Mushrooms are the fruiting structures of higher fungi. They produce spores by which the fungi reproduce. The vegetative parts of fungi are made of the thread-like mycelium which grow in or around other organic matter from which fungi are obliged to derive their energy since they lack chlorophyll. Toadstools is a term usually reserved for poisonous mushrooms but is sometimes used for any mushroom by those less knowledgeable.
Very few mushrooms are pathogenic - meaning they cause harm to a living host. The only one commonly encountered is Armillaria mellea which causes the familiar armillaria root rot and various polyspores which produce shelf-like brackets called corks on branches and trunks. The vast majority of mushroom are either Saprophytic, living on dead organic matter, or are Symbiotic, having a mutually beneficial relationship with other living plants. Mushrooms less than two inches in diameter are usually Saprophytes and feed on such things as animal manures, sawdust, bark, leaves, needles, thatch, or other plant debris. By releasing nutrients tied up in these materials, they can indirectly benefit surrounding plants. Symbiotic fungi benefit plants more directly by extracting nutrients and water from surrounding soil, and in many cases manufacturing hormones and other substances for the fungi's use. These are generally greater than two inches in diameter.
Since there are no chemicals registered for eliminating them, control measures, if desirable or practical, depend on the mushrooms living habits. Symbiotic fungi can only be controlled by removing the host plant which is usually unacceptable. Saprophytes decompose dead organic matter in the soil such as wood, leaves, thatch manures or other substances that were once alive. They release nutrients so other plants can use them, thus providing a beneficial function. De-thatching will reduce the growth of mushrooms feeding on thatch and will allow water to better penetrate into the soil.
Other useful techniques are aerification and using wetting agents which help water (and air) get through the thatch and the dense mycelium which also often impedes water movement. While acid conditions do not cause mushrooms, the decaying organic matter produces acidic compounds, thus applying lime will counteract the effect as well as acidic tendencies due to rainfall and fertilizer. It also encourages a better growing conductor for organisms that break down thatch and for the grass itself.
Where mushrooms are undesirable, frequent picking or mowing will remove them from sight and generally reduce their spread to other areas suitable for their growth. Questions as to edibility should be referred to those who can properly identify them.
For more information contact your local WSU Extension Office.