Wheat Disease Update for Nebraska - May 11, 2012, UNL CropWatch

Wheat Disease Update for Nebraska - May 11, 2012, UNL CropWatch

Wheat at grain filling stage

Figure 1. A wheat field in the grain development stage in southeast Nebraska on May 10. (Photos by Stephen Wegulo)

Fusarium head blight Stem maggot injury  
Figure 2. A wheat head affected by Fusarium head blight in a field in southeast Nebraska on May 10. Figure 3. Stem maggot injury  
Barley yellow dwarf  Barley yellow dwarf  

Figure 4. Barley yellow dwarf symptoms:
Left, yellowing; right, purpling.

May 11, 2012

Fungal Diseases

Stripe Rust. Dry weather during the last seven days slowed down the development of stripe rust. The risk for yield loss due to stripe rust has been reduced in southeast and south central Nebraska for wheat that is further along in development. A May 10 survey of wheat fields in Saline, Jefferson, and Gage Counties showed that wheat in most fields was past the flowering stage and in the grain development stages (milk to dough stages, Figure 1). Wheat in western Nebraska is still at risk for yield loss due to stripe rust should conditions become favorable for disease development (cool temperatures and rain). Scouting of fields in this part of the state should continue and producers should be prepared to apply a fungicide if the potential exists for stripe rust to develop to damaging levels.

Leaf Rust. Leaf rust was first confirmed on May 1 in Lancaster County. Since then there have been reports of the disease in south central and eastern Nebraska. Currently, levels of leaf rust are very low. However, incidence and severity can increase if favorable conditions (rain and warm weather occur).

Leaf Spots and Powdery Mildew.  The most common leaf spot diseases observed in wheat fields are Septoria leaf blotch and tan spot. Powdery mildew is more prevalent in fields with thick stands and in irrigated fields. A fungicide application for control rust diseases will also control fungal leaf spots and powdery mildew.

Head Diseases. Trace levels of Fusarium head blight or scab (less than 1% incidence, Figure 2) were found in Saline County and Jefferson County on May 10. The risk of Fusarium head blight rises with continuous moisture for seven days or longer. Because we have not had continuous moisture in the scab-prone areas of the state (south central and eastern), the risk of damaging levels of scab remains low. Other head diseases/disorders observed are loose smut and stem maggot injury (Figure 3). For information on distinguishing between head diseases of wheat, see Extension Circular EC1872, Distinguishing Between Head Disorders of Wheat.

Pre-harvest Intervals
for Fungicide Application

Before applying a fungicide to wheat, be sure to read the label and observe pre-harvest interval restrictions. Pre-harvest intervals for wheat fungicides are 30 or 35 days depending on the fungicide. Some fungicides cannot be applied after the full heading stage (Feekes 10.5). See the fungicide table for pre-harvest restrictions for individual fungicides.

Wheat streak mosaic at 100% incidence
Figure 5. A wheat field with 100% incidence and high severity of wheat streak mosaic virus in the southern Panhandle on May 4.

Virus Diseases

Barley yellow dwarf virus continues to be widespread especially in south central and eastern Nebraska. Some fields are more severely affected than others. Yellowing and/or purpling of top leaves starting from the tip and margins are typical symptoms of barley yellow dwarf (Figure 4). Nothing can be done to control virus diseases during the current growing season.

Wheat streak mosaic virus has been observed throughout the state at varying levels of incidence and severity. A field with 100% incidence and high severity of wheat streak mosaic virus was observed in the southern Panhandle on May 4 (Figure 5). See the related story in the April 27 issue of CropWatch for strategies for managing virus diseases.

Will Wheat Diseases Impact Another Crop Planted After Wheat This Year?

There have been inquiries regarding the impact of wheat diseases on a subsequent crop such as soybean or corn planted in ground that had wheat during the current growing season. Most wheat diseases do not affect corn or soybean. The exception is Fusarium head blight (scab). The fungus that causes scab also causes stalk rots and to some extent root rot in corn. Therefore, there is some elevated risk for these diseases in corn if the corn is planted into no-till wheat ground that had scab. This growing season, however, the risk of scab is low. Therefore, those planning to plant soybean or corn into wheat ground this season should not worry about wheat diseases.

Stephen Wegulo
Extension Plant Pathologist