UNL CropWatch May 6, 2011 Wheat Disease Update
|Figure 1. Tan spot on an upper leaf in a state variety trial at North Platte, Lincoln County.||Figure 2. Wheat drilled into wheat stubble in an irrigated field in Perkins County exhibits tan spot.|
May 6, 2011
A survey of wheat fields on May 5 in southwest Nebraska (Chase, Furnas, Keith, Lincoln, Perkins, and Red Willow counties) revealed little or no disease in most dryland fields. Wheat in this region is mostly at the Feekes 6 growth stage (first node detectable).
The most common fungal leaf spot disease observed was tan spot (Figure 1). It was at trace levels (less than 5% incidence and less than 1% severity) in all dryland fields surveyed. However, in an irrigated field in Perkins County where wheat was drilled into wheat stubble (Figure 2), tan spot incidence was 100% and severity on the lower leaves ranged from 10% to 30% or more. Most of the lower leaves in this field have turned yellow due to tan spot, giving the entire field a yellow cast (Figure 3).
Irrigation and wheat stubble on the soil surface are ideal conditions for rapid development of tan spot. In this situation, an early fungicide application may be warranted to reduce damage and, ultimately, yield loss. Otherwise it is recommended that fungicide application be timed to protect the flag leaf when the risk of occurrence of damaging levels of disease exists. For more information on managing tan spot, see NebGuide G429, Tan Spot Disease of Wheat.
In a different irrigated field, also in Perkins County, there was severe stunting and yellowing (Figure 4) caused by wheat soilborne mosaic virus. This virus is transmitted by Polymyxa graminis, a root-infecting fungus that is favored by wet soils. Hence, the disease tends to be most noticeable in low-lying areas of a wheat field. Wheat soilborne mosaic is managed by planting resistant cultivars. Do not apply a fungicide to control a virus disease.
In a few fields, there was evidence of foliar burn most likely caused by fertilizer. Symptoms were uniformly distributed and consisted of a burn or dessication of leaves starting at the tips (Figure 5). The injury was minimal and is not expected to affect the wheat crop in the affected fields.
In the Nebraska Panhandle, Sidney received 1.71 inches of precipitation (mix of rain, snow, and sleet) between April 21 and 27, but it has been dry since then. There are some symptoms of tan spot on lower leaves; however, no major disease problem has been observed. Samples submitted to the diagnostic clinic have shown root and crown rot/winterkill.
There have been no reports of rust diseases in Nebraska to date.
Reports from Kansas indicate that low levels of leaf rust have been found in the central and south central parts of the state. This week a small "hot spot" of stripe rust was found in Labette County in southeast Kansas. In general, dry conditions in states south of Nebraska have considerably slowed rust development.
Fusarium Head Blight
Fusarium head blight (FHB) alerts are now available. They can be sent to your email or cell phone. I will send FHB alerts in Nebraska only when the FHB risk tool indicates a high risk. You can sign up for the FHB alerts at: http://scabusa.org/fhb_alert.php.
Extension Plant Pathologist, Lincoln.
Extension Plant Pathologist, Scottsbluff
Dryland Cropping Systems Specialist, Scottsbluff