Avocados contain persin which is toxic to dogs. Persin is found in the fruit as well as the seed, leaves and bark of avocados. Keep all parts of the avocado from your dog's reach, as well as any foods that contain avocado. Tomatoes and potatoes are part of the toxic "nightshade" family of plants containing glycoalkaloids and are poisonous to dogs. Green tomatoes are especially poisonous. Dogs can eat cooked potatoes, but no raw potatoes or potatoes with sprouts or green on them.