Question
my very large and old dendrobium has developed clear dots all over the leaves along the spike and the leaves are now turning yellow. There are flower spiles ready to open. The orchid is hanging in my oak tree where it has been very happy for many years, I really am concerened because this orchid has been with me for many years.

Answer
Cheryl, it sounds as if sucking insects have been attacking your leaves. You will need to examine the plant to see if you can find any suspicious insects.  The most common insects that attack orchids are scale, mealy bugs, and aphids and thrips. The leaves that are turning yellow will eventually fall off-- but that shouldn't damage your plant and it should still flower unless you have a serious infestation that starts to attack the flower buds. In addition to inspecting your plant, I recommend you spray it with a systemic insecticide such as "Merit" or "Bayer Advanced Garden".  Try to avoid spraying the buds or flower spikes. In order to work the insecticide must get absorbed into the plant tissues and that will take  a few days.