Stripe rust has hit Nebraska wheat fields once again. In surveys of wheat fields on April 8 and April 12-14, stripe rust (Figure 1) and leaf rust (Figure 2) were found at trace to low levels in south central and southeast Nebraska. Meanwhile, samples from several wheat fields in Banner County in the Nebraska Panhandle had both stripe rust and leaf rust. This is the same county in which both diseases were active last fall, indicating the possibility that the two rusts overwintered.

RUST IN NEBRASKA: (From left) Figure 1. A hot spot of stripe rust in a research plot at the ARDC near Mead on April 14. Figure 2. Leaf rust in a research plot at the ARDC near Mead on April 14. Figure 3. A healthy wheat field in Thayer County on April 13. (Photo credit: Stephen Wegulo)

The surveys on April 8 and April 12-13 were in the southernmost tier of counties from Gage County in the southeast to Red Willow County in the west central part of the state. Low levels of disease, mainly fungal leaf spot diseases, were seen on the lower leaves in some fields. Other fields had very little or no disease and looked healthy (Figure 3).

On April 14, surveys were conducted in research fields at Havelock Farm in Lincoln and at the Agricultural Research and Development Center (ARDC) near Mead. The predominant disease at these two locations was powdery mildew that was active mainly in the lower canopy. A few hot spots of stripe rust (Figure 1) were found in one field at the ARDC. This same field also had trace levels of leaf rust (Figure 2).

Figure 4. Yellowing of the lower leaves caused by powdery mildew in a research plot at the ARDC on April 14

Symptoms to watch for
A symptom that was common in many fields is yellowing of the lower leaves (Figure 4). Several factors are involved in yellowing of the lower leaves early in the growing season. They include environmental conditions such as low temperatures and lack of adequate sunlight (lower leaves are shaded by the upper, actively growing leaves), natural senescence coming out of the winter, and inadequate nitrogen. If there are no symptoms or signs of disease on the yellow leaves, environmental conditions are the most likely cause.

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Yellowing of lower leaves can also be caused by diseases including powdery mildew (Figure 4) and fungal leaf spot diseases (Septoria tritici blotch or tan spot, Figure 5). If diseases are the cause of yellowing, symptoms or signs of disease will be visible on the leaves. These include dark brown or black spots, necrotic (dead) areas on the leaves or, in the case of powdery mildew, a white powdery fungal growth on the stems and leaves.

Figure 5. Yellowing of the lower leaves caused by Septoria tritici blotch in a research plot at the ARDC on April 14.

Getting a handle on rust
Given the presence of stripe rust and leaf rust in the state, it is recommended that wheat fields be scouted regularly for early disease detection. Dry conditions during the last two weeks have slowed or stopped stripe rust development. However, rain is forecast in the next several days for most of the state and especially the western half. Stripe rust, leaf rust, and fungal leaf spot diseases will become active and continue to spread following rain events.

If you see stripe rust in your field and favorable weather conditions (moisture, wind, and cool to moderate temperatures) are forecast, consider applying a fungicide to protect the wheat crop. The recommended timing is at 50% to 100% flag leaf emergence. However, if the risk of stripe rust development and spread is high, an earlier application at the jointing growth stage may be warranted. Consider the yield potential, resistance level of the wheat variety planted, and the price of wheat when making the decision to spray.

Keep in mind that stripe rust can form new races that can overcome the resistance in varieties rated as resistant, and resistance can be overwhelmed if disease pressure is high. Therefore, even if you have planted a resistant variety, consider a fungicide application as a second line of defense. Fungicides that are effective in controlling rust diseases are also effective in controlling fungal leaf spot diseases.

For more information, contact Stephen Wegulo, University of Nebraska Extension plant pathologist, at [email protected].

Source: UNL CropWatch