Quince blight, a nasty fungus that travels in on the wind. It is endemic to many areas and difficult to cure. Sometime a topical fungicide will work, but mostly it is a case of making the tree vigorous enough to resist real damage. If the tree is growing then I wouldn't remove it.
Clearing up dead leaves in the autumn and burning them is supposed to help against reinfection.

We get this despite giving the tree a real good drowning with systemic fungicide. All the leaves eventually drop off, but it then grows a new set which do not suffer in the same way. Never get any fruit though.