I have large areas (over 300 sq/ft) of pachysandra that has died. It appears to be scale (see pictures). There are other areas that have not yet died but appear to be under attack. This is the first time in over 30 years that I have ever encountered this problem. Will the pachysandra recover on its own or do I need to treat these plants? If so, what do you suggest and when should it be applied?
When stressed by poor site conditions, poor soils, environmental issues, etc. plants can be susceptible to disease and insect problems.
Pachysandra can be subject to a fungal disease and euonymus scale, a sucking insect. This looks like euonymus scale. Pachysandra grows best in moist well drained soils in partial to full shade. It will be stressed in the sun and will continue to decline. You did not mention the location. If in sun, you may need to select another groundcover for the site.
Be sure to clean out all old leaf debris and thin the plantings if too crowded. This fosters poor air circulation and fungal issues. Monitor for scale in all the beds.
Euonymus scale - This is a sucking insect that can cause decline and looks like a severe infestation. You will have to prune back the plants severely affected and clean up any old debris. Monitor for crawlers in May through June and August through September. Wrap double sided tape on the stems and look for crawlers. When you see them spray with a horticultural oil at the summer rate. You will need to get good coverage. http://extension.umd.edu/hgic/armored-scales-groundcovers