type of research in addition to looking for new
chemistry. Ivany notes that many growers of processing potatoes on Prince Edward
Island and other parts of Canada have switched to bed-shapers to hill potatoes
to prevent greening and this is done at planting or soon after potato emergence.
This system precludes use of later cultivation to control newly-emerged weeds
and is dependent on good weed control with herbicides. If present herbicides
are lost, and no new herbicides become available, then growers may have to revert
back to the older system of planting and cultivating several times to control
weeds before the potatoes are hilled near row closure.

In 2004 at the Elora research station, Swanton evaluated a new product being
developed for registration in the US for weed control in potatoes. However,
he points out that testing a new product on a couple of rows of potatoes is
not enough evaluation to satisfy the Canadian registration process and it is
down to the will of the industry to push for more evaluation or to encourage
the search for alternate products.

In 2003 and 2004, Ivany examined the same product and another called Spartan,
but both caused too much injury when the potatoes emerged. It also caused yield
loss and did not control some weeds common to Prince Edward Island, so they
may not be viable for registration. There are no other new products for potatoes
in the pipeline and the 170,000 hectares of potatoes grown in Canada are considered
a minor crop, so few products will be developed specifically for potatoes, Ivany

"The Canadian potato industry needs to be proactive on this issue,"
states Swanton. "It requires grower support to get issues looked at and
to get new chemistry into Canada." He reminds growers of re-cropping issues
and soil residues that could harm certain varieties, such as Dual and the Superior

Ivany adds that growers have to be careful when using Sencor as well, which
could harm Shepody and Superior varieties.

Both researchers suggest growers need to evaluate what weed control options
they have. They believe, when serious evaluation is conducted, growers will
find they have few alternatives. The time to start considering new chemistry
for weed control in potatoes, alternate methods for weed control and developing
comprehensive weed management systems is now, according to Swanton and Ivany.
They fear growers may have an uncontrollable weed problem in the near future
that will have no cure. There is no way to predict the effect such a scenario
would have on the Canadian potato industry. -30-


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