With our recent unseasonably low temperatures and the early appearance of winter-like weather, I’ve been asked what impact this might have on wheat diseases. The really short answer to the question is – it’s too soon to know for sure. I’ll discuss some of the things we consider when trying to anticipate which diseases might be problems in the coming season and what to do about them if you want to continue reading.
There are about a dozen diseases of winter and spring wheat that occur across our region, but only a few of them are widespread across most production zones. Stripe rust probably causes the greatest concern because of several epidemics in recent years, but eyespot, Cephalosporium stripe, dryland foot rot, snow mold, and wheat streak mosaic are also ever-present potential problems. Of these diseases, only stripe rust is a significant problem in spring crop – all others are mainly winter crop diseases
Whether these diseases occur and how much damage they cause varies, largely due to cultural practices, especially seeding (emergence) date, variety selection, and, weather conditions, both temperature, and rainfall (how much and when it falls).
All of the diseases listed above, except snow mold, are favored by early seeding – usually the earlier the crop is seeded the more severe the disease. This is especially true for wheat streak mosaic and the reason it was very severe in a few very early seeded fields in eastern WA in 2020. Optimal seeding dates vary across our production region, based largely on when seedbed moisture is available, with generally earlier seeding in lower-rainfall zones and later seeding in higher-rainfall zones. So, when we talk about early or late seeding, we really mean relative to the optimal seeding dates for that particular region. Early seeding in Pullman is not the same as in Mansfield.
Variety selection also influences occurrence of diseases. A large focus of the WSU breeding programs is improving resistance to our common diseases. Growers and field consultants have a pretty good idea of the diseases that commonly cause problems in an area, so my recommendation is to select varieties that perform best in the WSU Cereal Variety testing plots nearest your farm. Be sure to check the disease resistance ratings for them on our website (smallgrains.wsu.edu) or in the Washington State Crop Improvement Seed Buyers Guide, and grow them.
Everything I’ve mentioned to this point happens before we have much idea of what the weather is going to do, so these decisions must be made based on past history of problems in a field. Once the crop is in the ground, weather takes over.
In those years when autumn rains are timely, we typically see earlier seeding and emergence of the winter crop, which increases the risk of overwintering stripe rust, and fall infection for eyespot and Cephalosporium stripe. The risk of these diseases increases more if temperatures remain mild and rainfall continues into November and December, and even more so if winter remains relatively mild and open. Snowfall and continued snow cover puts the brakes on these diseases, limiting their overwinter development; snow cover also protects the crop against winter injury caused by low temperatures.
Low temperatures during December, January, and February have a strong influence on the severity of stripe rust next year. Lower temperatures (single digits and below zero) reduce survival of overwintering stripe rust and reduce the potential for disease next year. Snow cover also plays into this for the reason I mentioned above – it protects plants and the stripe rust fungus from those low temperatures. So low temperatures with little or no snow cover will reduce stripe rust more than if there is persistent snow cover.
The fall 2020 growing season has been relatively dry and emergence has been normal to late, depending on the area, which so far suggests to me that our disease risk is average to below average. I checked with Dr. Xianming Chen, ARS plant pathologist and our resident stripe rust expert, to get his take on stripe rust potential. His team checked fields in several eastern WA counties including Adams, Douglas, Grant, Lincoln, and Whitman counties, and did not find any stripe rust, which seems consistent with our fall weather thus far. We will continue to monitor temperatures during winter and expect to have the first predictions for stripe rust severity in late January or early February.
We don’t have good predictive models for any of the other diseases I mentioned above, so we’ll have to wait until early spring to begin scouting fields to see if they appear. For eyespot, there is the option of applying a foliar fungicide if disease is severe enough and we’ll have more to say about that in spring.
For more information, visit the Wheat and Small Grains Extension Team Disease Resources webpage.