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Wheat Disease Update: Conditions Elevate Risk of Wheat Scab

June 4, 2013
wheat
Figure 1. Stripe rust. Figure 2. Severe powdery mildew in the lower canopy of wheat at the UNL Havelock Research Farm on May 30.  
 
 

By Stephen Wegulo, Extension Plant Pathologist, University of Nebraska-Lincoln

Wheat in eastern Nebraska is in the heading to flowering growth stages. Excessively wet weather in the eastern part of the state over the last week has favored development of wheat diseases, especially in fields with thick stands. Stripe rust (Figure 1) was observed in breeder nursery plots earlier this week at the UNL Agricultural Research and Development Center near Mead and on May 30 at the UNL Havelock Research Farm in Lincoln. Incidence and severity of the stripe rust were low, but conditions (wetness and night temperatures in the low 50s to lower 60s) are favorable for rapid development and spread of the disease.

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Figure 3. Numerous fungal leaf spots on a wheat leaf in the mid canopy at Havelock Research Farm on May 30.


An inspection of breeder nursery plots at the UNL Havelock Research Farm May 30 showed severe powdery mildew (Figure 2) in the lower canopy of wheat plots. Leaf spot diseases (Figure 3) were starting to develop in the mid to upper canopy. Head diseases also were observed, including Fusarium head blight in its initial stages of development on wheat (Figure 4) and in more advanced stages on barley (Figures 5 and 6). Overall incidence of Fusarium head blight was trace. However, it is likely to increase given the excessively wet weather that has occurred before and during heading and flowering. Loose smut (Figure 7) was observed at a low incidence. Virus diseases, mainly barley yellow dwarf, also were observed at low levels.

Risk for Fusarium Head Blight

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Figure 4. A wheat head with a single spikelet bleached by Fusarium head blight at Havelock Research Farm on May 30.

Figure 5. A barley head with a few spikelets bleached by Fusarium head blight at Havelock Research Farm on May 30.


The coinciding of recent heavy rains with heading and flowering has elevated the risk for Fusarium head blight in eastern Nebraska. This risk can be monitored at the Fusarium Head Blight Prediction Center. A moderate to high risk warrants application of a suitable fungicide at full heading to early flowering to suppress Fusarium head blight. Wheat fungicides and their efficacy ratings for various diseases are listed in the fungicide efficacy table developed and updated annually by the North Central Regional Committee on Management of Small Grain Diseases (NCERA-184).
Management

Damage caused by rust diseases, powdery mildew, and fungal leaf spots can be minimized by applying a fungicide to protect the flag leaf. Fusarium head blight can be suppressed by applying a suitable fungicide at full heading to early flowering. If wheat is fully headed or in the early flowering growth stage, applying a fungicide that is effective on Fusarium head blight is recommended as it will both suppress Fusarium head blight and effectively control rust diseases, powdery mildew, and fungal leaf spots. Loose smut can be controlled by treating seed with a fungicide before planting; however, once it occurs, it is too late to control it. Nothing can be done to control virus diseases once they occur. Controlling volunteer wheat and grassy weeds before planting in the fall and choosing resistant cultivars can help reduce damage caused by virus diseases.

Pre-harvest Interval for Fungicides

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Figure 6. A barley head completely bleached by Fusarium head blight at Havelock Research Farm on May 30.

Figure 7. Loose smut on wheat at Havelock Research Farm on May 30.


Before and when applying a fungicide, observe label instructions and restrictions. The NCERA-184 fungicide efficacy table has information on pre-harvest restrictions for wheat fungicides. Some fungicides can be applied up to full heading but before flowering (Feekes growth stage 10.5). Others have varying pre-harvest intervals specified in days. Fungicides for suppressing Fusarium head blight have a pre-harvest interval of 30 days. No fungicide should be applied within 30 days of harvest.

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RELATED TOPICS: Wheat, Crops, Fungicide

 
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