I have a Phal with 6 healthy leaves growing in sphagnum moss for the last 10 months. Last week was the first time I added fertilizer to it (Orchid Food 20-10-20) and over the weekend, I noticed that half of the oldest leaf started turning yellow, so I pulled away some of the moss and noticed that the roots are turning yellow with black spots on them!!

Is this root rot or a disease? Should I re-pot the plant and stop fertilizing?
Any help/advice would be appreciated.

thanks
Urveeka