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While the apple part itself may not be harmful , the seeds are as well as the leaves and stems. They produce a level of cyanide which can cause serious illness.
Here is what the aspca says about crab apples.
fruit pits such as apple, cherry pits, peach pits, plum and apricot also contain cyanide.
food that are dangerous to dogs. http://www.lacetoleather.com/fataltodogs.html