Yes. If your dog has access to a large amount of crab apples and eats them whole, or if they eat the leaves or bark of a crab apple tree, it is toxic to them.
That’s because crab apples, like ordinary apples, have a compound in their seeds called cyanogenic glycoside, which turns into cyanide when digested. And we all know that cyanide is toxic to dogs and people. But that’s not the whole story. Here’s the rest of what you need to know.