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Dry weather slows disease development in wheatDry weather slows disease development in wheat

As rains return, scouting will be warranted.

Stephen Wegulo

May 22, 2018

3 Min Read
DISEASE-FREE: This wheat field in Nuckolls County on May 9 is free of disease, partly due to dry weather. Diseases could develop in fields that receive moisture.Stephen Wegulo

A mid-May survey of wheat fields in southeast and south-central Nebraska did not reveal any diseases. Wheat growth stage ranged from Feekes 6 (first node detectable) in fields that are behind in wheat development to Feekes 10 (boot) in fields along Highway 136 in extreme southern Nebraska. Most fields were at two or three nodes detectable (Feekes 7).

In general, wheat development is behind where it should be for this time of year due to prolonged cold winter conditions that prevailed into April.

The counties surveyed were Saline, Jefferson, Thayer, Nuckolls, Webster and Saunders. All fields were lush green and, except for irrigated fields, dry. A few fields had large yellow areas, with patterns suggesting sulfur deficiency, which is easily confused with nitrogen deficiency.

Sulfur deficiency shows up
Sulfur deficiency is more common in soils that lack or have little organic matter, such as sandy soils or areas in fields such as slopes that are prone to soil erosion, leading to little accumulation of organic matter. Therefore, symptoms of yellowing limited to certain areas in a wheat field are more typical of sulfur than nitrogen deficiency. In addition, symptoms of sulfur deficiency are more conspicuous when soil temperatures are cold as in early spring because there is less mineralization of organic matter (which releases plant available sulfur) in cold than in warm soils.

SULFUR DEFICIENCY: Yellow areas in a wheat field in Nuckolls County are found May 9. These yellow patterns suggest sulfur deficiency.

On individual plants, sulfur deficiency causes general yellowing and stunting of the entire plant, with yellowing more apparent on the younger leaves, whereas with nitrogen deficiency yellowing is more apparent on older leaves. On older plants deficient in nitrogen, the lower leaves yellow and die, whereas they remain pale green on plants deficient in sulfur. This is because nitrogen is redistributed to the younger leaves due to its mobility in the plant, resulting in severe yellowing on the lower leaves.

On the other hand, sulfur is not as mobile, and therefore symptoms of yellowing are milder on the lower leaves and more severe on the upper leaves. See the resources listed below that include information on management of sulfur deficiency.

The dry weather has delayed development of diseases; however, diseases will start developing in fields that receive moisture in the form of rainfall or irrigation. Therefore, scouting should continue for early disease detection.

Reports from southern states (Texas, Oklahoma and Kansas) indicate the intensity and prevalence of stripe rust are low. To date, the farthest north stripe rust has been reported in the central Great Plains is three counties in southeast Kansas.

STRIPE RUST: Reports from Southern states are low of stripe rust on winter wheat. Because the wheat crop in Nebraska is behind in development, there is still potential for stripe rust to develop to damaging levels.

Because the wheat crop in Nebraska is behind in development, there is still potential for stripe rust to develop to damaging levels, especially with rainfall. And because temperatures are getting warmer, there is potential for leaf rust to develop.

Fungicide not warranted
At this time, the absence of disease (or presence of very low levels of it) indicates a fungicide application is not warranted. The best timing for a fungicide application to control foliar fungal diseases is at 50% to 100% flag leaf emergence. The decision to apply a fungicide should be based on:

 presence of disease

• favorable environmental conditions for disease development (wet weather)

• price of wheat

Foliar fungal diseases are most effectively controlled by applying a fungicide preventatively — after disease detection and before the disease starts to develop to damaging levels. Fungicide application to control foliar fungal diseases should be aimed at protecting the flag leaf.

A 2018 fungicide table developed by the multi-state committee, Management of Small Grain Diseases, NCERA 184, shows the efficacy of different fungicides and modes of action in controlling specific foliar fungal diseases of wheat.

Wegulo is a Nebraska Extension plant pathologist. This report comes from UNL CropWatch.

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