Weeds and Rain

The annual U of I Snake River Weed Control Tour

Published in the August 2009 Issue Published online: Aug 06, 2009 Tyler J. Baum
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It was an unseasonably wet-and relatively cold-early growing season in eastern Idaho. Attendees at the annual Snake River Weed Control Tour in Aberdeen were fully aware of that, and were actually surprised the sun was finally shining without cloud interference after almost two solid weeks of rain.

By June 16, the day of the tour, the Aberdeen Research and Extension Center had received 5-plus inches 13 out of the previous 15 days. Most years, it's only three-quarters of an inch during that same period. Throughout the year, annual precipitation averages a grand total of 10-11 inches.

Dr. Pamela J.S. Hutchinson, Potato Cropping Systems Weed Scientist for the University of Idaho out of the Aberdeen station, explained that she was receiving calls from concerned growers about the excessive rain and cold causing potato crop injury. Though rain is a blessing, the excessiveness of the rain coupled with long periods of cooler temperatures and lack of sunshine have been making herbicides readily available for uptake by weeds and potatoes, but preventing the potatoes from metabolizing some of those herbicides fast enough for good crop tolerance.

"Anything that slows down potato growth also slows down herbicide metabolism," she says. "Cold temperatures can be a problem since it's pretty tough for a potato plant to emerge and grow quickly under those conditions."

Eastern Idaho is notorious for cold temperatures-Aberdeen recorded 13 days between May 1 and June 11 with a minimum temperature below 38°F, compared to 19 days for the same time period in 2008 and 11 days in 2007. However, Hutchinson points out that the last time the region experienced cold temperatures combined with unusually high rainfall in the spring similar to this year was 2005. Even then, she says it wasn't as severe as it has been early this year.

After the weed tour concluded, however, the sun remained out and the clouds stayed behind. Hutchinson said that once it warms up and the sun shines, it usually doesn't take long for early herbicide injury symptoms on potato vegetation to start disappearing, because the potatoes begin to actively grow and are able to break down those herbicides. In fact, she's observed this chain of events in her research trials, with no symptoms visible by row closure and no effect on tuber set, bulking or yields.

"We expect the crop to recover fully and that the early injury won't translate to yield reduction," she said.


One of the trials Hutchinson reported on at the weed tour included a future new herbicide from Syngenta Crop Protection called Reflex (fomesafen), not currently labeled for use in potatoes. Hutchinson, Research Support Scientist and Scientific Aide Brent Beutler, and JaNann Farr conducted the study last year-and are repeating it this year-to determine weed control and potato crop safety with Reflex alone or in tank mixtures with several standard potato herbicides.

The conclusion reached at the end of last year was that Reflex applied pre-emergence to the weeds and potatoes alone or in combination with these herbicides usually provided 90 percent or greater redroot pigweed and hairy nightshade season-long control. A pre-mix of s-metolachlor and metribuzin alone or with additional metribuzin did not provide greater than 85 percent hairy nightshade control; however, while the addition of rimsulfuron to the mixture or applied later, early post-emergence resulted in 97 to 100 percent control of this weed.

Common lambsquarters control by Reflex in combined with the other herbicides except Dual Magnum (s-metolachlor) or Outlook (dimethenamid-p) usually improved compared with control by Reflex applied alone. Regardless of the Reflex rate in the tank mixture treatments, control of all three weeds present in the trial was similar and ranged from 95 to 100 percent. Repeating the trial this year, Hutchinson says, will provide more clarity.

Meeting-goers were able to enjoy the beautiful sunshine with a trip out to the field to observe the progress of a herbicide potato crop tolerance study with 10 different potato varieties, many newly released. The trial is funded by the Idaho Potato Commission, and all 10 varieties received one times or two times the labeled rate of Chateau (flumioxazin) or Outlook, or the proposed labeled rate of Reflex, applied pre-emergence. Also included in the trial for comparison were plots of each variety with no herbicide application.

Hutchinson commented that she is seeing some of the same symptoms in this trial as she has been seeing in commercial fields-slight leaf crinkling and stunting-with some varieties seemingly more tolerant than others. She also said that, as expected, every day it is warm and sunny, the symptoms become less and less visible and that her experience with this study and others conducted in the past, have definitely helped her answer grower questions and concerns.