Potatoes and herbicide (part 1)

Herbicide damage to potatoes caused by heavy rainfall

Farmers throughout southern and eastern Idaho were befuddled by the bizarre symptoms of crop damage that surfaced in their potato fields following a brief period of heavy rainfall in May of 2017. More odd symptoms – including swollen shoots, sprouts growing at a right angle, tuber folding and yellow veins in foliage – appeared in regional spud fields following another wet May in 2019.

Pam Hutchinson, the University of Idaho potato cropping systems Extension weed specialist, has diagnosed and studied the problem – heavy rains prior to potato emergence can move herbicides too deep into the soil, where they’re more accessible by shoots and tuber roots than usual, which could, in turn, cause crop damage. For her simulated rainfall trials, Hutchinson has been testing four preemergence herbicides: Matrix, Outlook, Prowl H2O and metribuzin.

“I needed to learn what symptoms a herbicide could cause in that type of a situation with all of that excess rainfall,” Hutchinson said. “Over the years we’ve looked at and learned about herbicide damage under normal growing conditions.”

Heavy rainfall before spuds emerge can also cause poor weed control, as herbicides move too deep and too quickly through soil to be absorbed by weed shoots and roots. In the wet years, many potato farmers had to apply postemergence herbicides they had not planned to use.

The risk of herbicide damage because of excess rainfall can be even more pronounced in fields with sandy soils, through which water and the more water-soluble potato herbicides such as Matrix and metribuzin, can infiltrate deeper than in heavier textured soils. Hutchinson works with growers who farm sandy soils and have developed weed control programs with the less watersoluble preemergence herbicides. The growers can keep a close eye on the initial weed control and follow up with postemergence herbicides if necessary.

“(The trials) reinforced even more so that if you have real sandy soil you’re going to have more downward herbicide movement than formerly understood when excess rainfall occurs,” Hutchinson said.

Ritchey Toevs, an Aberdeen potato farmer and a past chairman of the Idaho Potato Commission, believes lessons of Hutchinson’s research are valuable to farmers, even during normal growing seasons.

“If we have a windy day and we’re not getting much water on and the wind makes it difficult to irrigate correctly, we’ll add a few more hours to the set, and we can duplicate a heavy rainfall, unaware of the damage we could be causing,” Toevs said.

Weeds and rain

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.”

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.

One of the trials Hutchinson reported on at the weed tour included herbicide from Syngenta Crop Protection called Reflex (fomesafen). 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.

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.