Aside from the ripened pulp found around the seeds of this plant, all of its parts contain cyanide and are toxic to dogs. Signs of poisoning include trouble breathing, gums that turn bright reed, shock, dilated pupils, and even death. The prunus serrulata 'Gioiko' is at its most dangerous to dogs when it is in the process of wilting.
If you think there is a risk your dog has ingested this plant, consult your veterinarian as soon as possible.