Hi, Urveeka. It is normal for the oldest Phalaenopsis leaf to wither as you describe. The orchid is directing resources to other areas. I let mine go until it easily falls away. To rule out pest/sunburn/fungal problems, post a plant picture for better advice. Whitish roots and brown spots are not necessarily bad. Trim the root away if the root is reduced to the center wire, dried out like a dead worm or if it is mushy. New root buds should be sprouting from the stalk base at this time of year, so feeding is good. Phal roots need air circulation and to dry slightly so I admire your success in sphagnum. I have found that my Phals do best in a course bark mix. Joe