Answer: Boxwoods are susceptible to nematodes and some fungal wilts of the soil. Your description sounds like either of these problems. There is not practical remedy for either problem.
To determine if one of these maladies is the cause, carefully dig up or wash out some roots of the sick plant and examine them for swollen areas indicating root knot nematode infestation. Then scrape back the exterior of a major root at the base of the plant and look for darkened interior streaking, a sign of the fungal infection.
If either is found, the others will likely succumb in time. Therefore, it may be better to consider replacing the shrub with another plant that is not susceptible.
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