Having the lowest leaves turn yellow like this is very common. Orchids often sacrifice older growth to support and nurture newer growth. When you see that in the spring, it is a natural part of the plant's growth cycle and should be left undisturbed while the orchid reabsorbs it.

When you see it at a later point in the year, it may indicate trouble. It suggests that the orchid doesn't have enough resources and is demanding resources from the older leaves. One possible reason for that is that the planting medium has degraded and or the roots are damaged and can no longer provide for the plant. Your planting medium may be in need of a change and the roots should be examined.   Even if you are observing this change in the spring, to be cautious check on the condition of the roots and planting medium.

Roots inside the mass are often less healthy as the moisture dries more slowly there. Pull away the moss or bark chips and look for healthy roots that are firm and not slimy or mushy. If most of them seem healthy, cut away the damaged and rotten ones, dust with a fungicide and re-pot the orchid in a terracotta pot with a drain hole.

There are a couple of other things that can cause this sort of leaf yellowing. If the temperature at night is dropping below 60 degrees, that will cause lower leaves to yellow, and check under the leaf for insects that can drain the leaf of fluids and chlorophyll.

Refer to our Guide to Phalaenopsis for more information on re-potting and caring for your orchid.

Courtesy of NYBG Plant Information Service