Question
On my dendrobiums, I have seen on both sides of the leaves
small white dots on leaves, which makes plant look old and dry.
I have no seen it on the new growth leaves.
What is this disease?
Help

Answer
Hello Meka,

Those white spots on the leaves of your dendrobium could be boisduval scale.  Try wiping them with a soft cloth dipped in isopropyl alcohol to see if some or all of the white spots disappears.  If it does, they could be boisduval scale or possibly even mealybugs.  But, if the white spots remain, it probably is being caused by something else.  Other possible causes would be chewing insects such as thrips, aphids or spider mites which would leave holes within the leaves.  

If the spots are being caused by any of the insects mentioned above, go to your local garden center and ask for a systemic insecticide for orchids.  Use it per the instructions on the container.  It will require a minimum of 3 applications over a 3 week period to make sure any unhatched eggs are wiped out also.  

Unfortunately, if the spots were caused by chewing insects, the spots will remain until new growths and leaves appear.  

Hope this helps.  Good luck.

Jim Kawasaki
San Jose, Ca.