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Extension Crop Connection: The first step in the program is timely scouting, starting early in the growing season.

Stephen Wegulo

January 30, 2019

5 Min Read
Tan spot (left) and Septoria leaf blotch (right) are the two most common fungal leaf spot diseases in wheat in Nebraska.
COMMON PROBLEM: Tan spot (left) and Septoria leaf blotch (right) are the two most common fungal leaf spot diseases in wheat in Nebraska.Stephen Wegulo.

With the wheat-growing season approaching, it is time to lay out a plan for managing diseases. The first step in a successful disease management program is timely scouting, starting early in the growing season and continuing until the wheat crop is fully headed.

Scouting is critical because it enables early detection and identification of diseases. This information is used to implement precise, targeted and timely disease management measures.

Scouting frequency depends on weather conditions. If disease-favorable weather is forecast, scouting should be done more frequently, at least once every seven to 10 days. A representative area of the field should be scouted using one of several patterns to walk through the field, for example X, W or Z.

The diseases to look for early in the growing season include the following:

· Tan spot and Septoria leaf blotch are the two most common fungal leaf spot diseases of wheat in Nebraska. They are most severe in fields with wheat residue on the soil surface. Symptoms first appear in April and initially appear as small, tan to brown lens or diamond-shaped spots that progress to elliptical to elongate tan-colored spots with a dark-brown center and a yellow border. As the lesions increase in size they coalesce, producing large, irregular areas of dead tissue. The lower, more mature leaves are infected first, and the disease progresses to the upper leaves. In Septoria leaf blotch, tiny, black fruiting structures may be seen in older lesions, and on upper leaves the lesions tend to be straight-sided without a yellow border. Management of these diseases includes rotation with broad leaf crops, planting resistant varieties, foliar fungicide application and fungicide seed treatments.

stripe rust

INCREASING THREAT: Over the past several years, stripe rust has become a more common threat in Nebraska. It's identifiable by yellow to orange pustules with rust spores forming stripes on mature, upper leaves.

· Stripe rust has become increasingly common in Nebraska. It appears starting in April because it is favored by cool temperatures. Yellow to orange pustules consisting of rust spores form distinct stripes on mature, upper leaves. Entire leaf surfaces can be covered with stripes. Distinct stripes do not form on seedling leaves. Stripe rust can be managed by planting resistant varieties and timely application of foliar fungicides.

· Powdery mildew usually occurs starting in late April or early May. It is more common and severe in irrigated fields and in the south central and eastern parts of the state where moisture and humidity favor its development. It is characterized by white, cottony patches of mycelium on the surface of the plant. These patches can occur on all aerial parts of the plant, including stems and heads, but are most conspicuous on the upper surfaces of lower leaves. Management of powdery mildew includes planting resistant varieties, avoiding excessive fertilization and fungicide application timed to protect the flag leaf.

· Wheat soilborne mosaic is a virus disease of wheat that occurs more commonly in the eastern part of the state because its vector, a soilborne funguslike organism, is favored by abundant soil moisture. Symptoms appear in April as mild green to conspicuous yellow leaf mosaics and stunting. Diseased plants can be uniform in a field, but more often occur in low-lying wet areas. Wheat soilborne mosaic is managed by planting resistant varieties. There are no chemical treatments.

· Wheat streak mosaic is the most important and widespread virus disease of wheat in Nebraska. It is transmitted by wheat curl mites. It tends to be more common in the Panhandle, but it can occur anywhere in the state. Symptoms first appear in April as light green streaks that elongate to form discontinuous yellow to pale green stripes, forming a mosaic pattern running parallel to the leaf veins. The most severe symptoms usually appear at the edge of the field closest to the wheat curl mite source. Leaf yellowing intensifies as temperatures become warmer in the spring.  Wheat streak mosaic can be managed by controlling volunteer wheat before planting in the fall, planting resistant varieties and avoiding early planting. There are no chemical treatments.

Economic considerations
Stripe rust, leaf rust, tan spot, Septoria leaf blotch and powdery mildew are the primary foliar fungal diseases of wheat in Nebraska. Fusarium head blight is the major fungal disease affecting wheat heads. These diseases can be controlled by timely application of fungicides.

However, to be profitable, fungicide application must be cost-effective. Fungicide application will be cost-effective if environmental conditions favor development of damaging levels of disease, if it is done preventively, the yield potential is high, the variety planted is susceptible and the price of wheat is not too low.

Table 1. Potential net profit from foliar fungicide treatment of wheat based on the current average selling price of wheat ($4.56/bu) at the elevator in Nebraska
Table 1 illustrates the potential net profit from foliar fungicide treatment of wheat based on the current average selling price of wheat at the elevator in Nebraska.

The highest cost-effectiveness can be achieved with one application timed to protect the flag leaf. Depending on environmental conditions, two or three applications may be warranted if early-season diseases are severe or if the risk for Fusarium head blight is high. With the current price of wheat, more than one application may not be cost-effective.

Wegulo is a Nebraska Extension plant pathologist.

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