Dr. Robyn Roberts
Field Crops Pathologist and Assistant Professor
970-491-8239; @RobynRobertsPhD (Twitter)
*Email is the best way to reach me
I hope to see many of you at the Colorado Wheat Field Days June 10-15! Please email me if you’d like to bring samples for testing/diagnostics. I will have a cooler to bring some samples back to my lab and/or the Plant Diagnostic Clinic if formal diagnoses are needed.
Stripe rust was found at low incidence and low disease severity in several northeastern counties (Figure 1). The past two weeks of cool, wet weather has been favorable for disease development. The extended hot weather forecasted over the next week should help control disease development since stripe rust spores germinate poorly above about 77°F. Be sure to carefully and frequently scout your fields for symptoms, and look throughout the entire canopy since symptoms may appear in the lower canopy first where it is cooler and wet.
Should you apply fungicide? The decision to spray a fungicide should depend on the level of disease, the susceptibility of the wheat variety, the growth stage of wheat, and the weather conditions. It is important to apply fungicides only if recommended and following the manufacturer’s label. We do not recommend fungicide applications unless you see symptoms and if the flag leaf is at risk of infection. Over-applying fungicides can select for stripe rust spores that resist fungicides, causing fungicides to fail in the future. Since new stripe rust races can emerge over time that may overcome disease resistance, fungicides are an important tool for disease control that we want to protect for the future in case disease resistance becomes ineffective. Be sure to also consider the yield potential and price of wheat when deciding whether to make a fungicide application.
Cephalosporium stripe disease (caused by the fungus Cephalosporium gramineum) was observed in Morgan county (Figure 2). One field was highly affected with symptoms on flag leaves in a large portion of the field. Cephalosporium stripe is a fungal wilt disease that usually develops when there is high residue on the soil and limited or short crop rotations have been implemented. The fungus survives in wheat residue over the winter and infects crops in the spring when the weather is cool and wet. This disease has not been particularly problematic in Colorado in the past, but because we have experienced unusually extended cool, wet weather we are seeing more incidences of Cephalosporium stripe with greater severity than in previous years.
What should you do if you see Cephalosporium stripe in your fields? Unfortunately, there are no fungicides available to control the disease so applying fungicides will not help. Document the incidence and take note of whether there is a lot of residue build-up in your field, and if the crop was recently rotated away from wheat.
Tan spot was observed in several counties, and in some cases on the flag leaves. Tan spot appears as necrotic (dead, brown) diamond-shaped spots surrounded by yellow halos or borders. The extended cool, wet weather has been favorable for tan spot disease. Higher incidence of tan spot is typically observed in fields with high levels of wheat residue on the soil surface which harbors the fungus.
What should you do if you see Tan Spot in your fields? Fungicides are recommended for tan spot only if the flag leaf is at risk of infection. When scouting for tan spot, take note of the growth stage of wheat, the severity of the infection, and whether the flag leaf is at risk of disease, and monitor disease development closely.
Wheat streak mosaic virus (WSMV), Triticum mosaic virus (TriMV), and High plains wheat mosaic virus (HPWMoV) have been detected in samples from several counties. Symptoms appear as yellow streaks and mosaic, yellow and green patterns on leaves.
What should you do if you see virus symptoms in your fields? There is no treatment for virus-infected plants, and no miticides are effective against the vector (the wheat curl mite). Controlling volunteer wheat and planting WSMV-resistant varieties are the best control measures. If you think you see virus symptoms in a WSMV resistant variety, please send me photos.
Weather impacts and outlook for fungal diseases
The fungal diseases currently affecting Colorado (stripe rust, tan spot, and Cephalosporium stripe) depend on both wet conditions and cool temperatures. Much of Colorado has received significant precipitation over the past two weeks (Figure 3); however, weather forecasts predict hot temperatures this week which should help limit the development of these fungal diseases. Frequent scouting is still very important because symptoms can develop rapidly when the weather conditions are favorable. The lower canopy will also stay cooler than the upper canopy, so diseases may hide in the lower canopy.
Continue scouting for stripe rust and look throughout the entire canopy since symptoms may appear in the lower canopy first where it is cooler and wetter. Please feel free to email photos if you think you see stripe rust.
Other fungal diseases
The extended cool, wet weather has been favorable for other fungal diseases that are typically not problematic in Colorado (e.g., tan spot and Cephalosporium stripe). Watch for these and other fungal disease symptoms, especially in the lower canopy, and watch for flag leaf infection risk.
Be on the lookout for virus symptoms. If you believe you see virus symptoms on resistant varieties, please send me pictures.
Growers are strongly encouraged to frequently scout wheat fields for diseases. Particularly, scout for fungal and virus diseases in the coming weeks, as well as the virus vectors (mites and aphids). Be sure to look throughout the entire canopy for diseases.
The Colorado Wheat Entomology Newsletter, written by Dr. Punya Nachappa and Darren Cockrell, covers insect/mite pests and management tips. The newsletters are published bi-weekly during the growing season and are available here: https://coloradowheat.org/2021/06/wheat-entomology-newsletter-june-1-2021/
Do you have a disease that you would like diagnosed? Contact the Plant Diagnostic Clinic for sample submission: https://plantclinic.agsci.colostate.edu/ or firstname.lastname@example.org.
Additional stripe rust resources
Below are additional resources to learn more about stripe rust, yield loss risks, and fungicide applications.
- The North Central Regional Committee on Management of Small Grain Diseases (NCERA-184) Fungicide Efficacy for Control of Wheat Diseases Table: https://crop-protection-network.s3.amazonaws.com/publications/fungicide-efficacy-for-control-of-wheat-diseases-filename-2021-04-21-154024.pdf
- Wheat variety database with stripe rust resistance ratings from field trials: https://wheat.agsci.colostate.edu/database/
- Additional photos of stripe rust symptom development (Kansas State Extension): https://www.sunflower.k-state.edu/agronomy/wheat/stiperust.html
- Stripe rust fact sheet and yield loss risks based on crop growth stage (Kansas State Extension): https://bookstore.ksre.ksu.edu/pubs/EP167.pdf
- Chart of wheat growth and development (Kansas State Extension): https://bookstore.ksre.ksu.edu/pubs/MF3300.pdf
Many thanks to Dr. Wilma Trujillo, Ron Meyer, Todd Ballard, Dr. Esten Mason, and Dr. Ana Cristina Fulladolsa Palma for contributing to this report.