Stripe rust was found at low incidence and low disease severity at the ARDEC Research Station in Larimer county, mainly on triticale (Figure 1). The cool, wet weather has been favorable for disease development as free-standing water persists on leaves (rainfall, dew). Stripe rust is spreading across Texas, Oklahoma, and Kansas, and was found at low incidence and severity in Nebraska last week.
Be sure to carefully and frequently scout your fields for symptoms (Figure 2), and look throughout the entire canopy since symptoms may appear in the lower canopy first. Pay attention to your local weather, and please feel free to email photos if you think you see stripe rust.
What should you do if you see Stripe Rust in your fields?
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. Applying pesticides more often than recommended can select for stripe rust pathogen races that are fungicide resistant. This means that in future years fungicides may fail, so proper fungicide timing and application is important to help protect the efficacy for future disease management.
When deciding to apply a fungicide, be sure to consider the yield potential and price of wheat. The best time to apply a fungicide is when the flag leaf has emerged since the biggest yield determinant is the infection of the flag leaf. Stripe rust symptoms appear 7-10 days after infection, so if you observed stripe rust symptoms 1-2 weeks before flag leaf emergence you may consider a fungicide application to protect the flag leaf. Earlier fungicide application is only recommended in severe cases. Be sure to consult the North Central Regional Committee on Management of Small Grain Diseases (NCERA-184) Fungicide Efficacy table (URL link in Additional stripe rust resources section below) before applying fungicides.
Weather impacts and outlook for stripe rust
Much of Colorado has received significant precipitation over the past two weeks (Figure 3), and some areas have received more precipitation than average for 2021 (Figure 4). Temperature highs and lows have varied widely across the state, with some areas experiencing warmer-than-average high temperatures and other areas having cooler-than-average low temperatures.
With moisture comes increased risk of fungal diseases, including stripe rust which requires cool, wet weather to germinate its spores and infect plants. The current high soil moisture levels, frequent rain, and cool night temperatures have kept plant leaf surfaces wet for longer periods of time, which favors stripe rust disease development. There will likely be more reports of stripe rust in Colorado in the next week due to the forecasted disease-conducive weather. Frequent scouting is important because symptoms can develop rapidly when the weather conditions are favorable.
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- 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
Tan spot was observed in several counties. Tan spot appears as necrotic (dead, brown) diamond-shaped spots surrounded by yellow halos or borders.
What should you do if you see Tan Spot in your fields? Typically, as the weather warms tan spot does not continue to be a problem in Colorado, so fungicide applications are not usually recommended.
However, some sites reported more tan spot than usual due to the cool, wet weather. 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) and High plains wheat mosaic virus (HPWMoV) have been detected in samples from Weld county, and more sites across the state are reporting virus symptoms. WSMV and HPWMoV are transmitted by the wheat curl mite. Symptoms appear as yellow streaks and mosaic, yellow and green patterns on leaves.
What should you do if you see WSMV or HPWMoV 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.
Barley yellow dwarf virus (BYDV) symptoms were observed in Weld county. BYDV symptoms include plant stunting and yellowing of the leaves which may have purple or red tips (Figure 5). Unlike the other common wheat viruses, BYDV does not display any mosaic or streaking patterns. BYDV is transmitted by aphids.
What should you do if you see BYDV in your fields? Document the incidence of BYDV and scout for aphids in the coming weeks. If you observe a high incidence of aphids in your fields that is above the economic threshold for insecticide application, you may consider applying an insecticide. Do not apply prophylactic sprays because they can select for insect populations that are resistant to insecticides, causing insecticides to fail.
Cephalosporium stripe (caused by Cephalosporium gramineum) disease was observed in Washington County. Cephalosporium stripe is a fungal wilt disease with symptoms appearing as one to three distinct yellow stripes near leaf veins that develop on older leaves first (Figure 6). Brown streaks eventually appear within the yellow stripes as the disease progresses. The fungus survives in wheat residue and requires cool, wet weather. High residue, limited or short crop rotations, and very early planting favor disease development.
What should you do if you see Cephalosporium stripe in your fields? This disease requires cool, wet weather conditions so it is not usually problematic if the weather warms. Document the incidence and take note of whether there is a lot of residue build-up in your field.
Continue scouting for stripe rust and look throughout the entire canopy since symptoms may appear in the lower canopy first. We anticipate that stripe rust will be found in more Colorado fields this week. Please feel free to email photos if you think you see stripe rust.
Be on the lookout for virus symptoms. Wheat streak mosaic (WSMV), Triticum mosaic (TriMV), and High plains wheat mosaic (HPWMoV) are the most common in Colorado, and as insect pressures across the state increase, we may see more virus symptoms. There are no pesticides that are effective against mites, so controlling volunteer wheat, which harbor mites, is the best management practice. 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 tan spot, stripe rust, and viruses 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/category/news-events/wheat-pest- and-disease-update/
Many thanks to Dr. Wilma Trujillo, Ron Meyer, Todd Ballard, Emily Hudson-Arns, Dr. Esten Mason, and Dr. Ana Cristina Fulladolsa Palma for contributing to this report.