From my understanding, some plants acquire these when the leafs stay too moist for too long in shady conditions like over crowding in big box garden centres where they cram larges trees in small over lapping spaces.

If its not a huge issue with you, pluck the few affected leaves if there isn't many, and watch to see if its spreading. I would also wait and see if after fall when it loses its leaves if it returns next spring. Could just be a seasonal issue.