Grapes are indeed extremely toxic for dogs. Based on what I've read, there's something in grapes which can destroy the dog's kidneys. I get arguments about this from people who tell me they give their dogs grapes all the time with no issues. Well, the amount of grapes necessary to become toxic varies greatly on the specific grape and size of the dog. Why would anyone want to risk that?
For decades I've heard and read poinsettias are toxic to animals. But I have since read that's a myth.