Mycorrhizal fungi are ancient plant mutualists, i.e. they are engaged in a mutually beneficial physiological relationship with plants. The mycelia (the thread-like filaments that make up the vegetative, below-ground phase of mycorrhizal fungi) grow in close contact with plant roots, often fully enveloping them. The mycelia absorb minerals like phosphorous and nitrate from the surrounding soil which become available to the “host” plant ( usually a tree or shrub)and in exchange obtain organic carbon which the fungi then metabolizes for energy. The carbon can be traced back to the host’s foliage which has converted atmospheric carbon dioxide to organic carbon compounds by means of photosynthesis. Myc are widespread and form associations with most terrestrial plants including crops. One estimate is that perhaps 80% of all vascular plant species are mutually involved with mycorrhizal fungi! The mycorrhizae enable plants to colonize nutrient-poor soils that would otherwise be less favorable for plant growth. Most of our crops also benefit from mycorrhizal fungi thus incorporating land areas into cultivation that would be otherwise inadequate for human food production. Interestingly mycorrhizal fungi generally do not occur independently of host plants and in fact, some are found only in association with a single tree species.
Mycorrhizal fungi produce basidiomata (“mushrooms”), a cap (pileus), or fruiting bodies) like some other kinds of fungus. These can be found in walks through the woods or even in yards and fields. Some species are edible while others are poisonous to humans.
This group of fungi inconspicuously carries out the vital function of recycling of nutrients through the decay of wood and other plant tissues. Here is a photograph of a colony of Armillaria tabescens. a gilled mushroom that sprang from an expansive growth of mycelia that sprang from underground roots of a red maple in our back yard. Unfortunately for the maple, the mushroom is an aggressive invader of living and dead trees, and can be an important agent in the decline and mortality of forest trees. In cases where the fungus attacks living trees, they would be designated as parasitic rather than saprophytic. The host tree (not visible) is in decline, with dead main branches, a well-thinned canopy and sloughed-off bark at base. Downy and Red-bellied Woodpeckers are frequent visitors and the former species successively (and exhaustively) excavated a nest-hole in a secondary (dead) branch. The tree has hosted multiple colonies of Antennaria tabescens for at least 5 years, but likely was under siege via root-rot, for longer.
Other fungi such as the oyster mushroom have advertised their presence through their mushrooms but only the Armillaria ‘root-rot’ has appeared predictably every year. The Armillaria colonies occur mostly in autumn, quickly passing through the above-ground phase in a week or so. They are first seen in a small “button” phase, although undoubtedly closer inspection (hands and knees) would have revealed even smaller phases. Several days later the button stage has expanded into fully opened nearly flat caps with exposed gills lining the underside of the cap. Along the gill surfaces are lined countless small, microscopic cells, each bearing four spores capable of developing in to the next generation of fungus.