mtssa help, please

[VickiC]

Hi,
I have an orchid labeled "mtssa Shelby" (well, it looks like the word "shelby")....that I noticed day before yesterday had something wrong going on. I looked closer and found powdery white dust, so from what I have gathered, it is powdery mildew (??) There is also a yellow leaf. I had ordered some things from FirstRays and one item was Physan 20. I rec'd my order today, so I had planned to treat it with the Psysan 20. ?? Should I drench the roots/and the whole plant with it? Next question: Are the black roots dead? Also, should I leave the "inner root threads" of the mushy ones I removed? I attached 2 pictures, so if you don't mind taking a look at them and then let me know what to do, I would appreciate it.
Thanks in advance,
Vicki