The fungus Rhytisma acerinum overwinters on fallen acer leaves, then infects the new spring foliage. The first signs are yellow leaf spots that turn into large shiny black blotches up to 1.5cm across. Severe cases can mean the leaves drop early, but the health of the tree won’t suffer.
An outbreak of shiny black blotches, edged in a ring of yellow, disfigure acer leaves in early summer. This can lead them to fall prematurely.
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Rake up and dispose of all fallen infected leaves.