Crocosmias will form new corms on top of old, on top of old, on top of old, the originals not rotting or dying away, sometimes for a number of years and if dug, they look like a sort of ringed and layered sausage. For this reason, you may find that though the lower ones are not up to much, those nearest the top are probably OK and I think would be well worth planting. They may not flower or do a lot this season but I'd be surprised if they were dead so worth a go. They are incredibly tough and I had some of C. 'Lucifer' in a dry tray for about 3 years in a shed, before I found them and replanted eventually. Quite a decent clump now.
May be sensible to dip in fungicide or perhaps cut away the affected parts if possible.