Fungicide usage on soybeans is far less common than corn. However, the reasons for using a fungicide on corn or soybeans are exactly the same, stop disease and promote plant health. Any leaf infected with disease spots will have its ability to work at peak efficiency reduced, and plants must stay healthy to maximize yield.
On corn, applications work best when applied between the stages of tassel emergence and blister kernel stage. Fungicides are locally systemic products, meaning they will extend coverage through the leaf that encounters the fungicide, but not move to new growth. Since corn will not add any new leaves at tassel, this time frame works great to protect the top most leaves of the plant that are the most important in capturing sunlight and converting this energy into sugars and starches for grain fill. Since soybeans will continue to grow new leaves until late into reproductive stages, timing of the fungicide is a little more difficult to get full plant health benefits and protect new leaves from disease.
Ideal timing for fungicide applications on soybeans is just before the R3 growth stage. At this stage, there are new pods developing on at least one of the upper four most nodes of the plant. Treatments at this time are effective at management of Anthracnose, Frog Eye Leaf Spot, Cercospora Leaf Blight, and Pod and Stem Blight. If White Mold or Scleortinia Stem Rot are problems in your field, these diseases need to be controlled with an earlier application of fungicide. Just like corn, selecting fungicide products that have both curative and preventative properties work best. Different fungicides will also have better control of certain disease than others will, so timely scouting of the field is necessary.
Since soybean plants do continue to add new leaves well into reproductive stages, it is possible that most of the yield benefits come from improved plant health. Fungicides help plants produce nitric oxide and prevents the production of ethylene, which is the plant hormone that promotes ripening. Essentially, fungicides keep plants greener longer allowing more time for pods on the upper portion of the plant to reach full development, and fungicides reduce stress, which promotes better pod retention.
The R3 growth stage is also an ideal time for treatment of insects that feed on leaves or damage pods. R3 is also the time in which nitrogen fixation by the root nodules slow down, providing an opportunity for some yield increases by applying a foliar feed product containing nitrogen at this time. Soybeans usually respond positively to insecticide or foliar products when tank mixed with fungicides.