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Jacob Heitstuman (15 months) cheering on his daddy, Brian, and his papa, Dale, as they fill the sprayer
in Pomeroy.

Photo by Stephanie Heitstuman.

FARMER'S TOOLBOX

In the weeds

2021 Wheat College focused on resistance, stripe rust, Hessian fly

July 2021
By Trista Crossley


Resistance isn’t futile when it comes to managing weeds in wheat. You just have to be willing to mix things up.

Approximately 60 growers dialed in to the Agricultural Marketing and Management Organization’s 2021 Wheat College to hear Washington State University Extension faculty talk about herbicide resistance and weed management in wheat production systems. While Wheat College typically consists of both classroom and hands-on learning, this year, it was once again held virtually because of COVID-19.

Participants earned two pesticide credits. Gordon Gering of Ritzville was the winner of the Blackstone Grill drawing. A recording of the session is available on WAWG's YouTube channel.

Herbicide resistance

Drew Lyon, who holds the Endowed Chair in Small Grains Extension and Research, Weed Science, kicked off the session, tackling the thorny subject of herbicide resistance. Herbicide resistance develops over time as a process akin to natural selection weeds out the plants that are susceptible to a chemical, leaving mostly resistant plants to reproduce. If the genetic variation (which occurs naturally) that provides resistance is passed to a plant’s offspring, eventually, the resistant plants outnumber the susceptible plants, and the chemical stops being effective. Resistance can develop within just a few short years, even when the percentage of resistant weeds in a population is just .0001 percent at the beginning.

In order for natural selection to take place, three things have to happen. There has to be a struggle for existence, there has to be variation in the population, and that variation has to be inheritable. The amount of time it takes for resistance to occur depends on a number of factors, including cultural practices; the frequency of herbicide use; the herbicide mode of action (single site-of-action will be faster); the biology of the weed species; and the frequency of resistant biotypes among weed species.

“Eventually, weed populations will develop resistance to an herbicide given enough time and enough use,” Lyon said, adding that herbicide resistance has been around as long as we’ve had synthetic organic herbicides, but the pace of herbicide resistance is quickening. “We’ve relied more on herbicides in the last 20 or 30 years than we did before. We used to use more methods of weed control back in the 1950s, 60s and 70s.”

Chemicals rely on modes of action to be effective. A mode of action is the way a chemical controls a susceptible plant, with different chemicals using different modes of action. It used to be that when one chemical stopped being effective, all a grower had to do was switch to a chemical with a different mode of action. But Lyon said we are developing more weeds that are resistant to more than one mode of action, and growers are running out of the ability to control some weeds, such as Italian ryegrass in Eastern Washington.

“It’s a scary situation when you start developing weeds that are resistant to multiple mechanisms of action,” he said.

The current state of herbicide development is grim. Besides one possibility from FMC, there are no herbicides with new modes of action in advanced development trials. The last new mode of action was introduced more than 30 years ago. The number of weeds with herbicide resistance continues to increase within the U.S. and globally, and the number of weeds with herbicide resistance to more than one herbicide continues to increase. Part of the problem, Lyon said, is that the introduction of Roundup Ready crops in 1996 sucked all of the profitability out of the herbicide market in the U.S.

“Basically, companies make money by selling herbicides to the corn and soybean market, and if they can’t make a profit in the corn and soybean market, they don’t make a profit in the U.S.,” he explained. “When Roundup Ready soybeans came out, everything was cheap, easy and effective, and people no longer bought other herbicide products. Only the big players still had discovery programs, and they didn’t really see herbicides as a place to make money. That might be starting to change now that we have glyphosate-resistant weeds, but we’ll never be back to the heyday. We have to work with what we have today. If we lose the effective products we have today, there’s not much coming down the road to save us.”

So what can farmers do to avoid or delay herbicide resistance in weeds? Lyon pointed to integrated weed management, which has five components: sanitation, mechanical, biological, cultural and chemical. Dryland farming is limited in biological control options, and in direct seed or no-till operations, chemical has replaced mechanical.

“I’ve just shown you that our chemicals are becoming less and less effective. We need to spend more time on the cultural and sanitation, things like crop rotation,” he said.

General principles of integrated weed management include:

• Using agronomic practices that limit the introduction and spread of weeds.

• Helping the crop compete with weeds.

