I'm not sure I follow, is it the label on the fungicide that says that? The AHS Daylily Dictionary page on leaf streak has a link to fungicides for leaf streak. If you can tell us which of these (or another) you want to use it would help:
https://daylilies.org/daylily-...
Are you sure your information doesn't just mean to apply fungicides in anticipation of humidity/damp/wet weather (which is when fungal diseases tend to proliferate)?
