Timing refers to when and how often the spray must be applied to effectively control a disease. The first application usually is made at a time close to but before the pathogen arrives on the plant surface. This information is often provided on the pesticide label or is available in Extension literature from your local county Extension agent. In most situations, fungicides are not effective in controlling the disease if the pathogen has already entered (infected) the plant tissues. In many cases, specific information about the disease cycle may be needed to time the first application correctly.
After the first application is made, the pesticide barrier is established on the plant surfaces. Effective use involves keeping this barrier active and complete throughout the time that the pathogen can arrive on and infect the plant. Modern fungicides are developed so that they do not persist in the environment for long periods of time. Rainwater, sunlight, microbial action and oxidation decrease effectiveness of the fungicide. Reapplication of the spray is needed in many cases to keep the fungicide barrier active.
Plant growth also affects the completeness of the barrier. As new leaves and shoots appear, they are unprotected and may be subject to infection. If so, they must be recovered with the barrier.
The fungicide label gives reapplication guidelines, usually in ranges of 7-14 day intervals. If excessive rainfall or rapid growth of the plant occurs, the shorter interval between sprays should be used. If not, use the longer interval.

From：ohioline.osu.edu