Tag Archives: Agronomy

In some years, wheat streak mosaic virus (WSMV) can be a severe problem for Kansas wheat producers. Fields with very severe wheat streak mosaic can typically be traced back to a lack of control of volunteer wheat. Problems with wheat production the previous year can leave large amounts of seed on the soil surface. As this seed germinates, it creates a “green bridge”, allowing wheat streak mosaic and wheat curl mites to survive locally.

Challenges faced in 2020

This year, the wheat crop faced several challenges that might have increased the amount of seed left behind after harvest, which could also increase the amount of volunteer wheat (Figure 1). These problems included:

  • Freeze damage during stem elongation (which caused many delayed wheat heads to emerge)
  • Hailed out wheat
  • Some reports of head scab (Fusarium head blight)
  • Waterlogging conditions in parts of central Kansas
  • Drought-stressed wheat

The presence of later-emerged heads due to the freeze damage to main stems can cause differences in maturity between tillers that survived the freeze and later tillers, which can increase harvest losses. One of the recommendations to manage fields affected by head scab is to increase the fan speed of the combine and “blow” the diseased kernels out of the harvested grain.  Likewise, waterlogged conditions and drought stress both decrease wheat kernel weight and likely increase harvest losses of grain. These smaller kernels might germinate into volunteer wheat increasing the risk of severe wheat streak mosaic the following year.

Figure 1. Thick stand of volunteer wheat after wheat harvest. Photo by Stu Duncan, K-State Research and Extension.


Wheat curl mites will move off growing wheat as the green tissue dries down and dies. After moving off the existing wheat at or near harvest time, the mites need to find green tissue of a suitable host soon or they will die of desiccation.

Producers often like to wait several weeks after harvest before making their first herbicide application to control volunteer wheat. This allows as much volunteer as possible to emerge before spraying it or tilling it the first time. Often, a second application or tillage operation will be needed later in the summer to eliminate the green bridge to fall-planted wheat by making sure all volunteer is dead within ½ mile of wheat being planted in the fall. Wet weather through late summer often favors multiple flushes of volunteer wheat and also favors the growth of other grassy weeds that can also support moderate populations of the curl mites and virus. These weather patterns keep a lot more alternate host plants alive during the critical period when mites and virus would not have plants to survive on.

If volunteer has emerged and is still alive shortly after harvest in hailed-out wheat, wheat curl mites could easily build up rapidly and spread to other volunteer wheat that emerges later in the season. On the other hand, if this early-emerging volunteer is controlled shortly after harvest, that will help greatly in breaking the green bridge. However, if more volunteer emerges during the summer, follow-up control will still be needed.

Other hosts for the wheat curl mite

Volunteer wheat is not the only host of the wheat curl mite. Over the years, multiple research studies have evaluated the suitability of wild grasses as hosts for both the curl mite and the wheat streak virus. There is considerable range in the ability of a grassy weed species to host the mite and the virus. Barnyardgrass is among the more suitable hosts for both virus and mites, but fortunately it is not that common in wheat fields. In contrast, various foxtails, although a rather poor host, could be an important disease reservoir simply because of their abundance. These grasses may play an important role in allowing the mites and virus to survive during the summer months particularly in the absence of volunteer wheat.

The K-State Research and Extension publication, MF3383 – Wheat Streak Mosaic, includes information about grassy weed hosts of the mite and virus, and the contribution of these hosts to the risk of severe wheat streak mosaic infections. Take note of significant stands of these grasses in marginal areas and control them as you would volunteer wheat.

If volunteer wheat and other hosts are not controlled throughout the summer and are infested with wheat curl mites, the mites will survive until fall and could infest newly planted wheat. Wheat curl mite infestations of wheat often lead to wheat streak mosaic infections (Figures 2 and 3).

Figure 2. Volunteer wheat on the edges of a sunflower field were infested with wheat curl mites and caused a wheat streak mosaic infection in the adjacent wheat crop that fall. Photo by Stu Duncan, K-State Research and Extension.

Figure 3. Close-up of wheat showing symptoms of a wheat streak mosaic virus infection in the fall. Photo by Jeanne Falk Jones, K-State Research and Extension.

Genetic resistance to wheat streak mosaic can also reduce the risk of severe disease problems. There are currently a few varieties adapted to Kansas that have wheat streak mosaic resistance, including KS Dallas (red), Guardian (red), Oakley CL (red), Joe (white), and Clara CL. All of these varieties have the same resistance source (WSM2). This resistance helps, but does have some serious limitations.  For example, this resistance is effective against wheat streak mosaic but does not cover triticum mosaic or high plains (two other viral diseases also spread by the wheat curl mites). The resistance conferred by WSM2 is also temperature sensitive and is much less effective at high temperatures, although the resistance in KS Dallas seem to endure greater temperatures before breaking down.  If wheat is planted early for grazing or if high temperatures persist into October, the resistance is much less effective. KS Silverado (white) also has temperature sensitive resistance to wheat streak mosaic, although from a different source other than WSM2.

