holly bushes dying one by one
I don't know the variety of holly. Previous owner planted it about 19 years ago. Last summer one bush started turning brown branch by branch and soon was completely dead. At the time I suspected something had destroyed the roots, or maybe the roots were too wet from all the rain. But then the one next to it died later in the summer, and now the one next to that is dying too, during a drought. Others in the same area still look healthy. Those are all in the back yard in an area that is generally moist (not this year of course) and gets part sun, but now another one in the front (full sun) is dying too. I'm attaching a picture of two branches from the same plant in different stages of dying, and two pictures of one affected plant in context with its healthy neighbors.
We do not see evidence of a particular insect or disease problem. It's likely that what you see is the result of roots damaged by flood (drowning) last year, then drought this year. We had almost twice the normal amount of rainfall last year. This drowned many root systems and has continued to kill plants slowly this entire summer. The hotter and dryer it is in the summer, the more plants struggling with damaged roots get knocked out.
Prune out the dead branches. They will be stiff and there will be no green under the bark when you scratch it.
Early next spring, you can try to rejuvenate the partially dead shrubs by cutting them down to just a few inches from the soil. You'll see whether they start sending out new shoots. This means a decent root system has survived. If they still look bad, replace them.
Meanwhile, water deeply any shrubs you want to survive. Fall droughts often kill evergreens especially. Do not let them go into the winter in dry soil.
Though we can't see the first photo clearly, it may have some white bumps on the bark. Not sure if that's what we're seeing or not. If so, please take an in-focus close-up of them, and send.
Your photos were not that clear. We think the white growth may be a type of armored scale which is a sucking insect. There are seveal types and we cannot identify the type from your photos. Here is our website for more information. https://extension.umd.edu/hgic/topics/armored-hard-scale-trees-and-shrubs
Heavy infestations may cause leaf yellowing, stunting, and dieback. Scale insects can be difficult to control. We recommend that you prune out the dead and the worst affected branches now. In late March to early April you can apply a dormant rate of horticultural oil. Follow label directions. You will have to monitor the growth next season and look for more symptoms. Typically, eggs hatch in spring or summer. The stage after the eggs hatch is the immature stage or commonly called the 'crawler' stage because they walk about on plants to find new feeding sites. This is how a scale infestation spreads. You may have to spray a summer rate of horticultural oil during the growing season
to control the crawlers. You can send us more photos if you notice symptoms next season. You also may decide to replace the shrub.