PM likes the cold and humid conditions best... most moulds like humidity. they hit at night when humidity can hit as high as 100% :O even in rooms with good air exchange during the summer months. By the time you see the spores it is too late, the plant is diseased, the spores are just how it spreads itself.
I have grown many plants that are immune to PM, immunity can be strain specific or even pheno specific. some plants can also learnt o become immune, maybe a gene that lays dormant until the disease gets a grip. i don't know, but i've seen it happen. It was actually fdd's Hijack.