Powdery mildew [View all]

I've just discovered it on my zinnias. Not concerned about this season which is almost over.
but for next year -
I have read 2 contradictory suggestions:
1. is to clean up all debris, infected plants, etc. at end of season to get rid of the spores.
(I find this advice common).
2. one person said that if you clean up the garden you prevent the soil forming immunity to powdery mildew, which intrigues me.

anybody know more about this (forming immunity)?

thanks,
ellen