Janna Beckerman, Associate Professor, Botany & Plant Pathology Dept, Purdue University
Ganoderma is a group of fungi that produce some very conspicuous 'butt rots,' including the artist's conk fungus, G. applanatum (Fig 1), and the varnish fungus, G. lucidum (Fig 2). Both of these fungi, and many others in the genus Ganoderma, infect the roots and lower trunk (also called the 'butt') of many hardwood trees and even some evergreens, like hemlock, caused by G. tsugae (Fig 3).
Diagnosing Ganoderma root and butt rot is difficult because the symptoms of infection are vague: Leaves of infected trees are often reduced in size, and exhibit yellowing or wilting. As leaves drop, branches also begin to dieback (Fig 4), and the tree continues to decline as decay spreads. This decay results in the wood becoming spongy, flaky, or pulpy, and destroys the structural integrity of trees, making them prone to failure, breaking or uprooting during wind events.
Eventually, fruiting bodies called 'brackets' or 'conks' begin to develop, usually on the butt log (first 6 or so feet of the main stem as it emerges from the ground) or visible roots. Conks are shelf-like, with the varnish conk having a red lacquered top, and the artist's conk being usually buff or gray. Both types of conks have a cream-colored pore layer that is on the underside. The pore-layer releases millions of rusty brown colored spores. This cream colored pore layer can be etched or colored (and gave rise to the name 'artist's conk')(Fig 5).
Unfortunately, by the time these conks are visible, it is usually too late for the infected tree and the damage cannot be reversed. Trees can have branches removed to extend the time the tree can persist, but this is not recommended for larger trees that can pose a hazard to people.