Dr. Tessa Albrecht, Research Scientist and Diagnostician
Department of Bioagricultural Sciences and Pest Management
Colorado State University
Most of the wheat is heading now but still behind schedule in the northeastern part of the state. The rain keeps coming in eastern Colorado and with it comes more fungal disease.
Stripe rust has made an appearance in Colorado. We have had two reports thus far; one north of Stratton and the other southeast of Lamar, only low levels of the disease have been reported. Stripe rust is caused by the fungal pathogen Puccinia striiformis and is spread by wind over long distances. It can also overwinter on volunteer wheat and weedy grasses but there is no evidence that it overwintered here last winter. Symptoms include small oval shaped yellow-orange pustules that form in stripes along the leaf veins. When these pustules erupt spores are released and the disease spreads. On resistant varieties the infected areas often turn brown and die, limiting the number of spores released. Temperatures between 50-64°F with substantial dew periods are optimal growth conditions for this pathogen. The primary management practice for stripe rust is planting of resistant varieties (see CSU Variety Characteristics Table). Timely foliar fungicide application to susceptible varieties can be an effective control measure (see KSU Foliar Fungicide Efficacy Ratings). There are several considerations when making fungicide decisions such as the susceptibility level of the variety that is infected. Typically limited benefits are observed by treating resistant varieties. The severity of infection and timing of application are important factors as well. With stripe rust infections, minimal yield losses are observed when the flag leaf is protected so applying fungicide when the flag leaf is fully emerged is critical. The severity of disease will be affected by upcoming weather conditions for example hot dry weather would likely slow disease progression significantly making fungicide application unnecessary. Additional management strategies for stripe rust include control of volunteer wheat and weedy grasses to prevent overwintering, late planting and nutrient management.
Leaf rust has been observed in Colorado, also southeast of Lamar. Characteristic symptoms of leaf rust are small round orange pustules that are typically scattered on the leaf as opposed to the stripe pattern formed by stripe rust pustules. Leaf rust is typically less damaging than stripe rust. Dispersal of the disease and management strategies are the same as that of stripe rust.
We have had a few more confirmed reports of Cephalosprium stripe caused by the fungal pathogen Cephalosporium gramineum. Initial infections occur in the fall and winter in roots of germinating wheat. Disease symptoms are long yellow stripes along the veins of the leaves progressing downward from the leaf tips. As with several other foliar fungal pathogens the spores survive in crop residue. Cultural practices should be used to control Cephalosporium stripe such as late planting, control of wheat residue and rotation. There is no approved chemical control for this pathogen.
Tan spot has been observed in Colorado, which is caused by the fungal pathogen Pyrenophora tritici-repentis. Tan spot survives in crop residue and thrives under continuous damp conditions. Leaf symptoms are small tan oval-shaped lesions often surrounded by a yellow halo. Centers of lesions darken with maturity. As the disease expands lesions coalesce causing large regions of diseased tissue. Spores are dispersed by wind affecting larger areas as the fungus is spread to new host tissue. Management of tan spot includes foliar fungicide applications, genetic resistance, rotation with broad-leaf plants and control of crop residue. Foliar fungicide application considerations are as mentioned above.
Wheat curl mite-transmitted viruses are widespread across eastern Colorado. We have had samples test positive for Wheat streak mosaic virus (WSMV) from Adams, Arapahoe, Baca, Bent, Larimer, Kiowa, Kit Carson, Phillips, Prowers, Sedgwick and Yuma Counties. Some of these samples also have Triticum mosic virus (TriMV) and/or Wheat mosaic virus (WMoV). Characteristic symptoms of WSMV are small elongated chlorotic lesions on leaves. Symptoms are more severe when TriMV occurs together with WSMV. All three of these viruses are transmitted by the microscopic, wind dispersed, wheat curl mite. There is evidence that the wheat curl mite overwintered in several parts of Colorado suggesting many initial infections may have occurred in the fall of 2018. The most effective management strategy for WCMs is control of volunteer wheat. Planting of genetically resistant varieties (see CSU Variety Characteristics Table) and late planting are also good control methods.
Many thanks to Scott Haley, Ron Meyer, CSU Extension agents and the CSU Plant Diagnostic Clinic that have provided information and samples that contribute to these disease updates.
Sources of More Information
CSU Variety Characteristics Table:http://wheat.agsci.colostate.edu/wp-content/uploads/sites/85/2018/09/Characteristics-Table-8-18.pdf
KSU Foliar Fungicide Efficacy Ratings: https://wheat.agsci.colostate.edu/wp-content/uploads/sites/85/2019/04/EP130.pdf
Guide to Wheat Disease Symptoms: https://webdoc.agsci.colostate.edu/wheat/linksfiles/MF2994.pdf
If you think you have disease in your wheat, samples can be diagnosed at the CSU Plant
Diagnostic Clinic: https://plantclinic.agsci.colostate.edu/