Black spot infection by the sound of it, but you haven't said whether these are hardy fuschias planted in the ground or tender ones just for summer use. If the latter, its a bit late to worry about black spot, you'll be ripping them out in the next three weeks or so to put in winter bedding...

If they are hardy ones try to collect the fallen leaves and burn them in case the infection remains on the ground until next year. Leave the bare branches until next April and then cut them to the base when (and if!) new growth appears from the bottom.