Why must protectant fungicides be applied multiple times during a fungal disease period?

Protectant fungicides need to be applied multiple times during a fungal disease period because they only provide protection to the specific parts of the plant that they come into contact with at the time of application. Once applied, these fungicides form a barrier that can help prevent fungal spores from infecting the treated areas. However, as new plant tissue grows or if rain or irrigation occurs, the untreated parts of the plant or newly developed foliage become susceptible to infection. Consequently, repeated applications are essential to maintain protective coverage over the entire plant as it continues to grow and to ensure that all potential entry points for fungal pathogens are adequately defended.

While it's true that factors like degradation in sunlight and the duration of effectiveness in the field also play a role in determining application frequency, the main reason for multiple applications is related to the growth of the plant and the limitations of the fungicide's protective effect only on treated surfaces.