Why are my roots dying?

[scy]

Two of my phals recently started dropping leaves. I originally thought it was probably due to too much light as I have them in an east facing window with no sheer curtain. I spotted new root growth in both of them about a month earlier. I even unpotted to reposition them both in their pots, so I was able to confirm the nice, firm, plump, healthy green roots!

My large phal noid started dropping leaves very rapidly, so I finally unpotted it again to see if anything was wrong. I found that there were white moldy (?) spots all over some black roots. Yes, the roots were all dead except for the new root nubs that were growing. I soaked all the roots and PrimeAgra in physan dilution. Now I have it behind a sheer curtain to shield it. On a side note, I have been battling a fungus gnat problem, so I finally got some mosquito dunks and placing them on top of the PrimeAgra and in humidity trays to get rid of these things once and for all. I decided to go through and clean through the green moss(?) in the pots, because it was getting pretty unsightly and smelly.

I just unpotted my phal sogo grape only to find that the same thing is starting to happen with this one too. I did catch it before the roots died. There are 2 bright green ones left but they have dark spots and the ones that have turned black are still firm. I have not cut anything because I'm hoping they are not dead if they are still very firm. I did see some white spots but they don't come off when I rinse them under warm water. I have soaked the roots in physan along with the PrimeAgra and plan on potting them back up.

1st Photo: noid phal
2nd Photo: Phal sogo grape

Why my roots are dying? I can understand the noid phal's roots dying as they were part of the original roots and maybe they didn't take to s/h. However, the other phal's roots were all new roots that grew in after repotting to s/h so they should be tailored to the environment. I would like to do my best to save them. I am going to put them on top of a heating pad and have already watered with kln solution. I'll try to find something large enough to put my large phal in to help raise the humidity. Any ideas on what happened and how I can avoid this in the future? Sorry for the long post. Thanks for your help!

Susan