Mild Winter Weather and Wheat Diseases – What to Expect?

Last year at this time, we were wondering whether winter was ever going to end. Remember the 60+ days of snow cover that began in early December 2016 and lasted into February 2017 and beyond?  In stark contrast, the winter of 2017-18 is turning out to be a no-show. It’s 50°F outside in Pullman today (Feb 8) and we haven’t seen a daily average temperature below freezing since January 4 (31.6°F). Compare that to the same time period last year when average daily temperatures were above freezing just 6 days. Another way to look at temperature data is Growing Degree Days* (GDD: 32°F base); last year from January 1 to February 8 there were 29 GDD, but there were nearly 10 times as many (241 GDD) during the same time period this year!  What impact is this mild weather likely to have on the disease picture for wheat this year?
Two diseases are particularly impacted by winter temperatures – stripe rust and eyespot (strawbreaker foot rot). Low temperatures are less favorable to both diseases, but for different reasons. For stripe rust, fall infection of winter wheat allows the stripe rust fungus to survive the winter; very low temperatures reduce survival of the rust fungus in infected plants, but the temperatures needed for this are in the single digits and negative numbers with no snow cover.

For eyespot, the most important time for infection is also during the fall – October to December – when it’s raining. Low temperatures during winter prevents new infections from occurring and slows disease spread in already infected plants; any temperatures below freezing with no snow cover will suffice.

Last month I discussed Dr. Chen’s first stripe rust prediction of the season (published January 3) and although it was good news because there wasn’t much rust around, I also mentioned that we would know more in March since his rust survival models are based on temperatures from December through February. Consequently, the very mild temperatures during January and February have not only allowed the rust to survive, but to begin growing again in areas with very mild temperatures.

Eyespot is more difficult to predict; we rely on past history, seeding date, and varieties planted. Fields managed as fallow, seeded early relative to their production area with susceptible varieties, and a history of eyespot are likely to have the disease again this year. Fall 2017 was generally favorable for eyespot with good crop establishment and rainfall during October and November; December was mostly not favorable due to low temperatures. The mild temperatures so far in 2018 have allowed eyespot to begin spreading in infected plants earlier than usual.
The best control for both stripe rust and eyespot is to plant a disease-resistant variety. In the absence of a resistant variety, scouting to look for disease is the next option; when disease is severe enough, application of a fungicide may be justified. I’ve discussed scouting for stripe rust in previous Timely Topics; follow those guidelines if you have a less than highly resistant variety. For spring crop, consider planting the most stripe rust-resistant variety that performs well in your area. Eyespot scouting is different and requires that you pull plants from different areas of the field at spray time, which must be prior to the beginning of stem elongation. You can find photos of eyespot and a discussion of how to scout in the Disease Resources section.

Stay tuned for more rust updates as conditions change. In the meantime, you can find additional information on stripe rust, including photos showing rust percentage under the Foliar Fungal Diseases in the Disease Resources section of the WSU Wheat and Small Grains website.  For information on weather conditions near you, visit the WSU AgWeatherNet webpage and look for a weather station near you.

* Growing degree days are calculated by taking the average daily temperature and subtracting the base temperature (32°F for wheat) and adding the values for each day of the time period of interest.


Washington State University