Hello, I just noticed a few plants in my backyard have entire branches of leaves turning brown. I looked more closely and most of the branches on those plants are now bright white. Looks like someone dipped the branches in a big bag of flour. None of the leaves are white. Just the branches.
Is this a plant disease or a fungus of some sort? If so, know what it's called? Is it dangerous to the plant? Should I at least cut off the branches with the dead leaves? How do I get rid of it?

Looks like a serious infestation of scale insects.

Or maybe mealybugs but the key word here is 'serious'.

You need to prune out all of the affected branches to allow moe light and air around the plant

Mealybugs, huh? Is the white powder something the bugs produce? The dust is really thick on some branches, while the others are clear.
Is cutting the branches off the only solution? There won't be much plant left if I do that to all the bushes.

That does not sound like mealy bugs. I don't know whether you can just wash off scale, but I would try.

Horticultural oil application is often suggested as a relatively non-toxic way to smother this type of insect/infestation. No need to cut off branches if you choose to do this method.

Baltimore has a strong horticultural presence. I bet if you took a sample of one of the infested branches to your Cooperative Extension Service office, you could get a positive ID as well as normal recommendation for dealing with it properly.