Heterobasidion Root Disease

In landscapes in forested areas, several host-specific strains of Heterobasidion annosum can infect almost any native conifer. Certain broadleaves are uncommonly infected, including almond, apple, madrone, maple, oak, and poplar. Also called Annosus root and butt rot, the disease kills the basal trunk, root crown, and roots of infected hosts.

Identification

Hosts commonly show no aboveground symptoms for 1 year or more after they are infected. More than one-half of the root system of a tree may be killed before aboveground symptoms become apparent. Dying trees will have browning and yellowing foliage, needles growing in tufts, undersized foliage, and reddish to purplish, discolored bark. Diseased hosts are highly susceptible to being blown over by wind.

In pines, major indications of Heterobasidion root disease are bark that separates easily from the wood of recently dead roots and silvery white flecks of fungus present at the interface between bark and wood. In firs and certain other hosts, a stringy white rot beneath bark and the presence of fruiting bodies are the most important signs of disease presence.

The fungus produces button-shaped, white fruiting bodies about 1/2 inch wide. Other of its fruiting bodies are leathery, several inches wide, and bracket shaped, seashell shaped, or irregular. The upper surface of these larger conks is blackish, brown, or reddish with a white margin. The lower surface is cream colored or white. These perennial fruiting bodies commonly envelop needles and debris near the ground as they grow, a characteristic feature of this fungus.

Life cycle

The fungus spreads by airborne spores that infect wounds around the trunk base and especially freshly cut stumps. If plants of the same species grow near each other they can naturally form root grafts through which Heterobasidion can spread from plant to plant. Groups of hosts often die in a gradually expanding clump as they are infected through root contact with the initial host. The fungus can survive for several decades in the roots of killed trees.

Damage

The pathogen infects and kills the basal trunk, roots, and root crown of conifers and occasionally certain hardwoods. Pines and true firs are often killed within several years of their initial infection. Hosts such as Douglas-fir, hemlock, incense-cedar, larch, and spruce are not commonly killed outright, but their heartwood and sapwood decay, increasing the likelihood of insect attack (e.g., bark beetles) and tree failure (e.g., tipping trunk, wind breakage).

Solutions

Infected trees cannot be cured. Prevention is the only effective management strategy.

Avoid wounding trees, especially near the soil line. Promptly cut down dying and dead conifers. Apply a registered borate fungicide such as Tim-Bor to freshly cut pine and true fir stumps and perhaps to stumps of other conifers. This prevents Heterobasidion spores from infecting stumps and spreading the fungus to nearby hosts through root grafts.

See Heterobasidion annosum from North Carolina State University and Pests of the Native California Conifers from the University of California for more information.

Adapted from the publications above and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).