Good evening Kathleen,

There are several reasons why the leaves turn yellow & die off, which include soil with poor drainage or too much sun.

It appears that your Pachysandra have Volutella Blight. You can remove diseased leaves, as well as any leaf debris, and prune back or dig up plants with stem infections. Be sure to prune only when plants are dry to minimize the spread of the causal fungus. When disease is severe, remove diseased plants, then follow with one or more applications of a broad-spectrum fungicide (e.g., one containing chlorothalonil).

To be sure though, I'd recommend contacting & sending samples to the MSU Diagnostic Lab for confirmation. https://www.canr.msu.edu/pestid/

I have included a couple links:

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/insects-pests-and-problems/diseases/fungal-spots/volutella-blight

https://hort.extension.wisc.edu/articles/volutella-blight/

A link for pachysandra care:

https://hvp.osu.edu/pocketgardener/source/description/pa_nalis.html

Good luck & have a great day!