Stripe Rust of Wheat Reappears in Northern Utah
Stripe rust is increasing in its range of occurrence within the lower 48 states. The disease was reported in Utah’s Box Elder, Cache, and Weber counties in the 2005 and 2006 growing seasons. Observations of stripe rust, in both years, ranged in severity from mild damage to complete crop loss.
The disease occurs during cooler wet weather and can move rapidly from a small affected area to an entire field within a matter of days. Rate of spread depends on cultivar susceptibility, wind, temperature, and moisture. Initial infections are difficult to spot. Constantly scouting fields during early spring when moisture is prevalent will increase the odds of detecting the disease before it gets out of hand. Infections can occur anytime during the plants’ development, and spores can be blown in from adjacent or distant fields with rain showers. Hence, sudden occurrences of the disease are often seen, when only a week before fields appeared to be unaffected.
A single spore, called a urediniospore, of the pathogen is microscopic but en masse appear yellow to slightly orange. If a person were to walk through a severely infected field, their clothing and shoes would appear yellow, covered with inoculum.
Lesions appear on wheat leaves primarily but can occur on the stem and on head tissues as well. Damage to grain is considered severe when the disease is seen in head tissues, as other parts of the plant are surely infected as well. Symptoms appear as elongated lesions on leaves, appearing yellow to slightly orange in color. These lesions consist of numerous smaller lesions called uredia. Each uredium, a single spore-producing lesion, consists of thousands of urediniospores that can be spread by wind to inoculate healthy wheat plants in adjacent or distant fields.
Primary infections in fields can be spotted as an odd oval to circular area that is off color. Upon closer inspection, a scout will observe the lesions on leaves and their severity will depend upon the cultivar’s degree of susceptibility as well as temperature, moisture conditions, and how long the infection has been there.
The fungus reduces a plant’s vigor by utilizing the host’s water and nutrients for its own purposes: growth and sporulation. The fungus survives from one season to the next on volunteer wheat plants. No known grass, other than wheat, can serve as a host to the pathogen.
Depending upon the timing of infection, crop developmental stage, prevailing weather outlook, and cultivar susceptibility, a grower may choose to use a fungicide to prevent further damage to their grain crop. Fungicides containing the active ingredient propiconazole are recommended to control stripe rust. Fungicides with this active ingredient have a curative eradicative action. A slight to mild yellowing of the sprayed wheat can occur but green-up usually follows in a few days. Care should be taken to follow labeled instructions regarding the stage of crop development and when the fungicides can no longer be applied. Generally, these fungicides cannot be applied at, during, or after head emergence.
Previous years’ disease severity, changing weather patterns, wheat cultivar, prevalent races of the pathogen, and control of volunteer wheat all influence the likelihood of the disease occurring in any given year. Seeing that stripe rust has occurred in the last two years, growers and agents should keep a sharp eye on wheat fields in 2007.
-Kent Evans, Extension Plant Pathologist