I just repotted my Great White NOID, my first orchid, and I tried a different potting mix. I switched from pure sphag to a sphag and CHC mix. I've done this with several other orchids this year and none of them have had any problems. Quite the opposite, they're all thriving.
Anyways, my phal developed some sort of rot in one of the new roots. I treated it with hydrogen peroxide and, foolishly, left the root on the plant. With all the other new roots, I realize now that I should have just cut it off at the first sign of rot. Now the rot has moved up and has formed a sunken black spot just above where the root used to join the plant. It is spreading slowly, mostly back towards where the root used to be.
Sorry for the rambling. I just have to explain every little thing. Here's a pic of the problem. I'll let you look at that instead of going on for another 3 paragraphs.