Armillaria root rot or shoestring root rot is caused by Armillaria (Armillariella) mellea, a common and damaging soilborne fungus worldwide. Armillaria is used loosely to refer to a group of about 20 genetically distinct fungal species that can be distinguished most readily using serological techniques. Common names for this group include oak fungus, shoestring root rot, honey mushroom, and honey agaric. The latter two refer to the color of the mushroom fruiting structure of the fungus that can sometimes be seen at the base of infected trees.
Armillaria root rot is widespread in the relatively heavy soils of the cooler parts of the temperate zones in the United States and Canada, particularly in the Pacific Northwest. The fungi attack about 700 species of mostly woody plants. Herbaceous plants that are susceptible include blackberry, flowering bulbs, potato, raspberry, and strawberry.
Armillaria mellea - bark
peeled back showing rhizomorphs. Among shade and ornamental trees, oaks and maples are the ones most commonly infected. Other woody plant hosts include azaleas, beeches, birches, black locust, boxwoods, cedars, currants, dogwoods, Douglas-fir, elms, firs, golden rain tree, hemlocks, hickories, hophornbeam, Katsura tree, larches, lilacs, mountainashes, pines, planetrees, poplars, privets, rhododendrons, roses, sassafras, spruces, sycamores, tree of heaven, tuliptree, willows, yews, and many fruit and nut trees. The Armillaria fungi may infect many other kinds of woody plants if conditions are favorable for infection. Table 1 lists ornamental, fruit, and nut trees and shrubs that are adapted to Illinois and their relative resistance or susceptibility to Armillaria root rot.
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http://www.ipm.uiuc.edu/diseases/series600/rpd602/
Woody plants that have previously been weakened by drought, flooding, poor drainage, frost, repeated defoliation by insects or diseases, other poor soil conditions, excessive shade, polluted air or other chemical injury, or mechanical injury are most susceptible to attack. The loss of fine feeder roots from this disease deprives affected plants of sufficient nutrients and water, and often results in branch dieback and staghead. The fungi can be of considerable importance in the final death of weakened trees and shrubs. Serious radial and terminal growth reduction of affected plants may occur. The fungi are also responsible for butt rot in some species of trees. In fact, Armillaria mellea and other species have been identified as having a significant secondary role in disease complexes such as oak decline, maple blight, and ash dieback.
Armillaria is commonly found in most forest soils, so the disease may occur in forested areas or areas that were previously forested. Diseased trees may be found scattered throughout a forest stand; or infection centers composed of one or several declining trees may be scattered in the stand
