Mushroom (status effect)
From the Super Mario Wiki
Mushroom is a mostly negative status effect featured in the Mario series. It makes its only appearance so far during the events of Super Mario RPG: Legend of the Seven Stars, where it is one of two status effects that changes the form of its victim, the other being Scarecrow. If Mario or one of his allies gets affected by this status effect, he or she will transform into a pudgy mushroom of a specific color (depending on who gets transformed). While being in Mushroom form, a character recovers a small amount of HP every turn, but otherwise is completely petrified until the effect wears off or the character's status is restored. Mario and his friends can get affected by this status effect when they are hit by an attack like Petal Blast. It can be completely evaded if a character wears a status effect negating item like a Trueform Pin or a Safety Badge.
The concept of turning into a mushroom is similar to the effects of Bean Fever featured in Mario & Luigi: Superstar Saga, which turns its victim into a bean. However, while the former is a status effect obtainable during battle, the latter is a disease that is part of the game's story.