Issue No. 4, Article 7/April 29, 2010
Wheat Disease Update
As some of the wheat fields in Illinois are beginning to head, it is a good time to scout for diseases that are present and to better understand the risks of diseases that can affect the heads, including Fusarium head blight (scab) and glume blotch.
Foliar diseases. Very few reports of foliar diseases have been noted yet in Illinois wheat fields this season. Some low levels of the Septoria/Stagonospora leaf blotch complex have been observed, but most symptoms are on the lower leaves at this time. Reports of "mosaic" type symptoms on the leaves have also been reported at a low level. These mosaic symptoms could be caused by a virus or could be a bacterial disease known as bacterial mosaic.
Results of a wheat disease survey conducted in Illinois during the 2009 growing season indicate that bacterial mosaic was very common throughout the state. To determine the cause of mosaic symptoms in wheat, affected samples generally need to be sent to a plant diagnostic lab. Reports of stripe and leaf rust in states south of Illinois (such as Arkansas and Louisiana) have been common; however, no observations of these rusts have been reported in Illinois this season.
Leaf blotch caused by the Stagonospora nodorum/Septoria tritici complex.
Symptoms of bacterial mosaic on wheat leaves. (Photo by Nancy Pataky, University of Illinois Plant Clinic.)
Symptoms of leaf rust (left) and stripe rust (right) on wheat leaves.
Fusarium head blight and glume blotch. Fusarium head blight (aka scab) can be one of the most devastating diseases of wheat when conditions are favorable for it. When present, the disease can cause loss of both yield and quality. Quality losses can be due to lower test weights and contamination of grain by toxins (i.e., deoxynivelanol, or DON) produced by the fungus that causes the blight; both results can be a serious problem for producers and millers. Because the fungal pathogen that causes Fusarium head blight (Fusarium graminearum, also known as Gibberella zeae) can also affect corn, causing Gibberella stalk and ear rot, the pathogen is already present throughout Illinois in many fields.
Weather is generally the driving factor in the development of Fusarium head blight. Because wheat is susceptible during flowering, the weather conditions from flowering through kernel development play a key role in the incidence and severity of the disease. Moderate temperatures (75 to 85°F), prolonged periods of high humidity, and prolonged wet periods favor disease development. A disease forecasting system based on weather conditions known as the Fusarium Head Blight Risk Assessment Tool is available online at www.wheatscab.psu.edu. You can obtain a "risk map" that allows you to see the risk of Fusarium head blight throughout Illinois and in other states (see below for a sample). This forecasting system, developed through collaboration among many university plant pathologists, was funded through the U.S. Wheat and Barley Scab Initiative.
Screen capture of the Fusarium Head Blight Risk Assessment Tool (April 28, 2010).
By the time symptoms of Fusarium head blight are visible on wheat heads, which appear as "bleached heads" or heads with both green and bleached areas, it is too late to manage the disease. Successful management of Fusarium head blight requires an integrated approach; it begins prior to planting when you are deciding which varieties to plant and which fields to plant into wheat.
Foliar fungicides are the only "in-season" option for control of Fusarium head blight. Although fungicides are a good control option, losses will still occur on a highly susceptible variety sprayed with a fungicide in an environment favorable for disease. A University of Illinois field research trial conducted at Urbana in 2008 and 2009 evaluated the effect of fungicides on 12 different wheat varieties that ranged from susceptible to moderately resistant to Fusarium head blight. The best control of Fusarium head blight was achieved when resistant varieties were applied with a fungicide. It is important to note that this trial had very high disease pressure; it was inoculated with the Fusarium head blight pathogen and the trial was mist-irrigated to provide a very favorable environment for disease.
Symptoms of Fusarium head blight on wheat (“bleached” heads).
Timing of fungicide application is very important to getting the best control. Fungicides should be applied at Feekes growth stage 10.5.1 (early anthesis, when the anthers are just beginning to extrude from the head). It is also important to spray with nozzles oriented to spray forward, which helps improve coverage of the wheat head.
Only a few fungicides have Fusarium head blight listed on their labels, and these are only triazole fungicides (Prosaro, Caramba, and tebuconazole products). Products that contain a strobilurin fungicide (Headline, Quilt, TwinLine, Stratego, and others) can be applied earlier in the season to help protect against foliar diseases, but they should never be applied when heads are present. In some instances, strobilurin fungicides applied later in the season can cause an increase in DON toxin contamination in grain, so it is important to never apply them at the later growth stages.
Figure 1. The effect of Prosaro and Folicur fungicides compared to an untreated check on Fusarium head blight of 12 wheat varieties differing in susceptibility. From left to right, varieties range from susceptible to moderately resistant to Fusarium head blight. This trial was conducted at Urbana in 2009 by Drs. Carl Bradley and Fred Kolb, University of Illinois, and funded by the U.S. Wheat and Barley Scab Initiative.
Glume blotch, caused by one of the fungi involved in the Stagonospora/Septoria leaf blotch complex (Stagonospora nodorum), can cause yield losses and test weight reductions in wet years. Symptoms of glume blotch are seen as gray to brown lesions on the glumes and awns. This disease is favored by wet conditions and temperatures between 68 and 81°F. In 2009, glume blotch was present in a wheat fungicide trial at the University of Illinois Dixon Springs Agriculture Center in southern Illinois. Prosaro and Caramba fungicides applied at Feekes growth stage 10.5.1 significantly reduced glume blotch severity, indicating that a fungicide application at the proper timing for scab control may also provide some control of glume blotch.
Wheat head affected by glume blotch.
On a side note, the brand-new (3rd) edition of the Compendium of Wheat Diseases and Pests is available from the American Phytopathological Society. It can be ordered online. This publication is a great resource that can be used to help identify wheat diseases.--Carl A. Bradley