The primary cause of damage appears to be old Boxwood Leafminer feeding injury, given the blistered and discolored foliage. There is some Boxwood Mite damage as well on older leaves, but it is minor and generally not a pest needing control. You can use the information on our boxwood diagnostic page to look for other conditions that might also be present, as well as to learn about management for leafminer, which is a very common boxwood pest. To support overall boxwood health, the plants should not be sheared (when pruning is desired, it should be more selective, only trimming the branch tips needed to maintain a symmetrical shape), since this can reduce air circulation within the plant's canopy and make it more vulnerable to pest or disease outbreaks. We mention this because many gardeners shear their boxwoods, as this plant appears to be. Stressful conditions from the planting site itself -- reflected heat or reduced air circulation from the closeness to the wall and pavement -- can also contribute to vulnerability to ailments.

For now, little intervention is possible save for perhaps using certain types of insecticide labeled for leafminer control, though pruning out the damage would essentially reduce the current population of those insects well enough for this year. Granted, it will take the plant some time to regrow from that much foliage removal, though it will eventually be shedding those damaged leaves anyway. Keep monitoring it for watering needs as we move into summer (making sure it's also not being inundated from runoff from a nearby roof downspout outlet) to keep stress levels low in case of drought or high heat.

Miri