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.