Powdery Mildew

I learn so much from a couple of the guys at my local garden center! 

I was opening the window in the area where I grow, trying to let some fresh air in. bad idea. The guy at the shop said that powdery mildew spores can get in your house that way! I have a False Sunflower and a Lilac bush outside that had it. I'm  wondering now if that's how I got PM almost a year ago. I never thought of the possibility and am so glad he mentioned it. So until the frost hits- no open windows near this garden!