Fungus is a rather common affliction, or benefit, in household plant soil. Many plant potting soils come with fungus already in the bag, which spring to life as soon as the first watering of the plant is performed. Not all are harmful, but some are.
Many mushrooms appear in potting soil because soil-less mixtures have been contaminated by fungus. However, some fungus do appear in plant soil because of airborne spores, or spores from mushrooms that come off of clothing.
The most common indoor soil mushroom is the Leucocoprinus birnbaumii, or yellow houseplant mushroom. These mushrooms are not known to harm the plant, instead feeding off of dead material in the soil. Leucocoprinus birnbaumii are not poisonous, although eating many of them may cause stomach problems. Coprinus are also common and look quite similar to the yellow mushroom.
Removal of benign mushrooms is not a simple task. The simplest way to get the mushroom out of sight is to remove the caps from the soil. However, this will not kill the mushroom as mushrooms grow under the soil; the caps are only the manifestation of the fungi fruit. Scraping a thin layer of soil off the top may kill the fungus, but even this is not likely. Changing the soil can be effective, as long as the pot is soaked in a 10 percent bleach-water solution for a half an hour and the roots of the plant are rinsed.
Some fungi will infect the plant and cause great amounts of damage if not controlled. Blackened or wilted leaves can be an indication of a fungal infection of the plant. In extreme cases, fungi can rot the roots of the plant, making the plant unable of absorbing water and nutrients of the soil. This is hard to control and a plant that has begun to show symptoms is often too far gone to be saved.
In the case of harmful fungus in the soil, a fungicide can be applied to control and kill it. An all-purpose fungicide can be purchased from your local garden center and applied directly to the soil according to the instructions. Fungicide may not kill the fungus right away if the fungus has spread throughout the soil. New applications every 7 to 10 days will help solve the problem.