Your plants are showing symptoms of Volutella, a common disease on pachysandra that attacks both the leaves and stems. It is more often on plants growing in sunny locations. It is most severe in overgrown plantings and is also often associated with scale (insect) infestations. The best advice we have is to put a mower on its highest setting and mow over the pachysandra with a bag attached to catch debris or rake up the symptomatic debris and discard it. Avoid overhead irrigation and thin new growth to increase air circulation that will promote faster leaf drying. Anything that keeps the leaves more dry than wet is what you are after. The longer leaves stay wet, the more likely that diseases can get out of hand. Fungicides suppress the disease but do not cure it.
See more information here: https://extension.umd.edu/hgic/topics/volutella-dieback-pachysandra-groundcover