Its hard to see clearly, the picture doesn't magnify well, but on current showing, if the yellow areas aren't deposits of some sort, and there's no question if it being weedkiller damage, I suspect its a viral infection. If it is, there's nothing you can do about it, no treatment. If you have other dahlias, it's probably best to remove this one and dispose of it away from your compost heap. If you don't have any others, then leave it be - it looks to have some flowers about to come out, but don't save it for use next year.

Thanks Bamboo. I have cut off the diseased bits and hope for the best!