Explanation: Watermelon without seeds or rind is safe for dogs! Lemons and limes, however, can be toxic their skins contain a substance called psoralen that can cause gastrointestinal symptoms like vomiting and diarrhea. In more severe cases, psoralen can cause muscle tremors, difficulty walking, liver failure and death. Rhubarb should not be eaten by dogs either. Rhubarb contains oxalates, which can cause problems to the digestive tract, nervous system and kidneys. It can also lower the calcium levels, causing renal failure and other issues. Lastly, avocados should not be consumed by dogs because they contain a very poisonous toxin called persin. Persin is found in all parts of the avocado (the fruit, pit, and bark) and can cause fluid accumulation in your dog's lungs and chest. This can result in breathing difficulties, oxygen deprivation and even death.