According to WebMD, plums are dangerous to dogs because dogs sometimes eat the pits. In a small dog, the pit has the potential to cause an intestinal obstruction. In addition, pits contain the poison cyanide.
WebMD identifies other fruits that dogs must avoid. Persimmons and peaches both have pits that have the potential to block the intestines. Cyanide is also present in peach pits. Grapes and raisins cause vomiting and, more seriously, kidney failure.
Some fresh fruits do make good dog treats, WebMD explains. In all cases, though, seeds, stems and leaves must be removed to prevent bowel blockage. Healthy choices include slices of banana, apples, oranges and watermelon.