Though not an expert for Indian insects and plants (which plant species is this?), it looks like galls. They could be induced by mites or insects (e.g. Diptera: Cecidomyiidae or Hymenoptera: Cynipidae, and others) . Bacteria and fungi are possible too, but look different usually. Anyway you if it's from arthropods you'll find mites or insect larvae within the galls, when you dissect them. They also have sometimes an opening at the underside which can be concealed and tiny.

Though not an expert for Indian insects and plants (which plant species is this?), it looks like galls. They could be induced by mites or insects (e.g. Diptera: Cecidomyiidae or Hymenoptera: Cynipidae, and others) . Bacteria and fungi are possible too, but look different usually. Anyway you if it's from arthropods you'll find mites or insect larvae within the galls, when you dissect them. They also have sometimes an opening at the underside which can be concealed and tiny.

Just quash or cut one of the galls.swelling mass, and watch the presence of larvae-may be under class insecta

Infestation of Gall insect

I agree with Peter. Leaf galls are caused by very tiny gall-making insects (Family Cecidomyiidae) when the adults lay their eggs in the leaf tissues. Certain chemicals introduced during the process stimulate the tissues to develop abnormally, thus forming the galls seen on the leaf (above left). When the eggs of these gall-making insects hatch, the larvae feed on these nutritive gall tissues. The leaf gall provides an almost perfect place for these larvae to escape predation.

I agree with above that it is due to insect not disease. These are sort of galls and i am sure that these are by midges. Normally in mango called as mango midge infestation. 

Gall are also produced by psyllids. I've seen very similar galls on avocado leaves, caused by Trioza aguacatae (see image). The image was taken from http://biocontrol.ucr.edu/hoddle/trioza/trioza.html

Try dissecting one under the microscope, take a photo and post it. If it is an eryophyid mite, you may not see anything unless you use a high magnification.

The gall formation is due to insect not by any pathogen. It found in any plantation or fruit crop. We found here in mango, pipal etc trees

Occurs on many perennial dicots -- globally.

Alstonia scholaris (Apocynaceae) plant infested by a gall inducing psyllid- Pseudophacopteron tuberculata

seems to be gall midge insect infestation.

Dear Prosenjit Pramanick

These are "leaf galls of Alstonia scholaris" caused by an insect Pauropsylla tuberculata.

Please have a look at these useful links.

Article Morphological, anatomical and biochemical studies on the fol...

Article Seasonal occurrence of Pauropsylla Tuberculata crawford (hom...

http://www.indianforester.co.in/index.php/indianforester/article/view/126623

https://www.omicsonline.org/sds-page-analysis-of-leaf-galls-of-alstonia-scholaris-l-r-br-2157-7471.1000121.pdf

http://www.ijprb.com/vol%2008%20(2)/10%20%20pankaj%20kumar.pdf

http://anubooks.com/wp-content/uploads/2018/02/voyager-dec-Vol-VIII-2017-7.pdf

http://agriinventionjournal.com/wp-content/uploads/journals/journal1.1/Studies%20on%20the%20histomorphology%20of%20leaf%20galls%20on%20Alstoniascholaris%20Linn.%20(Apocynaceae%20)%20induced%20by%20PauropsyllatuberculataCrawford.pdf

http://krishikosh.egranth.ac.in/bitstream/1/5810001068/1/M.V.Sc.%20Thesis.pdf

Good luck!

Not a disease- it is a condition "GALLS" Caused by psyllids ('jumping plant-louse"), could be midges or any other gall causing insect.

They lay eggs on the lower side of the leaves, egg hatch into nymph stage, which cause galls.

Solution- insecticide (not fungicide).

Dear Doctors who are answered to Dr. Hanif, Yes, for me it is not caused by a gall-midge but by one Psyllid. I work on galls produced by many groups of insects. But not in India; in France. My opinion is Hemiptera Psylloidea because some of you has identified the plant as Mangifera indica (mango). Sincerely yours, Dr. André Panis, Associated Scientist at the French National Research Institute for Agriculture

Insect induced foliar galls of Alstonia scholaris (Devil's tree) growing on the Banaras Hindu University main campus, India.

Seems insect infestation. These publications may help.

http://baloun.entu.cas.cz/png/PercyEtAl2015.pdf

https://is.muni.cz/repo/1339360/Malenovsky_et_al_2015_Aspidosperma_Pseudophacopteron.pdf

http://www.ijprb.com/vol%2008%20(2)/10%20%20pankaj%20kumar.pdf