If you are thinking this is Fusarium, I don't think the one photo of a purple stem is diagnostic enough to confirm a diagnosis of Fusarium.
Many of my Phals develop purple coloration in the stems and leaves and are perfectly fine.
Here are a few links that include pictures or descriptions of Fusarium; read and judge for yourself. I hope it all works out!
This link is in German but Google translates it. Follow the series of photos, starting from healthy plants through a series of infected plants:
Fusarium orkide Note the black lesions on the basal parts of the leaves which sheath the stem.
This is a link to a pdf on orchid diseases by Sue Bottom:
http://www.phelpsfarm.com/OrchidPestsandDiseases.pdf scroll down to information on diagnosis of fusarium (Obviously for a Phal, there is no rhizome to cut through, rely on waht is happening with the leaves).
This abstract explains in technical terms the symptoms you may see in one form of the disease:
SHEATH AND ROOT ROT OF PHALAENOPSIS CAUSED BY FUSARIUM SOLANI