It sounds like you are growing the non-native Japanese Pachysandra (botanical name Pachysandra terminalis) and not the native Allegheny Pachysandra (P. procumbens), is that correct? We ask because they might not necessarily succumb to the same issues. The description of the dark green foliage sounds like the Japanese species, especially if it's somewhat glossy and/or remains deep green all winter.

Sharing photos of the symptoms would help us make a diagnosis. Yellowing can be caused by a wide range of stress factors, including exposure to too much sun (for shade-loving species like this), too much root moisture or poor drainage, a high pest population (Euonymus Scale being the main issue for Pachysandra), or infection (Volutella Blight being common for Pachysandra). Root dieback from prior drought, as much of Maryland experienced last year (unless you were watering periodically), can also contribute to yellowing since it reduces the root system and makes the remaining roots more vulnerable to additional stresses.

The solution might be as simple as mowing (probably more like weed-whacking) the patch in question down almost to the ground, raking out any accumulated fallen leaves, and letting them regrow on their own. This is the typical remedy for Volutella outbreaks, since the reduced air circulation in a dense planting that gathers leaf litter (a great mulch material otherwise) can promote conditions conducive to infection.

Did the canopy above the yellowing section thin in the past year or two, such as from having storm breakage or tree dieback remove/kill some branches and reduce the shading of the Pachysandra in the summer?

Miri

You're welcome!