By Erick DeWolf, Extension Plant Pathology
Stripe rust is an emerging concern for many wheat growers in the state. The threat of yield losses to stripe rust has many growers looking into fungicide options. Here are some common questions that others are asking about wheat fungicides and their use.
Figure 1 Stripe rust killed by fungicide.
Q: How does the efficacy of the different fungicide products compare?
A: We provide an efficacy rating of fungicide products in Foliar Fungicide Efficacy Ratings for Wheat Disease Management 2017, K-State Research and Extension publication EP-130: http://www.bookstore.ksre.ksu.edu/pubs/EP130.pdf
In this publication, you can compare the efficacy ratings of many different products (including products that contain more than one mode of action) for stripe rust and many common wheat diseases. In general, wheat growers have many very good or excellent product options. In my experience, correctly identifying when a fungicide is needed and timeliness of the application are more important than which product is being used in most cases. Control of Fusarium head blight (scab) is the exception. For Fusarium head blight control, triazole fungicides are the best option. This includes products such as Prosaro, Caramba, and Folicur (or generic tebuconazole). See the fungicide efficacy publication mentioned above for more information.
Q: How do generic fungicides compare with other products options?
A: In tests conducted by universities throughout the Central Plains and Midwest in recent years, researchers have found no significant differences in the efficacy of products with identical active ingredients. In other words, the generic fungicides are equally effective when used at the same rates as other products with the same active ingredient.
Q: What about residual activity of the different fungicides?
A: Fungicide efficacy can be influence by many different factors including product, rate of application, the method, the disease targeted and weather conditions following the application. Therefore, it is hard to nail the residual life down exactly. After years of testing, we can make some general statements about residual life that can help growers know what to expect from their fungicide application. In general, all the fungicides shown in the product efficacy publication provide at least 21 days of residual activity (including tebuconazole products). When the application is made between flag leaf emergence and heading, this 21 days of residual life is enough to get the crop well into the grain filling stages of development. The disease may begin to increase again after the fungicide effect has diminished, but this late season disease generally has little or no effect on yield. Some of the products containing mixed modes of action may provide a little longer residual life, but in research tests that extra residual life does not consistently translate into more yield.
Q: Are there other issues to consider when selecting a product?
A: Yes. There is a growing concern about fungicide resistance in some parts of the country. For a long time, those of us growing field crops didn’t really have to worry much about this issue, but that is no longer the case. The development of fungicide resistance can be slowed by alternating modes of action between years, by using a product that contains multiple modes of action, or tank-mixing different modes of action. Products containing only strobilurin fungicides are most at risk for fungicide resistance.
Another factor to consider is the maximum amount of any one active ingredient that can be used per season. If an early application of tebuconazole is made, for example, you will not be able to apply the full rate of a product now if that product would put you over the limit for tebuconazole for the season. This is one of the potential downside risks of making an early-season application of a fungicide.
Q: What is the difference between a “curative” and “preventive” fungicide?
A: Honestly, I don’t really like to use these terms when describing fungicides because I think they can lead people down a confusing path. All fungicides are best applied before the disease becomes established or very early in the development of disease within crop. From this perspective, all fungicides work best in “preventive mode”. The triazole fungicides are generally considered to have some limited curative activity but they cannot restore leaf tissue already damaged by the disease. It would also be a mistake to think that a fungicide with curative activity does not provide any preventive activity. The different fungicides just stop the infection at slightly different times in the infection process.
Q: Is it best to use a product that combines multiple modes of action?
A: Fungicide products like Absolute Maxx, Nexicor, Quilt Xcel, and TwinLine, that combine multiple modes of action offer very good to excellent efficacy against stripe rust and other important foliar diseases in Kansas. As mentioned previously, using a fungicide with a mixed mode of action can also help reduce the risk of fungicide resistance developing in a fungal population. It is hard to say these are the “best” options and, I think growers have a lot of product options with very good or excellent efficacy on stripe rust and other leaf diseases. I suggest that growers consider efficacy ratings, cost, and availability when selecting products to use on their farm.
Q: Which fungicides can be applied latest in the season on wheat?
A: Always consult the label on this since any label violations could have unwelcome consequences. In general, the triazole fungicides can be applied the latest. Tebuconazole products (Folicur and generic products), Caramba, and Prosaro can be applied through the flowering stage. But these products have a 30-day preharvest interval as well, so producers have to keep that in mind and make sure they’re not applying it so late that they will have to delay harvest to meet the preharvest interval. Other fungicides have a growth stage cut off that prevents application during and after the flowering stages of growth.