Some of my orchid's leaves in the bottom of the plant are getting withered and leathery. Will you please tell me what is happening and how I can avoid that? Thanks.
This is likely cause by lack of hydration or heat stress.
Either the plant is under watered or the roots are rotted and unable to take up water.
You can carefully remove the orchid from the pot and check the condition of the roots. If they are dark and soft, you can trim away the rotting roots with a clean tool. Paint the cut ends with rooting hormone and repot the orchid.
The leaves may improve in appearance but will not likely return to a perfect condition.
If you have the orchids to close to the light source, it could be causing overheating of the plant.
Also if it is to cold for the orchid the leaves can become wilty.