@0soyoung
I have to respectfully disagree with you about fungus. Fungus infections are NOT permanent in all cases. In fact a large percentage are transitory. They come, they go. It is fungal species dependent, so without a proper identification of just which fungus we are dealing with, It is difficult to say anything with certainty.
Plants do not have an immune system in the sense of a mammalian immune system, but they do respond to infections and can indeed fight off infections by mechanisms completely different than a mammalian immune system. Plants can exude certain carbohydrates, plants can alter their internal pH, plants can wall off infected areas with cellulose, they can encourage other non-pathogenic species of fungi to colonize their tissues in order for the non-pathogenic fungi to out compete the pathogenic fungi. Similar with encouraging beneficial bacteria to attack the pathogenic fungi. The whole system is not very well documented, but the ''body of knowledge'' is being developed.
The Blights, such as Chestnut blight, and Dutch elm disease are 2 fungal infections that indeed are pretty much permanent, once infected it is difficult or impossible to eradicate them. But these are the extreme cases.
Other fungi are ubiquitous in the environment. The ''Black Tar Spot'' of maples, only infects the leaves. When the leaves fall in autumn, the tree is essentially free of tar spot. But the tar spot lives in the soil over winter, spores are blown into the leaves of the trees in late spring, or early summer, and the trees become infected again. Only way to break the cycle is to move the tree to an area where the soil is not infected.
Systemic fungicides such as Cleary's 3336 and Daconil will clear the tissues of the trees of the fungi they target. They will not repair old damage, but there will be no new damage once the fungicide has done its job. This is why they are the go to fungicides for pine needle cast fungi.
Those points above are my areas of disagreement. I won't belabor the points, I don't have the time to dig up references.
This portion is mostly to the OP,
@mdsai
More often than not, when I see fungal infections they are secondary, to an insect pest attack. Insects such as sucking insects like aphids or mealy bugs bite and leave wounds on the undersides of the leaf, then opportunistically a minor pathogen fungus gets in these open wounds and spreads a small zone of brown or black rot. Often if you clear up the insect pest, the fungus will not progress. I do like the idea of removing all the damaged leaves. With maples, this time of year, even small seedlings, a partial or even complete defoliation will not kill the tree. A new set of leaves will develop and they should be clean if you got rid of the insect. Always dispose of diseased leaves in a plastic bag and have it hauled away. Don't just drop them on the ground where they can serve as a source to inoculate the next round of leaves.
Pathogenic fungal infections can largely be avoided by altering the environment around the tree. If a tree seems to be picking up a fungal infection try moving the tree to a position with better air flow and more light. Trees on the ground should be put up on a bench or shelf. Don't crowd trees together, make sure there is enough space for a breeze to get through. Sun is excellent at discouraging fungi. Morning sun and late evening sun for otherwise shade loving plants is helpful. Full sun plants will more easily fall prey to fungi if grown with too much shade.
Hope these thoughts help. I would check and spray your little maple for insects first, then worry about fungus later, as I strongly suspect your problem started with insects on the underside of your leaves.