Thank you for the excellent question, Stephen. Fungicides are most effective when applied at the early stages of disease development, and tend to be more effective when they are applied as a preventative, rather than a curative, measure. That being said, unnecessarily applying fungicides increases the risk of developing fungicide resistance. This is why scouting and monitoring on a regular basis are so important. By regularly monitoring fields, you can spot disease symptoms early on and apply fungicides to prevent the disease from spreading further within the field. The impact of disease is highly dependent on the environmental conditions and these need to be taken into account when determining the need for fungicides whether preventatively or even curatively. If the weather conditions aren’t conducive for disease it will likely not impact yields and thus applying fungicides is an unnecessary input cost. As always, fungicides should only be applied by a trained individual and personal protective equipment should always be worn.
You can learn more about pesticide safety and integrated disease management in SIL University's free, online Integrated Pest Management and Pesticide Safety training course: https://soybeaninnovationlab.getlearnworlds.com/course?courseid=ipm