During this past season, we have seen widespread wheat streak mosaic virus across the state. Because of this, abundant curl mites will be active in the state. Producers should be vigilant about volunteer management between now and planting. Recent moisture may lead to higher levels of volunteer wheat emergence in the coming weeks.
Volunteer wheat control is one of the most important preventative measures for wheat streak mosaic virus. If volunteer wheat is allowed to stand, it creates a “green bridge,” allowing wheat streak mosaic and wheat curl mites to survive locally. Volunteer wheat should be terminated at least two weeks before planting to allow sufficient time for mites to die off. Growers should be mindful of volunteer wheat that may “hide” in double-cropped soybeans or cover crops and may be difficult to control.
Figure 1. Volunteer wheat that has emerged in wheat residue. Photo by Sarah Lancaster, K-State Research and Extension.
Breaking the “green bridge”
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 (death of the whole population will take approximately 2 weeks).
Producers often like to wait several weeks after harvest before making their first herbicide application to control volunteer wheat. This allows as much volunteer wheat as possible to emerge before spraying it or tilling it the first time. Glyphosate and atrazine are two herbicides that are often used for this purpose. Additional information about controlling volunteer wheat can be found in a recent eUpdate article: “Controlling weeds after wheat harvest.” Often, a second application or tillage operation will be needed later in the summer to eliminate the green bridge to fall-planted wheat by ensuring all volunteer wheat is dead within ½ mile of wheat being planted in the fall. Wet weather through late summer often favors multiple flushes of volunteer wheat (Figure 2). Also, it favors the growth of other grassy weeds that can support moderate populations of the curl mites and virus.
Figure 2. A thick stand of volunteer wheat after wheat harvest (left panel) and detail of volunteer wheat development (right panel). Photos were taken in Edwards County, KS, by Romulo Lollato, K-State Research and Extension.
Management with genetic resistance: One tool in the toolbox
Other than timely control of volunteer wheat, genetic resistance is also an important tool for WSMV control. Genetic resistance to wheat streak mosaic can also reduce the risk of severe disease problems. There are currently several that have wheat streak mosaic resistance, including KS Mako (red), KS Bill Snyder (red), KS Dallas (red), KS Hamilton (red), Guardian (red), Oakley CL (red), Joe (white), and Clara CL (white). Some of these varieties have a gene named wsm2. This resistance gene helps but has limitations. For example, this gene is effective against wheat streak mosaic virus but not against Triticum mosaic virus (https://bookstore.ksre.ksu.edu/pubs/ep145.pdf) or wheat mosaic virus (high plains disease), two other viral diseases spread by wheat curl mites, sometimes simultaneously. The resistance conferred by wsm2 is also temperature-sensitive. It is much less effective at high temperatures, although resistance in KS Dallas and KS Hamilton seem to endure greater temperatures before breaking down (~ 70º F). The resistance is much less effective if wheat is planted early for grazing or if high temperatures persist into October. KS Silverado (white) also has temperature-sensitive resistance to wheat streak mosaic, although from a different source than wsm2.
In addition, there are a handful of varieties with resistance to the wheat curl mite, including TAM 112, Byrd, Avery, Langin, Kivari AX, KS Western Star, Whistler, Canvas, Guardian, Crescent AX, Incline AX, Fortify SF, TAM 115, TAM 204, and T158. These varieties remain susceptible to 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. Still, it will not provide enough protection if wheat is planted close to volunteer wheat or other hosts infested with large populations of the curl mites and virus.
Unfortunately, many of these varieties are adapted to western Kansas and may not be the best options for production systems in the central corridor. Some central Kansas-adapted varieties, including Rockstar, SY Wolverine, LCS Photon AX, LCS Helix AX, and LCS Julep, tolerate WSMV infection better than others. KS Mako is a new release out of the K-State Manhattan breeding program that is adapted for the central corridor of the state and does carry the wsm2 gene. These varieties will still show symptoms of WSMV but may yield better than other varieties that are more susceptible.
More information on variety selection can be found here: MF991 Wheat Variety Disease and Insect Ratings https://bookstore.ksre.ksu.edu/pubs/MF991.pdf
Figure 3. Close-up of wheat showing symptoms of a wheat streak mosaic virus infection in the fall. Photo by Kelsey Andersen Onofre, K-State Research and Extension.
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 the virus and mites, but fortunately, it is not that common in wheat fields. In contrast, although relatively poor hosts, various foxtails could be important disease reservoirs simply because of their abundance. These grasses may play an important role in allowing the mites and the 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 the 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 become 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).
Kelsey Andersen Onofre, Extension Wheat Pathologist
andersenk@ksu.edu
Romulo Lollato, Wheat and Forages Specialist
lollato@ksu.edu
J.P. Michaud, Entomologist, KSU Agricultural Research Center-Hays
jpmi@ksu.edu
Sarah Lancaster, Extension Weed Science Specialist
slancaster@ksu.edu
Tags: disease volunteer wheat wheat streak mosaic wheat curl mite wheat green bridge