My screen is kind of small, but it looks to me like decay that set in after the branches died--maybe not what killed them at all.

Without more info it is hard to tell, if it crushes when you touch it, it is the aphids, if it can be dusted off a fungus, if you have to scratch it off it is most likely scale Fungus and scale can both be fatal but can be treated,

They're called snowy or woolly aphids. They're hard to exterminate. I recommend you spend $10 or so, and buy a few thousand ladybugs (technically not a bug; but also called a "ladybird beetle" )

[amazon.com]

Aphids

Aphids

Could be mealy bugs...if you touch them, do they move? You can order ladybugs as a natural way to control them...

When did you prune them last and how old are they? From the little I can see it seems to be dead wood from lack of pruning. I can't see it well enough to give you a simple answer but if they haven't been pruned it's easy enough read up on how and when to prune them. If that's the problem.

It's powdery mildew. Its a fungal disease. I've had it on a few of my plants this year. Water the plant well under the foliage then mix 1 tablespoon of baking soda, 1/2 teaspoon of any liquid soap into a gallon of water. Then put in a sprayer. I also use a tablespoon of horticultural oil if the fungus is real heavy as that helps the mixture to stay on the leaves and plant stems. You have to turn leaves over and be sure to coat the top and bottom of the leaves. It helped my plants. I started watering the plants with my hose right at ground level keeping the water off the leaves and stems. That seems to help.

Maybe Lichen - because we used to see that growing in the forest behind my house - on trees that were damaged - but on some that were healthy? [namyco.org]