I think its one of the Spiraeas, possibly S. Goldflame or Magic carpet (depends on height and spread). It would appear its suffered an attack of powdery mildew which hasn't been treated, and this is the result. I can see new leaf growth starting which looks quite healthy so far. These plants like a fair amount of sun, so if yours is in a shadyish spot, or somewhere it doesn't get a good half a day's sun, if you can move it somewhere more appropriate, then do, otherwise, cut it back or clip it over after its flowered and let it regrow. If it gets mildew again, spray with 1 part cows milk to 1 part water, spray thoroughly all parts until run off.

Homebird - in America, they use a higher ratio of milk to water, lots of experimentation going on, but I'm advised 1 to 9 is best. Also in America, they seem to be using it on downy mildew and wondering why it doesn't work , which I'm guessing might be why they're experimenting - but it won't work, not on downy mildew, its a different beast altogether....

Nellie61 - I've just noticed a mistake in my answer to you - the milk remedy should be 1 part milk to 9 parts water, not one to one...how I managed to type 1 when I meant 9 I've no idea...