Conifer root and butt rot (Heterobasidion): what the sources record

This page describes what four published sources say about one group of fungi, Heterobasidion, that decay the roots and base of conifers. It does not identify a fungus from a photo and says nothing about any single tree. Forest Research is a UK government agency; text adapted from its pages is used under the Open Government Licence v3.0. Contains public sector information licensed under the Open Government Licence v3.0. The parent page, wood decay fungi on trees, compares the fungi that have pages here.

Names, status and hosts

Forest Research says the disease in the United Kingdom is mainly caused by Heterobasidion annosum, once named Fomes annosus, and that a second species on fir, western hemlock and Norway spruce in England is still being studied. It lists the disease as present in the UK and not notifiable. The Southern Forest Health sheet says the species common in the eastern United States is called Heterobasidion irregulare, formerly H. annosum, and Natural Resources Canada names Heterobasidion occidentale for British Columbia. So the same disease sits under three scientific names in these sources. Forest Research, Southern Forest Health and Natural Resources Canada each also give the older name Fomes annosus.

UC IPM says several host-specific strains can infect almost any native conifer in forested landscapes and that almond, apple, madrone, maple, oak and poplar are uncommonly infected. Natural Resources Canada lists Douglas-fir, Sitka spruce and western hemlock as main hosts.

Where the fruiting bodies show

Forest Research: describes creamy white and brown perennial fruiting bodies at the base of infected living trees and on stumps, from popcorn size to clusters of brackets over 20 cm across. It says they often sit at or below ground level and can be buried in soil or litter.
UC IPM: describes button-shaped white bodies about 1/2 inch wide and leathery brackets several inches wide that often wrap needles and debris near the ground.
Natural Resources Canada: says the fruiting bodies are at the base of stumps and of dead or dying trees, white and pored underneath, and that immature "popcorn conks" can appear on upturned roots.

Forest Research says fruiting bodies are not always produced, and that their apparent absence should not be taken as evidence that the fungus is not there.

How the sources say it spreads

All four sources describe airborne spores landing on wounds and freshly cut stumps, then growth into the roots and on to neighbouring trees through root contact or root grafts. Forest Research says the fungus can survive for decades in stumps and root systems. UC IPM says groups of hosts often die in a gradually expanding clump. Natural Resources Canada describes the same spread as a roughly circular mosaic of dead trees.

What the sources say it can indicate

UC IPM: says hosts often show no signs above ground for 1 year or more, and more than half the root system can be dead before they do. It names browning and yellowing foliage, tufted needles and reddish to purplish bark among the signs.
Forest Research: lists root decay that raises windthrow risk and lowers resilience to drought, with more damage from bark beetles.
Southern Forest Health: says infected trees can survive for many years and that weakened roots are at more risk of windthrow.
Natural Resources Canada: says that in mature trees the fungus causes a butt rot and the outside signs are not readily visible, while groups of windthrown trees may point to pockets of the disease.

Where the sources differ

UC IPM says pines and true firs are often killed within several years of first infection, and that Douglas-fir, hemlock, incense-cedar, larch and spruce are not commonly killed outright. Forest Research lists death of young seedlings, reduced growth and root decay, and Natural Resources Canada describes crown symptoms in trees under 15 years old and a butt rot with few outside signs in older ones. The sources are written for forests, landscapes and plantations, so their figures do not carry over to a single street tree.

Management, as the sources put it

UC IPM says infected trees cannot be cured and that prevention is the only effective strategy. Forest Research calls its main approach preventative rather than curative, with treatments to fresh stump surfaces after thinning or felling, one biological and one chemical. Natural Resources Canada says few products are registered for stump treatment in Canada. For who may apply such products, see pesticide licences by country. For the people who examine trees, see tree risk assessment methods.