Fungi are a group of microorganisms that are classified into a separate kingdom than plants and animals. They are abundantly found in a host of environments, and live an independent, dependent (parasitic) or mutually beneficial existence. Fungi are vital in maintaining the proper functioning of the ecosystem by decaying dead matter and releasing vital nutrients in the soil and atmosphere.
Saprophytic fungi (or decomposers) are the largest group of soil-dwelling fungi. They convert dead and/or decaying matter into organic acids, carbon dioxide and fungal biomass (or their own bodies). They decompose hard organic matter, such as wood, by consuming its nutrients and immobilizing it in the soil. Saprophytic fungi are capable of breaking down cellulose, lignin (the breakdown of which releases essential carbon that is used by many organisms) and proteins. They are typically found around dead animals, insects, leaves and fallen trees. Examples of saprophytic fungi include Pleurotus ostreatus (oyster mushroom), Lentinula edodes (shiitake) and Stropharia rugosoannulata (king Stropharia).
Mutualists are a group of fungi that form mutually beneficial relationships (relationships that benefit both species involved) with plants. They typically grow on the roots of plants and help them to obtain nutrients (such as phosphorous) from the soil. Mycorrhizal fungi are a group of mutualists that grow within the roots of plants. There are four main groups of mycorrhizal fungi, including orchid, ericoid, ectomycorrhizal and arbuscular. Almost 90 percent of all plants form mutually beneficial associations with mycorrhizal fungi. Examples of mutualists include fungi of the Zygomycota and Basidiomycota families.
Pathogenic fungi invade a host plant and decompose its living matter. They strip their hosts off vital nutrients, causing the plant to weaken and eventually die. Pathogenic fungi enter a plant either through its outer skin (epidermis) or by growing within its stomata (breathing holes). They either infect a plant and cause it to die, or allow it to live while stealing vital nutrients from the living plant. Plant diseases caused by soil-dwelling fungi include bottom rot, black spot, canker, rust, Rhizoctinia disease, root rot and potato wart. Pathogenic soil fungi are commonly found at the onset of the cropping season. Examples of pathogenic fungi include Pythium, Rhizoctonia, Phytophthora and Verticillium.