Split fungicide treatments on wheat: Should producers consider an early application?
In general, fungicides have proven to be most effective in controlling foliar diseases on wheat in Kansas when applied in a single application sometime around flag leaf to head emergence stages, depending on level of disease risk (disease pressure and predicted weather conditions), variety resistance to the most threatening fungal diseases, yield potential, foliar fungicide efficacy, and other factors.
Fungicides can also be applied in a split application, with an early application made around early spring greenup followed by a later application at flag leaf to early heading stage. That approach adds a little extra expense, and may or may not pay off compared to the single application approach, as the majority of the yield response is normally associated with the flag leaf application. It is also important to remember that fungicides will only protect the leaves present at time of application; thus, an application during jointing does not substitute for a flag leaf application, as any leaf that emerged after the application will not be protected.
When making split applications, the early application often uses a low rate of product to save money. This provides a shorter length of control than a full-rate application. With the prevalence of low-cost generic fungicides on the market now, some producers are using a full rate of fungicide for the early application. The full-rate of most fungicides provides about two weeks of good protection, followed by a third week of partial protection to the leaves present at the time of application. Using a full rate early, however, could have implications for the second, later application. Growers will need to select a product and rate that stays within the labeled limits on the amount of each active ingredient used in a single season. You don’t want an early fungicide application to remove the ability to apply your preferred product at flag leaf.
Advantages and limitations of split applications
There are some advantages to making an early application, and some limitations. The advantages of early-season fungicide application include:
- Low cost. There is no additional cost for application if the fungicide is tank mixed with other products, such as liquid nitrogen fertilizer or herbicide. Often, however, the optimal timing for an early fungicide application is not until after the wheat has jointed – with one or two joints present. This is usually sometime in mid- to late-March in southern Kansas and a little later in northern Kansas, although the warm winter during this growing season has made some fields start spring development earlier than usual. Topdressed nitrogen and many postemergence herbicides should be applied before this stage to be most effective, so the optimal timing of both applications may not match. If a separate trip is made for an early fungicide application, that adds to the cost. Since the payoff for an early application is less certain than with later applications, it is perhaps best to consider using a low-cost generic fungicide for the early application and saving more expensive products, if desired, for the later application.
- Provides suppression of early-season diseases such as tan spot, powdery mildew, speckled leaf blotch, and possibly stripe rust.
The limitations of early-season fungicide application:
- Leaves not present at the time of application will not be protected. Therefore, these applications will not control leaf rust or stripe rust epidemics that come in from the south at later stages of growth. The early applications are most effective when combined with a second, later application of a fungicide.
- Additional product cost may not pay off under some conditions, especially this growing season when the wheat prices are low. Remember, the second application does the heavy lifting in the dual-application approach. If capital resources are limited because of low prices, it may be best to invest your money where you are likely to see the largest yield response.
K-State test results of early, low-rate fungicide applications indicate this practice is most likely to be effective in continuous wheat grown in high-residue conditions, and with varieties that are susceptible to either tan spot or powdery mildew. The value of the early applications is diminished in other rotations, conventional tillage systems, or with a variety that is moderately resistant or resistant to the targeted disease – usually tan spot or septoria leaf blotch, and possibly stripe rust or powdery mildew some years. K-State has not tested the practice of making split applications using a full-rate of product at both times.
Which diseases are present early?
Stripe rust and leaf rust rarely overwinter in Kansas. These diseases typically blow into the state from Texas and Oklahoma during the spring, and often do not begin to cause significant infections on wheat until about flag leaf emergence at the earliest.
However, stripe rust (Fig. 1) bears watching. It is primarily a cool-weather disease and starts a little earlier in the season than leaf rust. Stripe rust infections can quickly become severe and damage leaves. Leaf rust does not act quite as quickly, nor does it damage leaves as readily as stripe rust. Tan spot and septoria leaf blotch (Figs. 2 and 3), can also damage leaves quickly and may need to be controlled early if present in a field. Powdery mildew can become established early in a growing season, but this disease does not normally cause severe yield losses in Kansas, except on varieties that are very susceptible.
If a field has some hot spots of stripe rust at jointing or earlier, that would be an additional factor to take into consideration when deciding whether a split application of fungicide would be helpful. The same applies to tan spot, septoria leaf blotch or leaf rust. An early infection of or powdery mildew, however, would not be as important in making that decision.
Figure 1. Symptoms of stripe rust on wheat. Notice the blister-like lesions arranged in stripes. Photos by Erick DeWolf, K-State Research and Extension.
Figure 2. Symptoms of tan spot on wheat. Lesions are tan, with yellow margins, and mature lesions often have a darkened spot in the center. Photos by Erick DeWolf, K-State Research and Extension.
Figure 3. Symptoms of septoria leaf blotch. Lesions are tan, elongated, with thin yellow margins. Black speckles in the center are key identifying features. Photos by Romulo Lollato, K-State Research and Extension.
Product rates and restrictions
Producers considering the use of split applications must pay close attention to label restrictions. Every active ingredient in a fungicide has a maximum total amount that can be applied during the season.
For example, if an early application of a generic form of tebuconazole is applied at 4 oz/acre, a subsequent application of any fungicide containing tebuconazole around heading would put you over the 4 oz limit for the crop season. In addition, a subsequent application of a fungicide premix that contained tebuconazole would put you over the limit.
Thus, be sure to read the label to determine the maximum amount of a chemical that can be applied in a single season and the exact amount of a chemical(s) that is in a fungicide.
For information the efficacy of different foliar fungicide products, refer to K-State Research and Extension publication Foliar Fungicide Efficacy Ratings for Wheat Disease Management 2016, EP130.
The main conclusions we can draw from recent studies in Kansas and Oklahoma:
- In K-State studies, the greatest average profit has come from the flag leaf application of fungicides. Fungicides applied during jointing have rarely shown positive profit.
- The likelihood of profit for an early-season fungicide application is greatest for susceptible varieties in continuous wheat systems with a high level of surface wheat residue, and when the level of disease risk for stripe rust is high.
- Tan spot, septoria leaf blotch, and stripe rust found in hot-spots in a field are candidate for an early fungicide application, provided environmental conditions are conductive for further disease development.
For information on disease susceptibility of wheat varieties, see K-State Research and Extension publication Wheat Variety Disease and Insect Ratings 2016, MF991.
For information on assessing the need for wheat foliar fungicide, refer to K-State Research and Extension publication Evaluating the Need for Wheat Foliar Fungicides, MF3057.
Another publication providing good information, from which a few excerpts were used in this article, is Oklahoma State University’s Split Versus Single Applications of Fungicides to Control Foliar Wheat Diseases, PSS-2138.
Erick DeWolf, Extension Plant Pathologist
Romulo Lollato, Wheat and Forages Specialist