Heterobasidion annosum

A root rot and butt rot, mainly of conifers, that causes serious loss of merchantable timber and (on some sites) the death of trees in forestry plantations. It can spread from tree to tree via the roots, while the airborne spores can infect freshly cut stumps.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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