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Scout wheat fields now for early disease detection

Nebraska

To make timely disease management decisions in wheat, it is crucial to scout wheat fields regularly to detect diseases early, a University of Nebraska-Lincoln plant pathologist said.

In Nebraska now through May, early season fungal diseases include tan spot, powdery mildew and Septoria tritici blotch, said Stephen Wegulo, UNL plant pathologist in the university's Institute of Agriculture and Natural Resources.

Tan spot occurs throughout the state and the risk of its occurrence is highest in fields with pseudothecia-bearing wheat straw on the soil surface, Wegulo said. Pseudothecia are the sexual fruiting structures of the tan spot fungus.

Powdery mildew and Septoria tritici blotch 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," he said.

Rusts usually appear starting in May, Wegulo said.

"Stripe rust occurs sporadically and usually is the first one to appear because it is favored by cool temperatures," he said. "Leaf rust is favored by moderate to warm temperatures and occurs every year starting round mid-May. Stem rust also occurs sporadically."

Stem rust is more common in 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," he said.

Fungal leaf spots and rusts can effectively be controlled with fungicides.

The maximum benefit is obtained by timing fungicide applications to protect the flag leaf. An early season application may be warranted if disease pressure is high and environmental and local field conditions favor disease development.

The most common virus wheat diseases in Nebraska are soilborne wheat mosaic, wheat streak mosaic and barley yellow dwarf.

"These diseases are characterized by yellowing and/or a mottling or streaking of green and yellow," Wegulo said. "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 degrees, 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," Wegulo said. "Do not apply a fungicide to control virus diseases."

Soilborne wheat mosaic can be managed by planting resistant cultivars.

Wheat streak mosaic is managed by planting resistant/tolerant cultivars, avoiding early planting and controlling volunteer wheat.

Barley yellow dwarf is managed by planting resistant/tolerant cultivars, avoiding early planting and controlling volunteer cereal grains.

Fusarium head blight occurs in early June. It is characterized by premature bleaching of heads during or shortly after flowering.

"If excessive 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," Wegulo said. "Once bleaching of heads occurs, it is too late to apply a fungicide."

Root and crown rot diseases are easily overlooked, Wegulo said.

Stunting, reduced vigor and/or yellowing can be due to root and corn rots.

"To determine whether these diseases are present, dig up a few plants and examine the roots and crowns," he said. "A brown to black discoloration of the crown, subcrown internodes or the entire root system is indicative of root and crown rot diseases."

These are controlled best 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," he said.

More information, including close-up photos of the wheat diseases, can be found at CropWatch, UNL Extension's crop production newsletter, at http://cropwatch.unl.edu/. More information on managing and treating foliar diseases of winter wheat can be found on UNL's Surviving High Input Costs website http://cropwatch.unl.edu/survivinghighinputcosts.htm.



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