It is early June and warm weather is moving wheat in the southern part of the state into the soft dough stage, while wheat in the north is moving past flowering. By now, fungicide decisions should have already been made for the season. Fusarium head blight (head scab) symptoms have started to show up in southeast and central Kansas. Most reports and fields we have visited have had very low levels of disease (less than 1 in 100 heads affected).
Although infection of the wheat head by the fungus Fusarium graminearum occurs during flowering, symptoms only begin show up in the late milk and early dough stages. Cool, wet weather during flowering favors disease development. A fungicide application to control head scab must be made during flowering, before symptoms are present. When symptoms are visible, a fungicide will no longer be effective for control. Also, fungicide label pre-harvest intervals would restrict fungicide applications at this point in the season.
Symptoms of head scab include one or more spikelets that have become tan or bleached in color, while the stem typically remains green (Figure 1). In some cases, pinkish-orange structures of the fungus are also visible on infected spikelets. Symptoms can be quickly masked by maturity and may be easily missed.
Figure 1. Typical symptoms of Fusarium head blight (head scab). Photo taken on June 2, 2020 in Marion County, Kansas. Photo courtesy of Kelsey Andersen Onofre, Kansas State Research and Extension.
Diseased kernels will have a white, chalky appearance (Figure 2). Kernels may also have a pinkish discoloration. The diseased grain may also contain mycotoxins that may be harmful to humans and livestock. Deoxynivalenol (DON) is the most common mycotoxin associated with Fusarium damaged wheat. This mycotoxin is also called “vomitoxin”.
Figure 2. Grain from heads with Fusarium head blight will be lightweight, chalky, and may have a pink coloration. Photo courtesy of Erick DeWolf, Kansas State Research and Extension.
Controlling Fusarium Head Blight
Other late-season disease considerations
Stripe rust severity (how bad the disease is in any given field) this year has been lower overall than in previous years. Part of this lower severity could be attributed to lower levels of stripe rust in Texas and Oklahoma early in the season and a later arrival of the pathogen to Kansas this year. Additionally, weather may not have been as favorable for disease development in parts of the state.
Leaf spotting diseases (tan spot, Stagonospora nodorum blotch, Septoria tritici blotch) may be showing up more frequently as plants approach maturity on lower and upper leaves (Figure 3). These fungal diseases live through the winter on wheat stubble, so may be worse in fields with a wheat on wheat crop rotation. Reactions to these diseases will differ between varieties. Fungicides are an effective control option.
Figure 3. Classic symptoms of leaf spot diseases such as tan spot and Stagonospora nodorum blotch. Definitive diagnostics typically require laboratory diagnostics to distinguish between pathogens in this group of diseases. Photo courtesy of Kelsey Andersen Onofre, Kansas State Research and Extension.
Root diseases (take-all, common root rot, Fusarium crown rot, sharp eye spot) may be more pronounced this time of year, presenting as white plants scattered in fields that are otherwise green. When plants are pulled up from the roots, there may be discoloration of the lower stem and poor root development (Figure 4).
Figure 4. Bleached wheat plants in an otherwise healthy wheat plot in Saline County, Kansas. Plants appear to have talk-all and sharp eyespot. Photos courtesy of Kelsey Andersen Onofre, Kansas State Research and Extension.
Kelsey Andersen Onofre, Extension Plant Pathologist
Erick De Wolf, Extension Plant Pathologist