Wild mushrooms can be extremely poisonous to animals and humans, especially if you do not know what type of mushroom was eaten. Your dog should be taken to a veterinarian immediately. Take a sample of the mushroom eaten (in a wet paper towel or a paper bag) and if your dog has vomited, take that with you, too. Signs of mushroom poisoning can be diarrhea, lethargy, or jaundice. Other effects of mushroom poisoning can include unusual changes in behavior such as increased barking or chasing imaginary flies, gastrointestinal discomfort, liver failure, and even death. Onset of symptoms may be delayed, but it's important that your dog get medical attention as quickly as possible.
To prevent mushroom poisoning, you should eliminate mushrooms from your yard. It's best to pick them out of the ground and safely throw them away. If the mushroom is mashed into the ground, the spores are still present enabling more mushrooms to grow.