Air movement and humitiy is your problem. You may need to treat with chemical such as floramite from growshop. The shit is very powerful and toxic to humans so watch out. Also cut the infect leafs off and dispose immediately.
if you wanna go cheap just use very low ph water and cover the plants.
??????????? Floramite is a miticide and has pretty much zero to do with PM.
First off, you're not going to kill PM, it's IN the plant. But, you can deal with it.
There are only a very few, very toxic sprays that will work and a shit box full of *feel good* remedies that really won't do anything in the long run. This includes, but not limited to, baking soda, neem, milk and some others I'm sure that will pop up.
The most effective, least toxic, method that actually works is a sulfur burner/vaporizer. Not the cheapest, unless you DIY. Google it, or search right here or in the sick plants forum.
I'm not going to argue for it, or say much more for that matter. Let the *experts* go at it. This is the answer to your situation, end of story.
Wet