Rotting Oncidium sharry baby

[Oranagra]

Hi,
I recently moved my Oncidium sharry baby to semi-hydro, as it needed repotting (overgrown it's pot), and didn't seem to do good in it's previous bark mix setup, the pseudo bulbs were quite thin and shrunk, but there were new growths, and no rotting.

now, a few months later, the back bulbs started rotting one by one.. at first i removed the rotted bulbs, but now when more and more are rotting, i tried to to cut it and try to save the healthy portions, and throw the rest to the trash.

while cutting i noticed a purple ring on the rhizome, which i remember reading that it might be a sign of a virus.

besides that, i have some black spots on the bulbs, and on the leaves, i think i had spider mites, but not a severe outbreak.