Cryphonectria parasitica is a member of the Ascomycetes group of fungi, many of which are parasitic, including Dutch elm disease and oak wilt fungi. The same fungus that attacks the Ozark chinquapin also devastated American chestnuts and is mildly pathogenic on Allegheny chinkapin and live post and scarlet oaks. C. parasitica can infect any part of the trunk or limbs, gaining access into the tree’s living bark tissues through wounds.

A common entry point is at a branch node where the constant sway and growth of the limb causes splits in the bark. Another entry point is wounds made by insects. Once the fungus penetrates the bark, filaments that are threadlike in appearance fan out through the tree (B). A raised (D) or sunken (C) canker is formed. When the infection reaches down to the vascular cambium and functional xylem and phloem, transport of nutrients and water are cut off to areas above and below the canker, growth is restricted, leaves turn brown and eventually, the stem/trunk above the canker dies. When the fungus prepares to reproduce, it erupts through the older portions of the canker as bright orange or yellow fruiting pimples called stromata (A). Each is the size of a large pin head. Two types of spores are produced in stromata: sexual spores called ascospores, which are forcibly ejected from black, vase-like structures called perithecia, and asexual spores called conidia, which ooze out of round, fruiting bodies called pycnidia after rains. Conidia can hitch a ride to their next victim on the bodies of birds and insects or be carried in water droplets, while ascospores are wind-borne.

Because the fungus has a mixed mating system, it is able to both self-fertilize and outcross. A mature Ozark chinquapin that has become infected may have one or many cankers deforming its bark. However, because the fungus does not affect the root system, the long-lived Ozark chinquapin can produce sprouts from stumps for many years..