Summary: Viruses were detected in Colorado. Watch for stripe rust, viruses, and other fungal leaf diseases.
Dr. Robin Roberts, Field Crops Pathologist and CSU Assistant Professor
Robyn.Roberts@colostate.edu, 970-491-8239
Disease Observations
Mosaic virus complex: Plants exhibiting symptoms consistent with wheat mosaic virus complex disease were collected from Kit Carson County (Figure 1). The wheat mosaic virus complex is caused by one or more viruses, including triticum mosaic virus (TriMV), wheat streak mosaic virus (WSMV), and/or high plains wheat mosaic virus (HPWMoV). It is transmitted by the wheat curl mite, and symptoms appear as yellow streaks and mosaic patterns on leaves.
Management and prevention: 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 mite- and virus-resistant varieties are the best control measures. Volunteer wheat should be eliminated between harvest and planting, both in fields and surrounding areas, and be dead at least 14 days before fall planting.
Disease Watch and Management
Stripe Rust
I expect that stripe rust risk will remain low this year, but now is the time to scout for stripe rust. There are currently no reports of stripe rust in Colorado or Kansas, and, while the weather has overall been conducive for stripe rust disease development in Colorado, the spores do not overwinter here and instead blow up in wind currents from southern states (Texas and Oklahoma) every spring. However, the fungus requires both cool and wet weather, and the dry winter and spring in Texas has suppressed stripe rust sporulation. Stripe rust was initially reported in Texas in March, which is quite late in the season, and levels have remained low. Very low levels of stripe rust were reported in Oklahoma. The low inoculum levels limit the number of spores that can survive and migrate to Colorado to cause disease, reducing our stripe rust risk this season.
Viruses
We found that many of our samples in 2023 and 2024 had very high levels of TriMV, which is a closely related but different virus from WSMV (Figure 3). The Wsm2 resistance that works against WSMV does not work against TriMV.
Tan Spot and Stagonospora leaf blotch
Tan spot and Stagonospora leaf blotch diseases are caused by fungi that infect leaves and can begin to appear this time of year. However, both diseases are dependent on wet conditions, high humidity, and moderate temperatures (~68-75°F), and the fungi survive in infected residue. Typically, these diseases do not cause significant yield losses in Colorado and are often found together on leaves (Figure 4).
Management and prevention: Because the fungus survives in wheat residue, rotating crops will help reduce the number of spores in a field the following years. Fungicide seed treatments also help protect seedlings from infection. Foliar fungicides are generally not recommended in Colorado for treatment of Tan Spot or Stagonospora leaf blotch, except in very rare high-disease cases.
Additional resources
- Colorado Wheat Entomology Newsletter: https://coloradowheat.org/category/news-events/wheat-pest-and-disease-update/
- Fusarium risk tool: https://www.wheatscab.psu.edu/?ct=YTo1OntzOjY6InNvdXJjZSI7YToyOntpOjA7czo1OiJlbWFpbCI7aToxO2k6NjM7fXM6NToiZW1haWwiO2k6NjM7czo0OiJzdGF0IjtzOjIyOiI2NjA1OWYxMzk5MmJjMzY4ODcyMDIyIjtzOjQ6ImxlYWQiO3M6MjoiMjAiO3M6NzoiY2hhbm5lbCI7YToxOntzOjU6ImVtYWlsIjtpOjYzO319
- Information about the ‘green bridge’ and risks for viral diseases due to volunteer wheat: https://eupdate.agronomy.ksu.edu/article_new/spring-emerged-volunteer-wheat-should-producers-worry-about-wheat-streak-mosaic-virus-and-the-green-bridge-436-4
- Fungicide Efficacy for Control of Wheat Diseases Table: https://cropprotectionnetwork.org/publications/fungicide-efficacy-for-control-of-wheat-diseases
Contributors: Many thanks to Sally Jones-Diamond and Dr. Esten Mason, who contributed to this report.