Late Applications of Foliar Fungicides

Even though the 2019 season got a late start and seemed to drag on forever, we’re reaching the point in the season where things are finally wrapping up. Planting is complete, weed control is near finished and fungicide applications are being made as we speak. Now is the time of year we sit back and hope for August rains to make our soybean crop. And, we’re done having to make decisions on this year’s crop … or are we? The likely answer for 2019 is “yes”, but in any given year are there additional management decisions that can be made to help push the envelope on high-yielding soybeans?

Before making any recommendations on management decisions, let’s review what is happening to the soybean plant after R3, which is typically when fungicides are applied. At R3 the soybean flowering peaks and pods have begun to form. Any stress at this stage could reduce the number of pods, number of beans per pod and seed size, therefore it makes sense to protect the plant with fungicides and insecticides at R3. 

R4 begins when the soybean is at full pod (pods all the way up the main stem) and R5 begins when pods begin to fill with seeds. Rapid nutrient uptake is occurring, and nutrients are being redistributed from vegetative parts to pod. Once all pods are filled the soybean has reached R6 and will start the maturation process. The approximate time between R3 and the end of R6 is about 50 days. Any stress during this time can significantly reduce yield through the abortion of pods and seeds and the reduction of seed size. 

This leads to my question: “Is there more that we can do to help protect the plant after R3?” As we progress through growth stages, it takes less defoliation to notice a yield reduction with the most critical time being R5. Most fungicides available today will not provide 50 days of effectiveness and therefore are not protecting the plant through the critical time of pod fill and seed development. Is it possible to see additional yield gains from a sequential application of a fungicide at R5? I think it definitely could, especially in extremely high yield environments with disease pressure that arrives late in the season. 

To date I found little research to support this hypothesis (I did find that R5 applications could help with Phomopsis seed decay), however most of this research dates back to more than 15 years ago when yield levels were significantly lower than the yields we’ve been able to achieve the past few years and when no one was applying fungicides on soybeans. 

Most of this research was looking at one application and found that R3 was the ideal timing. I absolutely agree with this recommendation for a single application of fungicides in soybeans. However, for those growers looking for the next idea to maximize soybean yield potential, a sequential application at R5 may be something to try. And due to the rapid nutrient uptake during this same time it makes sense to consider adding foliar nutrition to the pass. A few nutrients to focus on would be phosphorus, potassium, sulfur and boron. Phosphorus is vital to seed formation, potassium improves water use efficiency and helps overcome the effects of disease, and sulfur and boron aid in seed production.

In summary, I understand that with the difficulties we’ve had this season, 2019 may not be the year for maximum soybean yields for many growers. If you are planning on making a single fungicide application the recommended timing is still R3—first and foremost. If you are interested in trying to raise the bar on soybean yields on your farm, protecting the soybean plant with a sequential application of a fungicide and foliar nutrition at R5 may help you to do so

Tracy Heuerman

Heuerman has worked within the Growmark system for nine years, as both a crop specialist and in her current role as a Field Sales Agronomist. She enjoys helping growers achieve maximum ROI by implementing strategies to increase yields. Raised on a family farm in south central Illinois, Tracy is still involved in growing corn, soybeans and wheat with her family. She holds a bachelor’s in agriculture economics and a master’s in plant and soil science, both from Southern Illinois University - Carbondale. She can be reached at



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