Factors for treating wheat rust

Robert Harveson

Wheat in Nebraska is affected by three primary rust diseases — stem, leaf and stripe rust. All share several common 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 do not regularly overwinter in this area although it has been documented on rare occasion. Thus the status of these rust diseases and their potential for causing problems in Nebraska can be estimated by observing and documenting the presence of the pathogens from Texas, Oklahoma or Kansas.

The three common rust diseases are caused by three distinct pathogens, and thus also differ in several fundamental ways. Symptoms for each are somewhat different and with practice are relatively easy to differentiate.

One of the most important is the differences in temperature required for optimal growth and infection. Knowledge of these differences may help make better decisions on managing the diseases when they appear.

Leaf rust — produces circular to oval orange-colored pustules and is favored by warm temperatures of 60-70 F.

Stripe rust — produces yellow linear pustules that run parallel with leaf veins and is favored by cooler temperatures of 50-60 F. Based on this trait, stripe rust usually appears earlier in the season than leaf rust.

Stem rust — produces reddish-brown oblong pustules with frayed margins on leaves and stems and is favored by warmer temperatures varying between 75 and 85 F.

When is treatment needed?

This question is very difficult to answer quickly and definitively. There are many factors to consider before deciding to apply some type of fungicide, and there is no one easy explanation for every situation.

Scouting wheat fields and learning to recognize these diseases is the first important step.

Wheat growers should scout their fields for any evidence of disease. If any type of rust is detected, the decision to apply a fungicide should be based on how widespread it is in the field, where on the plant it has progressed, and the 10-14 day forecasted weather conditions. Remember that for stripe rust to infect and cause disease, the required temperatures for infection are cooler that with leaf rust, ranging from 50-60 F. If dry, warm weather is predicted, it is unlikely that stripe rust will develop to damaging levels in dry-land fields, and therefore there will be little or no benefit from applying a fungicide to control the disease. Irrigated fields are at a higher risk for rust development because of the presence of more moisture. If conditions of 70-80 F with some form of moisture are expected, then the development of leaf rust would be enhanced. Therefore the ability to recognize and distinguish these two rust diseases is important for making the decision to use fungicides.

Stem rust is rare in Nebraska because the majority of 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 to 85 F).

Treating rust

If rust is detected on plants near the flag leaf in an irrigated field, the grower should consider applying a fungicide. Research has shown that no yield reductions occur in the absence of flag leaf infections. However, a fungicide should be applied only if disease is detected and environmental conditions favoring disease development are expected, and it is better not to wait until disease is observed on the flag leaf.

Anytime rust pustules are seen on plants, the original infection for those resulting signs and symptoms actually occurred up to 14 days earlier. Thus estimating the potential for disease development based on 10-14 day forecasts is an important consideration. This also illustrates the importance of only relying on local environmental conditions for making management decisions. Just because disease has been reported somewhere in Nebraska, it does not mean that disease will also be a factor for every grower or location in western Nebraska.

Concluding remarks

In summary, the decision to apply fungicides for managing wheat rust diseases is not an easy one to make, and requires the consideration of numerous factors for optimal results. Yield losses are dependent upon when infection occurs, and where on the plant it is noted. The flag leaf is the critical part of the plant that needs protecting. The presence of the disease lower on the plant is important if the projected environmental conditions are expected to occur with the next several weeks. Other critical factors to consider are the crop growth stage, the level of genetic resistance that may be incorporated into the cultivar being grown, and the reported presence of rust from the Great Plains wheat-growing states south of us.

Fortunately, when a fungicide application is employed, there are no apparent differences among the various labeled fungicides and their ability to control rust. Past research has indicated that all rust pathogens are equally sensitive to all tested fungicides. However, a good practice to remember is to rotate fungicides of differing modes of action to avoid or delay the development of fungicide resistant populations of the rust pathogens.

Before applying any fungicide, always read and follow labeled instructions and restrictions. This is particularly true with late season applications, as some fungicide may be applied legally at later growth stage than others. Yields of entire fields have been rejected at the elevators when fungicides had been applied in a manner out of compliance with the label.

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