Woolly velvet polypore

The woolly velvet polypore (Onnia tomentosa) is a destructive wood-decaying fungus that acts as a serious pathogen for various conifer species. This organism causes root and butt rot, which gradually weakens the trees, leading to growth suppression, windthrow, or premature death. It is recognized as one of the most economically significant pathogens in coniferous forests.

Primary hosts for this fungus include spruce, pine, and fir trees. The pathogen can affect trees of all ages, though it is particularly devastating in plantation settings. The infection often begins in the root system and slowly moves upward into the stem, eventually compromising the structural integrity of the tree's base.

Symptoms of an Onnia tomentosa infection include crown thinning, needle discoloration, and a noticeable reduction in annual growth rates. In advanced stages, small, brownish, velvet-like fruiting bodies appear at the base of the tree or attached to exposed roots. Internally, the roots and heartwood exhibit advanced brown rot, turning the wood into a crumbly, decayed mass.

The development of the fungus is favored by moist soil conditions and the presence of root injuries. The pathogen spreads primarily through spore dispersal or via root-to-root contact within a stand. Once established, the fungus can survive in stumps and woody debris for decades, acting as a reservoir for further infection of neighboring healthy trees.

Management strategies focus on minimizing mechanical damage to tree roots during thinning or harvesting operations. Cultural practices include the following:

• Removal and destruction of heavily infected stumps.
• Planting a diverse mix of tree species to slow pathogen spread.
• Regular forest health surveys to identify and quarantine infection centers.
• Promoting stand vigor through appropriate thinning and site selection.