Figure 2. Septoria tritici blotch	Figure 3. Powdery mildew

Figure 1. Tan spot

April 22, 2011

Recent heavy rains may favor development of fungal diseases of wheat. To make timely disease management decisions, it is essential that wheat fields be scouted regularly for early disease detection. A weekly scouting schedule starting in mid to late April is recommended.

Fungal Diseases

In Nebraska, common fungal diseases of wheat include tan spot (Figure 1), Septoria tritici blotch (Figure 2), powdery mildew (Figure 3), and leaf rust (Figure 4). Stripe rust (Figure 5) occurs sporadically. Stem rust (Figure 6) occurs sporadically on susceptible cultivars or breeding lines. Most commercial wheat cultivars grown in Nebraska are resistant to stem rust. Fusarium head blight (scab, Figure 7) also occurs sporadically; however, it has occurred every year since 2007.

Figure 4. Leaf rust

Figure 5. Stripe rust

Tan Spot occurs throughout the state and the risk of its occurrence is highest in fields with wheat straw bearing fruiting structures on the soil surface. Powdery mildew and Septoria tritici blotch occur most commonly in the south central and eastern parts of the state, but can occur anywhere where moisture favors their development. Early in the growing season, these diseases are most noticeable on the lower leaves which may appear yellow. Therefore, when scouting, be sure to look for disease symptoms in the lower canopy of the wheat crop.

Wheat rusts (stripe, leaf, and stem rust) usually start appearing in April/early May (stripe rust), mid May (leaf rust), and June (stem rust). Stripe rust is favored by cool temperatures, leaf rust by moderate to warm temperatures, and stem rust by warm temperatures on susceptible cultivars or breeding lines. Monitoring the northward movement (and severity) of rusts from southern states can help us prepare to manage them.

This year, drought-like conditions in the southern states have slowed the development of stripe rust. Therefore, it is likely that stripe rust may not develop to damaging levels in Nebraska. If it develops, it is not expected to be as severe and widespread as it was during the 2010 growing season. Leaf rust occurs yearly and is favored by wet conditions.

Foliar Diseases

Fungal foliar diseases of wheat 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 favor disease development.

Fusarium head blight starts occurring in late May to early June. It is characterized by premature bleaching of heads (Figure 7) during or shortly after flowering. Bleached heads may appear suddenly and are randomly scattered throughout entire fields or large areas in the field. If excessively wet weather occurs one to three weeks before flowering, a fungicide should be applied at early flowering to 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 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 planting certified, fungicide-treated seed into firm, well drained soil. Once they occur during the growing season, it is too late to control them.

Virus Diseases

Wheat soilborne mosaic, wheat streak mosaic, and barley yellow dwarf are the most common virus diseases of wheat in Nebraska. Triticum mosaic virus, a virus first discovered in 2006, was first detected in Nebraska wheat fields in 2009. Like wheat streak mosaic virus, Triticum mosaic virus is transmitted by the wheat curl mite.

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 68oF, development of wheat soilborne mosaic slows 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. 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.

Stephen Wegulo
Extension Plant Pathologist, Lincoln