This looks like Volutella Blight, a very common fungal pathogen that can damage Pachysandra. The linked page provides more information. Suppression of the disease essentially entails mowing-down all Pachysandra growth in the affected area, raking-out debris to improve air circulation (and to remove spore-laden material), and letting the patch regrow at its own pace. Be sure to continue monitoring the area closely for watering needs, since the ongoing drought is stressing and hampering growth on many plants, and if Pachysandra is growing under a tree, the root competition will be substantial for what little moisture is present. There is no fungicide recommended for this condition, nor would use of one be possible at this point due to disease stage and the high heat.

Miri