Either Volutella blight or the beginning of boxwood blight. There are various cultural reasons for the first (a fungal pathogen) and it can be treated but no controls for the second other than removal.

@gardengal48 (PNW Z8/9) If it's the second, can I plant the same pachysandra here after removing the dead one?

Is there any landscaping company good at diagnose and treatment? All landscapers I met did not seem to be clear about the cause, and I'm not confident that they can do the job well. Thank you.

Landscapers landscape - they dig, plant and build. Rarely will you ever encounter those skilled enough for plant diagnosis or treatment. It is just not in their brief or skill set.

Take samples of the dead, diseased and healthy plants into your county extension service. They have access to plant pathologists that will be able to discern what exactly is the problem and recommend a treatment or replacement option.

I'm sure any landscapers will be happy to remove and replant. That IS their skill set!

@gardengal48 (PNW Z8/9) Thank you for the information on the extension office, and I'll try that. Besides the extension office, do you know any type of company also doing the diagnosis plus treatment? I'm trying to hire someone to do the whole job if possible, because it's easy to manage as the hired takes the sole reponsibility. Thank you very much.