May 16, 2013

Target Treatment to the Type Present

Wheat in Nebraska is affected by three primary rust diseases: stem, leaf, and stripe rust. All share some fundamental characteristics, including being favored by wet, humid conditions. They also normally arrive in Nebraska from the south on wind currents that move up through the Great Plains in the spring. They are not thought to regularly overwinter in this area although it has occasionally been documented.  Disease reports from Texas, Oklahoma, and Kansas can be used to estimate when or if rust diseases will likely appear in Nebraska.

These diseases are caused by three distinct pathogens and differ in several important ways. With practice, symptoms for each are relatively easy to differentiate.

One of the most important differences is the optimal temperature for development of the particular rust. Knowing these differences can help you make better decisions on managing these diseases when they appear.

Stripe rust produces yellow linear pustules that run parallel with leaf veins (Figure 1) and is favored by temperatures of 50-60°F. Based on this trait, stripe rust usually appears before leaf rust.

Leaf rust produces circular to oval orange-colored pustules (Figure 2) and is favored by temperatures of 60-70°F.

Stem rust produces reddish-brown oblong pustules with frayed margins on leaves and stems (Figure 3) and is favored by temperatures of 75-85°F.

Need for Treatment

Prolonged cool conditions this spring have delayed wheat development as well as the onset of wheat diseases. (For more information see the Wheat Disease Update in this week's CropWatch.)

Stripe Rust

Stripe rust has been detected in eastern Nebraska, but at low levels. It has not been observed in western Nebraska to our knowledge. Its late arrival, should it come, will likely result in little damage due to the warmer temperatures that normally predominate in late May and June.

Wheat growers should scout their fields for stripe rust. If stripe rust is detected, the decision to apply a fungicide should be based on

• how widespread stripe rust is in the field,
• where on the plant it has progressed, and
• the 10-14 day forecasted weather conditions (temperatures of 50-60°F are required).

If dry weather with temperatures of 60-70°F is predicted, stripe rust is unlikely to develop to damaging levels in dryland fields. There would be little or no benefit from applying a fungicide to control the disease. Irrigated fields are at a higher risk for stripe rust development due to the presence of more moisture. If stripe rust is detected in an irrigated field, consider applying a fungicide to protect the flag leaf. Research has shown that no yield reductions occur in the absence of flag leaf infections. A fungicide should be applied only if disease is detected and environmental conditions favor disease development.

Leaf Rust

Leaf rust had not been detected in Nebraska as of May 15 and only trace levels had been detected in Kansas. Current forecasts indicate that leaf rust, like stripe rust, is unlikely to develop to damaging levels in Nebraska. However, wheat fields should still be monitored for this disease. As with stripe rust, irrigated fields are at a higher risk than dryland fields for development of leaf rust. If leaf rust is detected and local environmental conditions favor its development (60-70°F), consider applying a fungicide to protect the flag leaf.

Stem Rust

Stem rust is rare in Nebraska because most wheat varieties grown in the state have good resistance to the disease. It is usually detected in mid to late June. Of the three rust diseases of wheat, stem rust is favored by the warmest temperatures (75-85°F). Stem rust has not been detected in Nebraska, Kansas, or Oklahoma, and is not expected to be a problem this season. However, if it is detected, a fungicide should be considered based on the criteria noted above.

Label Restrictions

Before applying any fungicide, read and follow label instructions and restrictions. This is particularly true with late season applications, as some fungicides may be applied legally at later growth stages and some may not. Yields of entire fields have been rejected at the elevators when fungicides had been applied in a manner out of compliance with the label.

Robert Harveson
UNL Extension Plant Pathologist, Panhandle REC
Stephen Wegulo
UNL Extension Plant Pathologist, Lincoln