In addition, there are a handful of varieties with resistance to the wheat curl mite, including TAM 112, Byrd, Avery, Langin, KS Western Star, Whistler, Canvas, Guardian, Crescent AX, Incline AX, Fortify SF, TAM 115, TAM 204, and T158. These varieties are actually susceptible to the viral diseases, but they generally slow the development of the mite populations in the fall.  This resistance can help reduce the risk of severe disease but will not provide enough protection if wheat is planted in close proximity to volunteer wheat or other hosts infested with large populations of the curl mites and virus.

ST. LOUIS (June 22, 2020) — Farmers often face detrimental losses when it comes to yield-robbing pests, but the soy checkoff and its partners in the Take Action program released free tools to use this week to mitigate crop damage and stave off resistance. From June 22 through June 26, university experts, weed scientists and advocates will come together for PEST Week (Pest Elimination Strategies and Tactics) to break down the pesticide-resistance challenge into manageable and realistic steps for farmers.

“PEST Week is really a reminder to invest in best management practices now to protect our yields, so we don’t pay for it later at the elevator,” said Tom Oswald, a United Soybean Board (USB) farmer-leader from Cleghorn, Iowa. “The resources that the Take Action program provides are unbiased and backed by the experts across the country and give us, as farmers, the information we need to manage our pests and defend against increasing resistance. After a tough spring, it’s more important than ever to take this seriously.”

Take Action is an industrywide pesticide-resistance management initiative funded in part by the soy checkoff and other endorsing partners including commodity groups, academic institutions and the leading trait and agrochemical companies.

It’s crucial to conduct midseason steps now, such as in-field scouting and using a different site or mode of action from the previous application, to minimize weed competition, disease and insect damage. Pesticide resistance can be even more costly and stems largely from ineffective applications and management — which the Take Action program has vowed to correct.

“It’s getting to the point where if farmers don’t deal with resistance, they will see major losses,” said Christy Sprague, a Michigan State University professor and weed extension specialist for Take Action.

Take Action’s partners compiled all the latest information from weed, insect and disease experts into a ready-to-read kit that farmers can download from the Take Action website or reference on the go in the Take Action app. Every small step farmers take brings large rewards. Every day during PEST Week, a different step in effective and responsible pest management will be showcased across Take Action’s social channels, encouraging farmers to follow along and take the week to level up their management plan.

To get a head start, Take Action identified and included a profile on each of the “Big Four” weeds, highlighting why they can spell trouble along with best management practices farmers can implement this season:

– Waterhemp — Catching waterhemp early is a necessity. That’s why Take Action has a weed ID tool both online and in-app. After it germinates, it can grow over an inch a day and becomes much more difficult to control after reaching 6-8 inches tall. Waterhemp is a high-volume seed producer and can continue to produce into the summer. Being small and light, seeds can be carried by the wind, so dedicated attention to identification and elimination is incredibly important, especially midseason.

– Palmer Amaranth — Palmer amaranth, also called pigweed, presents similar issues to waterhemp, being a high-volume seed producer. Germinating the entire growing season, one Palmer amaranth plant can produce 250,000-500,000 seeds from early May to mid-August, making this weed a high priority to scout for and manage during the season. Effective herbicide application is critical to control this invasive species.

– Giant Ragweed — With a large seed and later germination than other weeds, it is incredibly important to follow up a pre-emergent herbicide with a post-emergent application and different site of action to limit yield loss from giant ragweed in your fields.

– Horseweed — Also called marestail, horseweed has two primary periods of emergence: late March through June, and late summer into fall. This weed competes with soybeans throughout the growing season, reducing yield and interfering with harvest. Like others, it can produce up to 200,000 seeds that are transportable by the wind, which can lead to big problems. Using herbicide treatments in fields where horseweed seedlings are observed or with a history of horseweed control problems is one potent way to manage this pest.

If farmers consistently see large weed populations or weed escapes — i.e., weeds that survive after herbicide applications — they may be managing them ineffectively, which can increase the spread of herbicide-resistant seeds and make matters even worse. Prevalent resistance directly threatens farmer profitability by posing economic challenges, decreasing land values, increasing crop losses and other challenges.

The Take Action program resources are segmented by the specific disease, insect or weed problem that farmers may be facing in addition to providing content on general best practices and common mistakes.

Get the Take Action kit through the website, IWillTakeAction.com/kit, as well as follow the daily steps and tips for responsible management on Take Action’s Facebook and Twitter pages.

About Take Action: Take Action: Pesticide-Resistance Management is a farmer-focused education platform designed to help farmers manage herbicide, fungicide and insect resistance. The goal is to encourage farmers to adopt management practices that lessen the impacts of resistant pests and preserve current and future crop protection technology. The program is endorsed by major ag chemical and trait providers, experts affiliated with land-grant universities, scientific professional organizations and soy, corn, cotton, sorghum and wheat commodity groups. Keep up with the latest updates by following Take Action on Facebook and Twitter or visiting IWillTakeAction.com.