Bishops weed, Aegopodium podagraria can be susceptible to a leaf spot disease.  We have had a wet summer which can encourage this. The foliage keeps moisture within, and the longer leaves are wet and the less good air flow there is, the better the conditions for disease to flourish.

Remove any affected plant material and any fallen affected leaves to reduce the pathogen load. Prune nearby shrubs and trees, or anything to improve the air flow around these plants, which would facilitate the drying out of the patch. Keep mulch thin and away from the base of the plants. There is not much else you can do.  

You will have to see if it recovers and if not consider planting some native species.
http://extension.umd.edu/hgic/flowers/groundcovers

mh