Good question, Ethel. It’s always a good idea to check first! The seeds, leaves and stems of the crabapple species contain substances known as cyanogenic glycosides, which can cause vomiting, diarrhea, a lack of oxygen in the blood, a decreased heart rate, respiratory congestion, seizures and coma. A large enough ingestion could ultimately lead to respiratory failure and even death. Typically, we do not see any problems beyond gastrointestinal irritation in small animals who consume a few pieces of the plant material. Severe clinical effects usually occur in grazing animals such as horses or other livestock, who consume large quantities.
The bottom line? A nibble or two of a fallen crabapple is not likely to pose problems beyond minor stomach upset. Just don’t let your dogs eat them in quantity or allow them to consume the leaves, stems or other parts of the tree.