Armillaria ostoyae

Armillaria ostoyae (synonym Armillaria solidipes) is a species of plant-pathogenic fungus in the Physalacriaceae family. It is the most common variant, in the western U.S., of the group of species that all used to share the name Armillaria mellea. Armillaria ostoyae is quite common on both hardwood and conifer wood in forests west of the Cascade crest. The mycelium attacks the sapwood and is able to travel great distances under the bark or between trees in the form of black rhizomorphs ('shoestrings'). In most areas of North America, Armillaria ostoyae can be separated from other species by its physical features. Its brown colors, fairly prominent scales featured on its cap, and well-developed ring on its stem set it apart from any Armillaria. (Herink, 1973) It is known to be one of the largest living organisms, where scientists have estimated a single specimen found in Malheur National Forest in Oregon to be 2,400 years old, covering 3.4 square miles (2,200 acres; 8.8 km2) and colloquially named the 'Humongous Fungus'. Armillaria ostoyae grows and spreads primarily underground and the bulk of the organism lies in the ground, out of sight. Hence, the organism is invisible from the surface. In the autumn this organism blooms 'honey mushrooms', evidence of the organism beneath. Low competition for land and nutrients have allowed this organism to grow so huge; it possibly covers more geographical area than any other living organism. The species was long known as Armillaria ostoyae Romagn., until a 2008 publication revealed that the species had been described under the earlier name Armillaria solidipes by Charles Horton Peck in 1900, long before Henri Romagnesi had described it in 1970. Subsequently, a proposal to conserve the name Armillaria ostoyae was published in 2011 and has been approved by the Nomenclature Committee for Fungi. This fungus harms conifer trees in the U.S and Canada. This fungus, like most parasitic fungi, reproduces sexually. The fungi begin life as spores, released into the environment by a mature mushroom. Armillaria ostoyae has a white spore print. There are two mating types for spores (not male and female but similar in effect). Spores can be dispersed by environmental factors such as wind, or they can be redeposited by an animal. Once the spores are in a resting state, the single spore must come in contact with a spore of a complementary mating type and of the same species. If the single spore isolates are from different species, the colonies will not fuse together and they will remain separate. When two isolates of the same species but different mating types fuse together, they soon form coalesced colonies which become dark brown and flat. With this particular fungus it will produce mycelial cords also known as rhizomorphs. These rhizomorphs allow the fungus to obtain nutrients from far away. These are also the main factors to its pathogenicity. As the fruiting body continues to grow and obtain nutrients, it forms into a mature mushroom. Armillaria ostoyae in particular grows a wide and thin sheet-like plates radiating from the stem which is known as its gills. The gills hold the spores of a mature mushroom. This is stained white when seen as a spore print. Once spore formation is complete, this signifies a mature mushroom and now is able to spread its spores to start a new generation. The disease is of particular interest to forest managers, as the species is highly pathogenic to a number of commercial softwoods, notably Douglas-fir (Pseudotsuga menziesii), true firs (Abies spp.) and Western Hemlock (Tsuga heterophylla). A commonly prescribed treatment is the clear cutting of an infected stand followed by planting with more resistant species such as Western redcedar (Thuja plicata) or deciduous seedlings. Pathogenicity is seen to differ among trees of varying age and location. Younger conifer trees at age 10 and below are more susceptible to infection leading to mortality, with an increased chance of survival against the fungus where mortality can become rare by age 20. While mortality among older conifers is less likely to occur, this does happen, however, in forests with dryer climates. The pathogenicity of Armillaria ostoyae appears to be more common in interior stands, but its virulence is seen to be greater in coastal conifers. Although conifers along the coastal regions show a lower rate of mortality against the root disease, infections can be much worse. Despite differences in how infections occur between these two regions, infections are generally established by rhizomorph strands, and pathogenicity is correlated to rhizomorph production.