Tomentosus root rot

Last updated on January 30, 2025

Onnia tomentosa

Onnia tomentosa is a fungal pathogen that infects primarily Engelmann, white, and black spruce. Tomentosus root rot is often found in spruce forests and causes mortality in younger trees. Root disease centers are typically small in size (2-3 trees) but may overlap with other centers making them appear larger.

Description

Tomentosus spreads both by spores and root to root contact. While younger trees can be killed more rapidly by infection, it may take between 15 and 20 years for an older tree to succumb to Tomentosus root rot. This root rot can cause extensive root decay as well as decay extending up the tree trunk which decreases the net volume of harvestable timber. It can effectively reduce the culmination age (the age at which mean annual increment is at its highest) of a spruce stand. Mortality in young seedlings is closely related to distance from an infected spruce stump which means that stump avoidance can be an effective planting strategy in some cases.

Tomentosus root rot is found across the province but particularly in the spruce forests of northern and interior British Columbia. Drier and warmer biogeoclimatic subzones below 1200m are at greatest risk. View the distribution map (PDF, 485KB)

Host tree species

All species of spruce are particularly susceptible. Lodgepole pine is also moderately susceptible when associated with spruce.

Damage symptoms

Young trees with advanced Tomentosus root rot show reduced height and leader growth, thin yellowing crowns, and stress cone crops. While early stage decay causes red-brown staining of the heartwood, advanced decay is shown as relatively long pits that appear honeycomb-like in cross section. Early stages of the decay look very similar to decay caused by red ring rot (Porodaedalea pini).

Fruiting bodies grow on the ground and are funnel-like, stalked, and have a yellow-brown, rust-brown, or dark brown velvety upper surface. The top surface of the fruiting bodies is easily confused with species of Hydnum or Hydnellum that have needle length teeth rather than pores.

Read a detailed list of symptoms (PDF, 205KB)

Management

There are several interventions to address tomentosus root rot to minimize future losses. Regenerating with less susceptible species that are ecologically appropriate, the removal of stumps post-harvest when appropriate, and stump avoidance while planting are recommended treatments for tomentosus root rot. Read a detailed guide on root disease management. https://www2.gov.bc.ca/assets/gov/farming-natural-resources-and-industry/forestry/forest-health/forest-health-docs/root-disease-docs/rootdiseaseguidebookjune2018_4.pdf

Identification images

tomentosus causing advanced decay

tomentosus fruiting bodies at the base of a tree

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