Time to keep an eye out for diseases in wheat

As wheat comes to a head, it is important to protect wheat from diseases such as fusarium head blight (also known as scab), and stripe rust.

This year has been mostly dry as the wheat crop developed. However, rains have returned to some areas as heads have emerged. During flowering (anthesis), warm, humid weather encourages the spread of scab, so it’s good to keep an eye on risk as heading begins.

Wheat scab

Fusarium head blight (scab) infects wheat during flowering, beginning at Feekes 10.5.1. During periods of continuous moisture and temperatures ranging from 77°F to 86°F, head blight symptoms can appear within three days after infection. A crop that appeared normal several days earlier can suddenly show widespread symptoms.

According to Carl Bradley, University of Kentucky field crops pathologist, this fungus survives through the winter in infected corn residue. High humidity and frequent rainfall promote the production and dispersal of spores from residue. The wind can blow spores onto wheat plants. Warm, humid weather promotes infection and secondary spread.

Symptoms of scab will appear as bleached spikelets on the head later in the season. Infection can lead to small or shriveled grain kernels referred to as “tombstones.” In addition to shriveled grain this fungus produces mycotoxins such as deoxynivalenol (DON), which can accumulate in the infected grain. Infected grain can be docked or rejected at the elevator. 

There is a tight window for effective fungicide application, so preparation to spray or having custom application lined up is necessary for when first flowers appear. “The window for management starts at anthesis,” explains Bradley. “This is when the heads are out and you can just start to see some anthers coming out of the middle part of the head.” 

The Fusarium Head Blight Prediction Center, housed at Penn State University, produces daily risk maps that indicate low, medium, or high-risk areas for scab development in the Midwest. This tool estimates the risk of a fusarium head blight epidemic (> than 10% field severity) using weather conditions (temperature, rainfall, and relative humidity) measured 15 days prior to flowering. See below for the current risk map — much of the Midwest is low risk.

Bradley recommends that to achieve the best management of wheat scab, different management practices must be implemented, such as planting wheat into fields that were previously cropped to soybean (rather than corn), planting wheat varieties with moderate to high levels of resistance to scab, and applying foliar fungicides at the proper timing. Of these different management practices, the application of foliar fungicides is the only one that can be done during the growing season.

“A fungicide application might be considered if a susceptible variety is planted, or if you are worried about scab on your farm,” said Bradley. “These applications should be made at early flowering to suppress FHB.”

Stripe rust

Even though it looks like we've got low risk for scab, the thing that throws a wrench is  stripe rust,” warns Bradley. “Stripe rust got an early start this year in southern states and has recently made its way into Tennessee and Illinois.”

Beginning symptoms of stripe rust appear as yellow streaks on leaves.  Orange pustules will then appear in a “striped” pattern where the yellow streaks first occurred.

Yield loss is related to how early stripe rust appears.  In many years, stripe rust may not occur until after heading.  In those cases, a fungicide specifically for stripe rust control will likely not be needed.  If stripe rust appears prior to heading on a susceptible variety, then yield loss may occur. 

Fungicides that are registered for control of scab also have efficacy against stripe rust,” says Bradley. “So an application of an effective fungicide for protection against scab will also provide protection against stripe rust.”