Stripe rust and leaf rust were rapidly increasing in incidence and severity in parts of central Kansas this week. Stripe rust and leaf rust became established in the upper canopy of wheat in south central Kansas a few weeks ago. Observations this week indicate that stripe rust has increased in severity in many fields of susceptible varieties that were unprotected by fungicides. In some cases, more than 30 percent of the flag leaf area has been damaged by the disease. Leaf rust has also moved to the upper leaves on susceptible varieties in the south central region.
Stripe rust and leaf rust were also reported in additional areas of the state this week with many new reports of the disease in middle canopy in west central and northwest regions of the state (Figure 1 and 2). There are a few reports of stripe rust moving to the upper leaves in these areas also, but for the moment this seems to be rare. Many producers in western Kansas are still trying to assess potential damage from freezing temperatures and heavy snowfall a few weeks ago. This damage creates tremendous uncertainty about the yield potential of the crop and complicates the decision to apply fungicides for rust management.
These decisions will need to be made on a field-by-field basis. It may be helpful to prioritize seed production fields and fields of susceptible varieties that appear to have escaped the worst of the freeze/snow damage. Other fields may also benefit from a fungicide application, but the outcome of this investment is uncertain because we just cannot predict how the damaged fields will recover.
In central Kansas, growers should be watching the harvest restrictions on fungicide applications. The flowering stages of growth indicate that the timing window for fungicide applications is coming to an end for many products. Some fungicides have growth stage restrictions and others have a fixed number of days that must be waited after application before the crop can be harvested. The wheat foliar fungicide efficacy publication provides a summary of these restrictions. https://www.bookstore.ksre.ksu.edu/pubs/EP130.pdf. Be sure you understand these limitations before applying fungicides at these critical growth stages.
Figure 1. Stripe rust distribution in Kansas as of May 12
Figure 2. Leaf rust distribution in Kansas as of May 12
Erick DeWolf, Extension Wheat Pathologist