Question

orchid   QUESTION: Sir,

 Your advice please.

ANSWER: The yellowing area of the leaf plus water droplets on the leaf suggests here that water has gotten into the crown at that leaf and generated a fungal infection.  To prevent spread, remove as much of the tissue as possible (the entire leaf if necessary) and seal the cut ends with a powdered fungicide.  When you water, try to dry any wet leaf surfaces as soon as possible. In nature, plants often grow in a manner such that rain water will run off from the leaves. I like to use a fan with and after watering early in the day to dry the leaf surfaces by nightfall.

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QUESTION: Thank you for great advice! If I were to repot, must I discard the entire original potting mix? Can I use sphagnum moss and charcoal as my new potting medium as materials like peat moss, perlite or fir bark are quite hard to obtain at my location. Just to confirm, crumbling and blacken roots are signs of rotting right? Is it also advisable to use plastic pot(with draining holes at the bottom) to repot the orchid? Thank you once again for your attention.

Answer
I recommend that you not use sphagnum moss as that seems to be keeping your potting mix too wet.  Most symptoms shown in your photo suggests that the potting mix is staying too wet.  Crumbling roots are rotted roots and are the result of potting mix problems. Yes, plastic pots with lots of holes in the base would be helpful.  As for the difficulty in locating additional potting ingredients, there may be alternatives available in your area such as coconut fiber in several forms, osmunda or tree fern fiber.  Try googling "Singapore Orchid Growers"  and you may be able to contact some local growers and find out what they use.  They may also have some supplies or know where you can get them.