Answer: When the plants have stopped growing, trim away the old stems at the base. Remove all of the "plant debris" and destroy it to reduce the possibility of reinfection. You might also add a fresh layer of mulch. During the growing season, do what you can to increase air circulation around the plants (thinning them a bit rather than allowing a wild heavy thick tangle to develop might help) and avoid wetting the foliage, especially in the evening. In my experience, mildew just reappears on some plants some years (it can blow in on the wind) and unfortunately washing the inanimate objects would probably not help.
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