Hoodat? I'm not Hoodat but I've already had a dip!
I'm just going to say that I believe it is stress that permits mildew to get its start.
For taking over, moisture makes it possible or ever, probable. However, bug damage, arid conditions, severe sunlight, maybe even wind - can all stress the plant and mildew can get started. If moisture then lingers on the leaves in the days after that, the mildew grabs the plant in a death embrace . . ! Yikes!
Now for a sprinkle of Johnson's Baby Powder and to bed . . . .
Steve