Scout Wheat Fields Regularly for Early Detection of Disease

Scout Wheat Fields Regularly for Early Detection of Disease

Photo of tan spot on the lower leaves of a young wheat crop. Pseudothecia (sexual fruiting structures) of the tan spot fungus on wheat straw.

Figure 1. Tan spot on the lower leaves of a young wheat crop. (Source: Photos by Stephen Wegulo)

All photos link to larger versions.

Figure 2. Pseudothecia (sexual fruiting structures) of the tan spot fungus on wheat straw.
Photo of powdery mildew on the lower leaves of a young wheat crop. Photo of septoria tritici blotch on a wheat leaf. Note the tiny, black pycnidia (asexual fruiting structures) which form in older lesions.
Figure 3. Powdery mildew on the lower leaves of a young wheat crop. Figure 4. Septoria tritici blotch on a wheat leaf. Note the tiny, black pycnidia (asexual fruiting structures) which form in older lesions.

April 17, 2009

To make timely disease management decisions, it is essential to scout wheat fields regularly to detect diseases early. A weekly scouting schedule starting in mid April is suggested.

In Nebraska, early season (April to May) fungal diseases include tan spot, powdery mildew, and Septoria tritici blotch. Tan spot (Figure 1) occurs throughout the state and the risk of its occurrence is highest in fields with pseudothecia-bearing wheat straw (Figure 2) on the soil surface. Pseudothecia are the sexual fruiting structures of the tan spot fungus.

Powdery mildew (Figure 3) and Septoria tritici blotch (Figure 4) occur mostly in south central and eastern Nebraska where moisture favors their development. Early in the growing season, these diseases are most noticeable on lower leaves which may appear yellow. When scouting, be sure to look for disease symptoms in the lower canopy of the wheat crop.

Stripe rust on a wheat leaf. Leaf rust on the flag leaf of a wheat plant.
Figure 5. Stripe rust on a wheat leaf. Figure 6. Leaf rust on the flag leaf of a wheat plant.
  Stem rust. Note the large, brick-red pustules and the flakes of ruptured stem tissue around them. These pustules also occur on leaves.
  Figure 7. Stem rust.  Note the large, brick-red pustules and the flakes of ruptured stem tissue around them.  These pustules also occur on leaves.

The rusts (leaf, stem, and stripe rust) usually appear starting in May. Stripe rust (Figure 5) occurs sporadically and usually is the first one to appear because it is favored by cool temperatures. Leaf rust (Figure 6) is favored by moderate to warm temperatures and occurs every year starting around mid May. Stem rust (Figure 7) also occurs sporadically. It is favored by warm temperatures and therefore occurs in June on susceptible cultivars or breeding lines. Monitoring the northward movement (and severity) of rusts from southern states can help us get prepared to manage them.

Fungal leaf spots and rusts are effectively controlled by applying a fungicide. The maximum benefit is obtained by timing fungicide applications to protect the flag leaf. An early season fungicide application may be warranted if disease pressure is high and environmental and local field conditions (e.g. wheat drilled into wheat straw) favor disease development.

Soilborne wheat mosaic

Figure 8. Soilborne wheat mosaic.

Severe wheat streak mosaic.
Figure 9. Severe wheat streak mosaic.
Figure 10. Barley yellow dwarf.
Figure 10. Barley yellow dwarf.
The most common virus diseases of wheat in Nebraska are soilborne wheat mosaic (Figure 8), wheat streak mosaic (Figure 9), and barley yellow dwarf (Figure 10). These 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 soilborne wheat 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°F, development of wheat soilborne mosaic slows down or ceases and wheat streak mosaic symptoms become more prominent. Barley yellow dwarf is characterized by yellowing from the leaf tip down and from the leaf edges to the mid rib.

Virus diseases, once they occur, cannot be controlled. Do not apply a fungicide to control virus diseases. Soilborne wheat mosaic is managed by planting resistant cultivars. Wheat streak mosaic is 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.

Fusarium head blight.

Figure 11. Fusarium head blight.

Field symptoms of Fusarium head blight.
Figure 12. Field symptoms of Fusarium head blight.
Wheat affected by root and crown rot diseases. Note the stunting, yellowing, and lack of vigorous growth.
Figure 13. Wheat affected by root and crown rot diseases.  Note the stunting, yellowing, and lack of vigorous growth.

Fusarium head blight (scab) occurs in early June. It is characterized by premature bleaching of heads (Figure 11) during or shortly after flowering. Partial bleaching of the wheat head is diagnostic of Fusarium head blight. Bleached heads appear suddenly and are randomly scattered throughout entire fields (Figure 12) or large areas in the field. If excessively wet weather occurs one to two weeks before flowering, a fungicide should be applied at early flowering to prevent or reduce infection of wheat heads by the Fusarium head blight fungus. Once bleaching of heads occurs, it is too late to apply a fungicide.

Root and crown rot diseases are easily overlooked. Stunting, reduced vigor, and/or yellowing (Figure 13) can be due to root and crown rots. To determine whether these diseases are present, dig up a few plants and examine the roots and crowns. A brown to black discoloration of the crown, subcrown internodes, or the entire root system is indicative of root and crown rot diseases. These diseases are best controlled by crop rotation and planting certified, fungicide-treated seed into firm, well drained soil. Once they occur during the growing season, it is too late to control them.

Stephen Wegulo
Extension Plant Pathologist