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Green Bridge Management is Key to Managing Wheat Streak Mosaic Disease

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What is Wheat streak mosaic?

Wheat streak mosaic (WSM) is a virus disease that affect small grain crops, such as winter wheat and spring wheat, but also barley, durum, oats, corn, and others. There are at least three viruses that cause this disease and are considered a complex. Most of you will know Wheat streak mosaic virus (WSMV), while Triticum mosaic virus and Wheat mosaic virus may be lesser known. All of these viruses are vectored by the wheat curl mite (WCM), which is very small and disperses in wind currents (Figure 1).

What are Wheat streak mosaic symptoms?

Yellow streaking and mottling are characteristic symptoms of WSM. These symptoms usually affect the entire plant (Figure 2). Plants that become infected at early growth stages (before jointing) are typically stunted and their tillers sprawl out rather than growing erect. Yield loss from WSM can range from insignificant to 100%. Early infection typically results in more severe yield losses and wheat heads may be sterile.

What is the green bridge and why is it important for WSM?

The green bridge plays an important role in the WSM disease cycle, because it supports WCM and virus survival between crops. The green bridge describes the presence of green plant material between harvesting of one crop and planting of the next. When an infected crop matures and dries down, WCMs carrying the viruses disperse in the area seeking green bridge hosts for “over summering.” The best green bridge hosts that support high WCM populations and virus replication are volunteer wheat, corn, and to a lesser extend downy brome (cheatgrass). Numerous other gassy weeds are intermediate or poor green bridge hosts for WSM, that essentially maintain a background level of the virus but generally don’t promote WSM outbreaks unless conditions are highly favorable.

The presence of green bridge hosts in your area increases the risk of WSM for your future crop. Areas with high volunteer wheat and/or high downy brome abundance are of particular concern, especially if those green bridge hosts display symptoms of WSM infection. Attention should be paid to green bridge hosts in the very field where winter wheat is to be planted and in fields that are located upwind of the new winter wheat crop. Wheat curl mites can travel for several miles in wind currents.

What is the best way to terminate the green bridge?

Green bridge management is the most effective strategy to reduce WSM risk and is a good best management practice in all areas, particularly those hit by hail after mid-milk growth stage in cereals which would have promoted volunteer emergence. When choosing a termination method, consider the time to plant death. WCMs can survive on a host plant for as long as there is green tissue present.

  • Roundup (glyphosate) herbicide kills slowly and can take up to 2 weeks. Glyphosate-treated wheat plants can be a source of dispersing WCM for up to two weeks post-application (Jiang et al. 2005; Thomas and Hein 2004). Moreover, short-term increases in WCM population size and dispersal have been observed in the 3 to 9 days following a glyphosate treatment of wheat plants, depending on the glyphosate dosage applied (Figure 3).
  • Paraquat herbicide acts more quickly and reduces WCM survival time to a few days post-application (2 to 5 days; Jiang et al. 2005).
  • Tillage or swathing can be very effective. This will rapidly kill plants and reduce the likelihood of WCM being caught by air movement. When conditions are dry and hot, these mechanical methods of green bridge termination will eliminate WCM populations within a few days (Jiang et al. 2005; Thomas and Hein 2004).
  • Grazing provides incomplete green bridge control and allows the WCM to survive and disperse for an extended period. Unless you plan on extensive grazing, this is not a recommended green bridge termination method.

How long are wheat curl mites dispersing in the environment?

WCM are active over a wide temperature range. The mites reproduce most rapidly at temperatures between 75° F and 85° F and WCM population size on the host plant is an important trigger for mite dispersal. WCM movement slows down at temperatures below 50° F. The most recent WSM epidemic in northcentral Montana in 2016 was, among other things, facilitated by an unusually long and warm fall that lasted well into October. History has taught us that a long, warm fall extends the period of WCM dispersal into the new wheat crop and increases risk of WSM outbreaks.

How does winter wheat planting date relate to wheat streak mosaic risk?

Do not plant your winter wheat crop until green bridge hosts are well dead. If you chose glyphosate termination, let at least two weeks pass between herbicide application and crop planting.

You want to avoid having your winter wheat seedlings emerge when WCM populations are still high and actively dispersing in the environment, which increases the risk of the mites infesting your crop at an early developmental stage. Recall that early infection with WSM viruses results in more severe symptoms and crop damage. Delaying winter wheat planting reduces the risk of WCMs immigrating into your wheat crop.

Do not plant before the recommended planting date for your area. If the weather forecast projects continued warm weather for the coming weeks, consider delaying planting past the recommended date. Of course, later fall planting reduces yield potential and has to be weighed against the risk of yield loss from WSM infection. We developed a WSM risk assessment tool AWaRe that can help you understand WSM risk factors at play in your area and support your decision process.

Where can I find more information?

Publications that were referenced in this AgAlert:


Picture Captions

Figure 1. The wheat curl mite is a small and cigar-shaped mite that vectors the viruses causing wheat streak mosaic disease. (Picture credit: Dai Ito, Montana State University, MT)

Figure 2. Characteristic symptoms of wheat streak mosaic infection in wheat include yellow streaking and mottling of all leaves. Plants are often stunted. (Picture credit: Mary Burrows, Montana State University, MT)

Figure 3. Number of wheat curl mites (WCM) on susceptible wheat cultivar over time in response to herbicide treatments. (This figure was published in Jiang et al. 2005)


Please don’t hesitate to email ( or call (406-994-557) if you have any questions. I’m here to help.


Uta McKelvy, Extension Field Crop Pathologist