Wheat Disease Update – May 23, 2017
Written by: Kirk Broders, Plant Pathologist; Department of Bioagricultural Sciences and Pest Management, Colorado State University; C030 Plant Science Bldg., 1177 Campus Delivery, Fort Collins, CO, 80523-1177; 970-491-0850 (work); firstname.lastname@example.org
The wheat crop in Colorado is on average between Feekes growth stage 9-10.5 (boot stage to heading complete). Much of the state has had cooler and wetter than average weather (including snow in Fort Collins!) over the last 2 weeks and this has resulted in the development of several foliar pathogens on wheat. In particular, I have seen an increase in stripe rust and Stagonospora leaf and glume blotch (SLB & SGB) (Fig. 1). While the number of reports of stripe rust have increased (now present in Logan, Sedgwick, Phillips, Washington, Yuma, Kit Carson, Cheyenne and Kiowa counties), the disease incidence (% of plants in a field infected) and severity (% of leaf area infected) remain relatively low. However, the unseasonably cool and wet weather provides the optimum conditions for the disease to spread. The question many growers are asking is “Will the cost of fungicide application be greater than the potential yield loss due to stripe rust?” The answer is context dependent. There are four factors that growers need to considers 1) price of wheat; 2) potential yield of a field; 3) price of fungicide application and 4) current or potential disease pressure. If yield potential and/or the price of wheat is high, the cost of a fungicide application is more easily offset. However, with current low market prices and average dryland winter wheat yields, disease severity must be relatively high to offset the cost of fungicide application. In most areas of the state, fungicide application will not be necessary, given moderate to low disease pressure. However, there are pockets where stripe rust and SLB and SLG are reaching levels that will impact yield. Grower’s in these areas may want to consider applying a fungicide to prevent yield loss. There are several fungicide compounds with very good or excellent efficacy against both stripe rust and Stagonospora (Table 1).
Stagonospora leaf and glume blotch (SLB & SGB), caused by Parastagonospora nodorum, are most commonly observed in fields where significant wheat residue is present from the previous season, as the fungus is able to survive the winter in this residue. The spores that survive in the reside are then dispersed onto the growing wheat plants in the spring. Moderately warm (68-82°F) temperatures coupled with frequent rain (which helps splash spores from the residue to lower leaves and then to higher leaves and finally the head) favor the development of SGL & SGB. SGB symptoms are distinct from those of other wheat head diseases. When the fungus that causes SGB infects wheat, distinct portions of the glumes and lemmas (chaff) will turn dark brown to black, often with a purplish tint (Figure 1 & 2). After initial infection, the fungus will continue to colonize the glumes. The brown-black edges of the lesions will continue to expand along the plant tissue. SGB can cause kernels to shrivel and shrink, which reduces test weight. Because SLB typically precedes SGB, observing SLB (Figure 3) in wheat can indicate the risk of SGB. If the foliar stage of this disease becomes severe during the season, applying a fungicide at flag leaf emergence or flag leaf (Feekes growth stage 8-9) can help prevent SGB. However, since much of the wheat is past this stage, you can apply fungicides up to Feekes 10.5.1 (early anthesis or flowering) to protect against SGB and (in most cases) even protect the flag leaf from SLB. Again this will have to be a decision made based on the level of disease present in the field. Fields with moderate to high levels of SLB should consider applying a fungicide to protect against the fungus spreading to the head and causing glume blotch.
Kirk D. Broders, PhD
Department of Bioagricultural Sciences and Pest Management
Colorado State University
Submitted to BARN Media by:
Dr. Wilma Trujillo