Dr. Kirk Broders, Plant Pathologist
Department of Bioagricultural Sciences and Pest Management
Colorado State University
This is the CSU Wheat Fields Days 2018 edition of the Colorado Wheat Disease Update. On June 7-13 the CSU Crops Testing team along with CSU wheat breeder Dr. Scott Haley visited CSU variety trials in eastern Colorado from Walsh in the south to Julesberg in the north and many places in between. If you were unable to attend the field day, you can find a copy of the field day guide here, and below I have provided a recap of the disease topics covered at the field day. I was able to visit many of the variety trial locations and saw a range of diseases and evaluated material from the CSU breeding program and PlainsGold varieties, as well as varieties from Kansas Wheat Alliance, LimaGrain, Syngenta and WestBred. Overall, disease pressure was much lower than in 2017. This was primarily the result of stripe rust never becoming established in eastern Colorado during the 2018 growing season. While the wet and cool conditions present in the state in early May were ideal for stripe rust infection (Fig 1), the limited amount of inoculum moving up from the hot and dry areas of Kansas, Oklahoma and Texas, prevented any early season infections. Therefore, fungicide applications were not necessary. However, as I said at the field day, each year stripe rust will be a threat to Colorado wheat when wet and cool conditions prevail in Texas, Oklahoma and Kansas. I recommend growers follow Clark Neely (@TXSmalGrains) and Erick DeWolf (@KSUWheatDisease) on Twitter to stay up to date on stripe rust reports from those states, as those are very good indicators of the likelihood of rust arriving in Colorado.
Much of my talk at the wheat field day focused on the mite-vectored viruses of wheat, as it was again the most widespread and significant disease of wheat in Colorado this year. While the disease was not as severe as in 2017, I was able to find symptoms of virus infection in many of the wheat fields I visited. In my opinion, the wheat-curl mite vectored viruses, including Wheat Streak Mosaic (WSMV), Triticum Mosaic (TriMV), and High Plains Wheat Mosaic (HPWMoV) represents the most persistent disease threat to wheat production in Colorado (Fig 2&3). Based on previous survey work, both the wheat-curl mite and WSMV are widespread and established in eastern Colorado, and we continue to see an increase in the incidence of coinfection with WSMV and TriMV. In addition, there are few economically viable chemical options for the control of the mite or the viruses they vector. This means that controlling this disease requires a multi-faceted approach which includes managing “green bridge” hosts, avoid planting early in areas with a history of mite-vectored viruses, and selecting resistant varieties.
Problems from this disease complex start when the wheat curl mite is able to oversummer in significant numbers. As the mite cannot survive off green plants for more than a day or two at the most; the mite needs a ‘green bridge’ to be able to survive until the new crop emerges in the fall. Typically, the mite survives on volunteer wheat, corn, or other weedy grasses until wheat is planted in the fall and then the mite moves back to wheat as it begins to emerge. In Colorado the greatest ‘green bridge’ risk results when pre-harvest hail rapidly results in pre-harvest volunteer wheat. The mites quickly carry the viruses to this volunteer and attempt to ride out the summer. Controlling volunteer wheat is perhaps the single most important thing ALL growers can do to reduce the wheat curl mite population. So make sure to remind your neighbors and friends. Also remember to allow at minimum of 2 weeks after herbicide application to kill volunteer wheat before planting the fall crop. There should be NO green tissue visible, as the curl mite will hold on for dear life until the last bit of living tissue is dead.
The mite and viruses can also survive on some other grasses present through the summer, but the next most significant risk for mite-virus oversummering is corn. Mite populations can establish on the corn and carry the viruses through corn and move to the new wheat crop in the fall. Damage around corn fields will be variable but will depend on how green the corn stays (irrigated corn is at greater risk than dryland). Significant August rains can improve the condition of dryland corn and extend its ‘greenness’ further into the fall when it can overlap with wheat emergence. The severity of this spread will depend on the extent of this corn-wheat overlap and on the fall weather conditions (e.g. greater risk with warm extended fall). Even though the risk of disease from adjacent corn is not as extreme as from pre-harvest volunteer, the gradient or border effects from mite and virus spread can be significant. Virus risk around these areas can be managed by avoiding early planting in these areas to minimize this overlap and planting resistant varieties.
In regards to planting date, research completed near Akron, CO over the last 5 years indicates that planting prior to September 15th puts wheat at a higher risk for curl mite infestation and viral infections in the fall. Keep in mind that this is a general rule and may vary from year to year. The most important factor to consider is the presence of a green bridge host, as mentioned above. That being said, in many places in Colorado if there is adequate soil moisture you often need to start planting, and the presence of volunteer wheat makes it likely for curl-mites to be present and active during planting. I recommend you review the Colorado Winter Wheat variety trial bulletin for updated Wheat Streak Mosaic Virus (WSMV) ratings. If you need to plant prior to September 20, I recommend planting one of the varieties that are more resistant (WSMV score <4). These include Avery, Byrd, Denali, Joe, Oakley CL and Snowmass. I would avoid planting the more susceptible varieties (WSMV >6) before September 20. These include Brawl CL, Larry, LCS Chrome, Settler CL, Sunshine, SY Monument, SY Rugged, WB4462, and WB Grainfield.
For more information, check out these resources on the wheat-curl mite and viruses they vector. Also check out this very cool “Wheat curl mite time machine” to simulate curl mite spread in your production system.
There were also a couple other foliar diseases I saw sporadically at several variety trials. These included the fungal diseases tan spot (Fig 4) and stagonospora blotch (Fig 5) and bacterial leaf streak caused by Xanthomonas translucens (Fig 6). All three of these pathogens survive in the residue and could continue to be present in future years as growers leave more reside on the field to retain moisture and prevent erosion. None of these disease will likely cause any yield loss this year, but under the right conditions they may be a problem in future years.
Finally, thank you to all the growers and the CSU crops testing program for putting on an excellent series of field days. The data collected from these variety trials every year is remarkably valuable and important to the success of the wheat program at CSU and to wheat growers in Colorado. Also many thanks to the families and local business and groups who prepared and provided multiple breakfasts, lunches and dinners during the field day tour. I really enjoyed meeting and visiting with everyone who came out to the field days.