When we roll up our awning and it rains, water picks up the roof dirt and seeps in the end of the awning to creates dark streaks of dirt at the bottom of each roll layer across the awning.
By spring it is usually a bit moldy so I scrub it, let it dry, then roll it up.
The problem of course is every time it rains, some water seeps into the ends and the streaks reform, though not moldy or quite as dark.
I can of course re-clean it every time it rains but I have discovered that the awning gives just as much shade with the streaks, as it does without the streaks.
Be safe