I have a large established circular bed of pachysandra under a crabapple in my front yard. I expand the circle about 18 inches a year to keep up with the drip line and keep it in proportion. Right now it's probably 15' in diameter.

The pachysandra usually looks healthy and green by this time of year. I say usually because there's some spots of yellowed leaves here and there. More disturbing is a 2.5 area of what looks like compressed and dead plants. We don't have a problem with animals running free in this neighborhood or I'd say a large dog layed there till the plants died.

I plan on removing the plant material from the area and replanting. Any ideas on how to prevent a reoccurance or what's going on?