Weather can always intervene and short circuit a disease epidemic. In these situations, by all means, go with your gut instinct. There is no possible way to develop disease thresholds appropriate for all situations. The table below shows the levels of disease control each material is capable of providing. Product labels will provide detailed use instructions and product limitations.
Fungicides are valuable only if yields and test weights are threatened by disease.
Similarly, fungicides will be of limited value if other diseases develop that are not controlled by foliar fungicides. Examples are all virus and bacterial diseases, and fungal diseases such as take-all, loose smut and head scab. Unfortunately, the above situations will always be a risk to the fungicide user. Some of this risk can be reduced by monitoring crop development throughout the season. This might allow you to pick up on yield limiting factors which indicate that applying fungicides may not be a good idea.