We have a 10 year old Robinia Frisia which came out with brown spotted leaves (normal size) this year. The lower part of the crown is worse, and has hardly grown this year, the upper part has grown branches and is also affected, but not as badly. We suspect this is a fungal infection "Phloeospora robiniae". A good description is given on The Telegraph newspaper website on 04/09/2008, and of course on the RHS website. For a very good picture, type the disease name into Google and scan down to see "images for...".
What to do?
Would a pesticide treatment help? If so, which product(s) and when?
Other than that all we could find is suggestion to prune the crown to shape, and thin it to allow drying airflow, and to clear up debris.