Timing of fungicide applications has a significant impact on yield response. Timing of application is basically directly tied to the type of disease you are trying to control as well as when the disease infection occurs. Once a plant disease sets in, the goal is to stop the progression in the plant so that no further damage can occur, there is no fungicide that will repair previously damaged plant tissue. The balancing act is then timing the application to minimize disease damage while providing residual protection during critical plant stages that impact yield and plant health.
From a corn standpoint, major diseases for our area such as Gray Leaf Spot (GLS), Common Rust, and Eyespot can infect the plant at different times depending on the weather. Generally speaking it is key to watch for disease onset and density after V12 and establish proper timing for application after that along with choosing the correct fungicide for the disease control and plant health benefits you are targeting. Read more here.
As for soybeans, major diseases such as Cercospora, Brownspot, Frogeye Leaf Spot, and White Mold can infect soybeans at different times as well. Again, to achieve proper control applications should occur prior to the disease advancing too far. Generally for fungal leaf diseases critical timing would be the beginning R3 stage compared to White Mold infections that occur closer to R1 and after. Hence knowing what disease you are targeting and picking the correct fungicide and timing can be critical for maximum ROI. Read more on Soybeans.