This approach will result in no economic benefit from fungicides if disease pressure remains low. Waiting too late to apply fungicides is equally ineffective and is also very common. Herein lies the greatest obstacle to effective, economical use of foliar fungicides. The above thresholds must be used along with some good old common sense. For example, if a threshold is reached for powdery mildew, you may want to hold off spraying if an extended period of hot, dry weather is predicted.
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.