• Using practices that keep weeds “off

balance.” In other words, don’t keep doing the same thing over and over again. Some things farmers can do are raise a new crop, change the timing and/or type of herbicide application or introduce a different tillage system.

Lyon also talked about research he’s done on herbicide effectiveness in different weeds, including Russian thistle, prickly lettuce and Italian ryegrass. His message to growers was to combine different modes of action to control weeds.

“The lesson is, we need to try to get more mechanisms of action working,” he said. “It would have been nice to do that before we developed resistance to some of these things, but now it’s almost imperative that we don’t rely on a single mechanism of action to control these troublesome weeds.”

Stripe rust

According to WSU Extension Plant Pathologist Tim Murray, there are three different wheat rusts in the Pacific Northwest: stripe rust, leaf rust and stem rust, with stripe rust being the most common. The most visible symptom of stripe rust is either pustules on seedlings or long yellow stripes on plants that are post jointing. Stripe rust can be very destructive on susceptible varieties, causing yield losses of more than 50 percent when it occurs early in the growing season. The primary control method of stripe rust is planting a disease-resistant variety.

The factors that affect stripe rust are:

• Favorable temperatures/moisture for infection (temperatures of 50-64 degrees F with six hours of dew). Murray said cool temperatures are best for disease development, but that is less important than infection.

• Fall infection. There has to be susceptible plants in the fall because stripe rust only survives on living wheat plants. It is carried over from fall to spring.

• Winter survival. Temperatures in the single digits without snow cover will be detrimental to stripe rust. Under those conditions, Murray said the disease will die but wheat plants will be fine. Snow cover, unfortunately, provides enough insulation that the fungus can survive.

Stripe rust control options include:

• Cultural controls such as green bridge management, avoiding early planting and excessive irrigation (furrow irrigation is better than using sprinklers).

• Plant disease resistant varieties, preferably those with high temperature adult plant (HTAP) resistance that has a 1-4 variety resistance rating. HTAP resistance is more effective against all stripe rust races.

• Monitor the stripe rust forecast from Dr. Xianming Chen, scout fields and spray fungicides when necessary. Spraying is recommended when planting susceptible varieties (5-9 variety resistance rating) or when 1 to 5 percent of plants in the field have active rust.

If growers do need to spray, Murray said there are lots of choices, most of which will provide about three weeks of effectiveness when used at full label rates. A product with two modes of action will give growers an extra week of protection, while three modes of action will likely give them another week. Murray cautioned growers that it is important to follow growth stage and harvest restrictions on herbicides.

“In general, we don’t recommend spraying after anthesis because of diminishing returns in the benefit of the herbicide,” he said. “Usually, an earlier spray application is going to be more effective than one that’s made later in the season.”

Hessian fly

Stephen Van Vleet, an Extension educator in Whitman County, called Hessian fly a silent pest that is becoming more widespread. It poses a serious threat not only to wheat, but also barley and rye. It has long been a problem in spring wheat, but is increasingly becoming a problem in winter wheat. Susceptible spring wheat varieties can typically see upwards of a 70 percent loss in yield. There is also some evidence that the resistant breeding source is losing effectiveness.

“You don’t see it or really notice it (Hessian fly) for a while, then we have these populations on susceptible varieties. When we notice this problem, they are already in the puparium down there and trying to get control of it is more or less impossible,” Van Vleet said. “The only way to solve this problem at this point in time is resistant varieties.”

Rotation and other cultural practices are helpful, and research has shown that insecticide seed treatments may help winter wheat more than spring wheat. Hessian fly susceptibility can vary by rotational system:

• Winter wheat/spring wheat/fallow is at higher risk.

• Annual spring wheat is at very high risk.

• Later planting is often higher risk in spring/lower risk in fall wheat.

• Direct seeding is a higher risk.

• Spring wheat adjacent to winter wheat is a higher risk.

• Poor volunteer control puts crops at a higher risk.

Other Hessian fly considerations include:

• Hessian flies don’t travel very far.

• The repeated use of the same resistance gene will/has led to a breakdown.

• Our knowledge/surveillance of Hessian fly is minimal.

• Winter wheat has served as refuge crop. Growers can learn more about all of these topics at smallgrains.wsu.edu. There are also variety selection tools that can help growers choose which varieties will work best in their area and for their conditions.