Some wild mushrooms are poisonous to dogs. Dogs are not too picky about what they eat, and very occasionally a dog will wolf down a poisonous mushroom before you can intervene. There are several reported cases each year, with occasional fatalities.
In general, dogs are not attracted to wild mushrooms as food. It is atypical for a dog to eat one (poisonous or not).
Since the majority of mushrooms aren't poisonous, even if a dog does eat a mushroom for some reason, the chances are good that it will cause no harm.
If a dog does eat a poisonous mushroom, it will most likely throw it up or become ill for a short time, followed by complete recovery.
Dogs frequently eat grass, and it is possible that a poisonous mushroom can be ingested accidentally while grazing.
Symptoms of mushroom poisoning in dogs are similar to those in humans. Depending on the species consumed, symptoms can include lethargy, excessive salivation, erratic behaviour, expression of pain (yelping, whining), vomiting, diarrhoea, coma and death. In most cases, immediate vet treatment can prevent death unless a deadly poisonous amanita or galerina is the culprit. Some lepiotas can also cause serious problems if eaten in sufficient quantity.