March 29, 2012

Figure 1. Leaf rust

Figure 2. Stripe rust

On March 28, stripe rust was observed in a grower’s field near Wichita in south central Kansas, and on March 29, leaf rust (Figure 1) and stripe rust (Figure 2) were observed in research plots in Manhattan in northeast Kansas. These follow reports of these rust diseases in states further south. The detection of leaf and stripe rusts in Kansas indicates that we are likely to see these diseases in Nebraska in the next two to three weeks.

This year the wheat crop in Nebraska is approximately two weeks ahead of normal crop development due to unseasonably warm temperatures in the winter and spring. Above normal temperatures are forecast for the coming weeks. If rainfall occurs, the combination of warm temperatures and moisture can lead to rapid development of foliar wheat diseases including powdery mildew and fungal leaf spots. These have already been observed in southeast Nebraska. There also have been reports of general yellowing of wheat fields. This condition is common at the current growth stage and can be caused by several factors, including inadequate fertilization and virus diseases. If the yellowing is of a general nature with no obvious virus symptoms (stunting and leaf mosaics, mottling, or streaking), it is most likely due to inadequate fertilization. The wheat crop usually grows out of this condition and greens up as the growing season progresses.

Scout Wheat Now to Detect Diseases Early

It is recommended that wheat fields be scouted routinely for early detection of diseases. This will facilitate timely management decisions. Except in fields where disease pressure is high early in the growing season, fungicide application to control foliar fungal diseases should be aimed at protecting the flag leaf. It may be necessary to apply a second spray at early flowering to suppress Fusarium head blight (scab) in scab-prone areas.

The most common virus diseases of wheat in Nebraska are wheat soilborne mosaic, wheat streak mosaic, barley yellow dwarf, and most recently, Triticum mosaic. Virus diseases are characterized by yellowing and/or a mottling or streaking of green and yellow. They are difficult to distinguish and often can be mistaken for nutrient deficiency. Symptoms of wheat soilborne mosaic are prominent early in the season and are more severe in wet, low-lying areas in the field. As the season progresses and day temperatures rise above 68^oF, development of wheat soilborne mosaic slows down or ceases and wheat streak mosaic symptoms become more prominent. Because of the above-normal temperatures that have prevailed during the winter and spring, symptoms of wheat streak mosaic may show up earlier than normal. Barley yellow dwarf is characterized by yellowing from the leaf tip down and from the leaf edges to the mid rib.

Virus Diseases are Best Managed Before They Occur

There are no control measures for virus diseases. Do not apply a fungicide to control virus diseases.

Wheat soilborne mosaic is managed by planting resistant cultivars. Wheat streak mosaic and Triticum mosaic are managed by planting resistant/tolerant cultivars, avoiding early planting, and controlling volunteer wheat, especially the volunteer that emerges just before harvesting. All volunteer should be completely dead at least two weeks before planting. Barley yellow dwarf is managed by planting resistant/tolerant cultivars, avoiding early planting, and controlling volunteer cereals. Controlling aphids (the vectors of barley yellow dwarf) can reduce infections. However, viruliferous aphids can still fly in from outside the field and transmit the virus to the wheat crop.

Stephen Wegulo
Extension Plant Pathologist