Figure 1. Winter wheat planted into previously a corn field in Douglas County, South Dakota. This winter wheat was found with no early leaf spots and would therefore not benefit from an early fungicide. Photo by Emmanuel Byamukama.
Winter wheat growers will soon be applying herbicides to manage weeds in their wheat fields. This is usually also the time when a fungicide may be tank-mixed with herbicide to save on the application cost. However, the question to pose here is whether the application of fungicide at the herbicide timing is profitable.
Research done at SDSU and other universities in the region show limited yield response to an early fungicide timing. This is mainly because the fungicide applied at this time protects the lower leaves but not the new leaves that develop after the fungicide application. Yet the upper leaves, especially the flag leaf and the leaf below flag leaf, are the main contributors to grain yield. For this reason, a fungicide applied to protect the flag leaf and the leaf below flag leaf consistently shows positive yield response.
The main diseases that develop in winter wheat around this time are tan spot and powdery mildew. These diseases develop in the lower canopy, starting with leaves closest to the soil, primarily because the pathogens that cause these diseases survive on last year’s wheat stubble. Tan spot and powdery mildew at moderate to high severity levels can cause significant grain yield loss.
When is it beneficial?
In certain field conditions, an early fungicide at herbicide timing may be beneficial. If wheat is planted no-till into wheat residue or into a wheat fallow system, an early fungicide application may be beneficial. Since the pathogens over winter on old wheat residue these fields will be subjected to high inoculum throughout the season. For rotated field (Figure 1), early leaf diseases pressure is low and usually does not require an early fungicide application.
Sometimes stripe rust can develop in wheat early in the season. In such circumstances, an early fungicide can protect plants from continued stripe rust development. This usually happens when there is a mild winter which favors the pathogen to overwinter in South Dakota. There have been no reports of stripe rust so far this season in South Dakota. Therefore, if stripe rust occurs, it will most likely be from spores blown in from southern states.
Another factor that may necessitate an early fungicide application is where there was substantial fall winter wheat growth (for early-planted winter wheat) and where heavy tillering due to moderate to high fertility occurred. These conditions encourage powdery mildew to develop early in spring and therefore an early fungicide may be beneficial.
The Bottom Line
Growers are encouraged to scout their fields in order to determine if an early fungicide application will be needed. The decision to apply a fungicide should be driven by the presence of disease symptoms and also current and forecast weather conditions. Wet and warm weather encourages disease build up. View Management of Small Grain Diseases: Fungicide Efficacy for Control of Wheat Diseases for a list of fungicides and their efficacies against wheat diseases. This information is put together by the North Central Region Wheat Pathology Working Group.