May 23, 2013

Rust Diseases

There have been no new reports of rust diseases in Nebraska since the observation of stripe rust at Mead on May 7. Warm temperatures during the week of May 13 considerably reduced the risk for stripe rust development. However, while the overall risk for stripe rust development is low, with recent cool temperatures the risk will be elevated where there is moisture. Even with high daytime temperatures (higher than 65°F), the risk for stripe rust development is significant if night temperatures fall below 65°F. Growers should continue to scout wheat fields for stripe rust and other diseases.

If stripe rust is detected and rainfall is forecast, be prepared to apply a fungicide to protect the flag leaf. Last week stripe rust was observed in south central Kansas to a level that warranted a fungicide application in a few fields. Very little leaf rust has been reported in Oklahoma and Kansas; therefore, the risk for damaging levels of leaf rust in Nebraska remains low. No leaf rust or stem rust has been observed in the state.

Most Common Diseases in Wheat

Figure 2: Powdery mildew in the lower canopy in a field in Lancaster County on May 22.
Figure 3: A wheat leaf showing virus symptoms in a wheat field in Saline County on May 21.	Figure 4: Bleaching of a wheat head caused by Fusarium head blight (scab).

Wheat fields surveyed on May 21 and May 22 in Lancaster, Saline, and Saunders counties looked healthy when observed from a distance (Figure 1). Closer examination revealed low levels of diseases, mostly Septoria tritici blotch and tan spot in the lower canopy. Powdery mildew (Figure 2) was present in the lower canopy in fields with thick stands. One field surveyed on May 21 showed a low incidence of plants with virus symptoms (mosaic of yellow and green on leaves; Figure 3) randomly scattered in the field.

Fusarium Head Blight

Wheat is at the boot or heading stages in south central and southeast Nebraska. The Fusarium Head Blight Prediction Center is currently showing a low risk for Fusarium head blight (also known as scab) in the state. The disease is recognized by the premature bleaching of wheat heads (Figure 4). During favorable environmental conditions (warm, wet weather prior to and during flowering), symptoms can appear suddenly. If heavy rainfall occurs prior to and during flowering, the risk for scab development will increase in those areas that receive rainfall. The risk for scab development can be monitored at the FHB prediction center website.

A fungicide application based on a predicted moderate to high risk and timed at early flowering can significantly reduce damage caused by scab. Once symptoms appear, it is too late to apply a management measure.

Stephen Wegulo
Extension Plant Pathologist