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According to a recent @ILSoyAdvisor Twitter poll, over half (56%) of the respondents indicated that they always spray fungicide on their beans. 

We want to know: do you use fungicide on your beans?

— ILSoyAdvisor (@ILSoyAdvisor) July 30, 2019

Another 27% responded that they “sometimes spray” based on disease pressure. Many of these growers have discovered what university research generally shows: A well-timed fungicide application will normally result in a positive ROI by protecting the yield potential of a soybean field.

University studies consistently show an average yield increase from fungicide application of 2.5 bushels or greater. Obviously, the yield response will generally be larger in fields with heavy fungal disease pressure. The response may also be higher in varieties that are susceptible to diseases such as Frogeye Leaf Spot, Cercospora or Septoria. However, it is important to note that even in the absence of fungal disease, we often see a yield response due to fungicides relieving plant stress. This stress mitigation helps the plants retain more blooms and pods that they might have otherwise aborted.

Although some fungicides have both curative (the ability to stop an infection from progressing) and preventative activity, remember that any damage that has already been done cannot be undone. Therefore, it is generally best to apply before symptoms appear. If not applying foliar fungicide to all your soybean acres, prioritize fields that have a history of fungal disease, susceptible varieties, or conditions favorable to disease development.

The optimal time to apply foliar fungicide to beans is from the R3 to R5 growth stages. As discussed in my previous blog, these stages are critical to determining final yield since the plant is filling out pods.  Flowering has been completed or reduced to a minimum at this point, thus there is no opportunity to add pods. Anything that can be done to keep the plants healthy during this time period will help protect the final yield potential of the field.

The exception to this timing recommendation is in areas where white mold is expected to be a problem.  White mold infects the stems through the flowers, so the best way to protect against this pathogen is with a fungicide application at R1. Fungicides are most effective at limiting the impact of white mold when used preventatively; results are inconsistent when applying after symptoms have appeared. In very high-pressure areas or with highly susceptible soybean varieties, a second application at R3 may be beneficial.

One thing to keep in mind is that foliar fungicides will not help with control or prevention of soil-borne early-onset diseases such as SDS or Phytophthora. Fungicides are also not effective against bacterial infections. Fungicides are very effective tools, but we shouldn’t expect them to do something that they aren’t capable of.

A final consideration when making fungicide application decisions is that the stems of soybeans in sprayed fields will often stay greener longer into the fall. Some growers don’t like seeing green stems at harvest time, but it’s important to remember that green stems later in the season are indicative of healthy plants. When a plant hangs on and continues to push nutrients into the seeds, we see a greater probability that we will maximize the yield of the field.

Growers have tough decisions to make throughout the year. Many factors influence the final yield of each field, and many of these factors are out of the farmer’s control. The decision to apply a well-timed foliar fungicide is one that can increase the bottom line in many cases.