Artist's conk and varnish conk (Ganoderma species): what the sources record

This page describes what four published sources say about the fungi in the genus Ganoderma that grow on trees. It does not identify a fungus from a photo and says nothing about any single tree. All four sources are from the United States. The parent page, wood decay fungi on trees, compares the fungi that have pages here.

Type of rot and hosts

The TCI Magazine article on important wood decay species and the Missouri Botanical Garden both describe Ganoderma as causing a white rot. The garden's page on Ganoderma root rots names oaks, honeylocusts and maples among the hosts, and the other sources add further hardwoods. The University of California integrated pest management pest note on wood decay fungi in landscape trees lists two species, G. applanatum and G. polychromum, which it says was formerly named G. lucidum.

What the conks look like, as each source gives it

The sources describe the fruiting bodies, called conks or brackets, in different words. UC IPM gives the conk of G. applanatum, which it calls artist's conk, as semicircular, 2 to 30 inches wide and 1 to 8 inches thick, brown above and white below, with the lower surface turning dark when scratched, and usually found near ground level. For G. polychromum it gives red-brown annual conks up to 14 inches wide with a varnish-like crust. The Missouri Botanical Garden describes shelf-like conks up to 14 inches wide on the lower trunk and exposed roots, with a white porous underside when fresh.

Where the sources differ

The sources do not agree on how long a conk lasts. The TCI article describes Ganoderma applanatum as a perennial conk and the related sessile species as annual. The Missouri Botanical Garden describes the conk as annual. A reader comparing two field guides can find the same species listed both ways.

What the sources say conks can indicate

UC IPM pest note: says columns of decaying wood can extend as far as 15 feet above and below the conk of G. applanatum, and that G. polychromum can kill a host during a period of 3 to 5 years.
Missouri Botanical Garden: says death can follow in as little as 3 to 5 years from infection, that the rate of decay appears to depend on tree vigour, and that by the time conks are noticed it is too late to reverse the infection.
University of Georgia field report: says these fungi can weaken roots and kill trees, and that they can spread between trees through root grafts.
TCI article: calls artist's conk a moderate to aggressive white rotter that is not a root rotter, so the extent of decay can more easily be found with a sounding hammer, resistance drill or sonic tomography, and says varnish conk, G. sessile, is less aggressive and usually has less decay with it.

For more on how conks are weighed against other signs, see the sources compared on white, brown and soft rot compared and tree risk assessment methods.

Management, as the sources put it in general terms

The UC IPM note and the garden discuss avoiding wounds to roots and trunk base, keeping the tree in good growing condition and, where decay is far along, having the tree examined by a qualified person. They do not describe a treatment that removes established decay. Where a source mentions any chemical use, see pesticide licences by country for who may apply products to trees. For credentials of the people who examine trees, see consulting arborist credentials.

The sources are US extension and trade publications. They do not cover other regions, and species lists differ by country.