Today 
The fungus Rhytisma acerinum overwinters on fallen 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.
Solution
Organic
Rake up and dispose of all fallen infected leaves.
Chemical
As a preventative measure use a recommended fungicide, containing either a Bordeaux mixture or copper, and spray before signs of infection and repeat at 10-15 day intervals.
