Based on your photos it looks like volutella, a fungal disease on the pachysandra.
See our website below for photos and more information on managing the beds culturally. Remove/trim back and thin plants/vines with severe symptoms. Removal of fallen debris and clean up of old leaves etc. will also help reduce the amount of moisture trapped and will promote faster drying of the planting bed. This is hard to control in wet weather. It is up to you if you want to consider a fungicide. They protect new foliage only. You would have to spray every 10-14 days until it dries out. Follow all label directions. http://extension.umd.edu/hgic/volutella-dieback-pachysandra-groundcover
Also, check the drainage in the area as Pachysandra can be susceptible to root rots if the area does not drain well. mh