Plant: Syzygium cumini. Appearance: blisters (gall) on both sides. After some time holes appear in the blisters. What kind of disease is this? This exists all year round and is not seasonal.
They're galls and are probably caused by insects or mites. Unfortunately you can't do anything about it I'm afraid (the leaf damage is permanent and irreversible and by the time you notice it chances are that the bug that has triggered the change has already left). The only possible "treatment" for the leaves is to remove them and a possible prevention is to spray oil on the tree in spring, just before new growth appears (to choke the insects). Besides the fact that they're ugly they actually don't pose any threat to the plant and don't impair it in any way.