What you have here is not a disease. There is a very small insect that is endemic to your particular plant/tree. It arrives when the leaf is just unfurling. (Endemic in this usage means that each plant or tree species has a type of insect that feed off it during sometime in its life cycle) Female insect lays its eggs on the underside of the leaf and then leaves for another leaf to lay more eggs and die. As the babies develop they build a protective house, if you will-your pimples, to develop into an adult. Then the cycle starts over again. Eggs, hatching, growths and hatching, flight and etc. this is not harmful to your tree and doesn't need expensive treatment. Treatments many times don't work anyway because of insect timing issues. Best advice here is to keep leaf litter picked up as that contributes to the insect problem in the future. Thank you for using the Ask an Expert Systym.

Dear Mr. Jim,
I thank you very much for the feedback. It was quite helpful in identifying the cause of the problem. I would like to know if Neem Oil sprayed on the trees be one of the  solution to the problem.
Regards
Santhosh

Hello Mr Santhosh. Generally your tree's problem is mostly cosmetic and shouldn't harm the tree other than appearing unsightly because of the insect homes on the leaves. The leaf that you have in the picture appears to be from a tropical or sub-tropical variety of plant. The difficulty in treatment comes with timing of treatment to get the adult insect that is causing the issue. Neem oil certainly can't hurt the tree. You might try treating the tree with a Systemic Neem product that is fed through the roots. This type of systemic product goes from the roots to the entire rest of the plant then when the insect bites or feeds on the plant it dies. Follow the directions on the container so you won't harm the tree while trying to help it. Thanks again for using the Ask an Expert system.