Rhizomorphs of honey fungus (Armillaria mellea) on fallen wood. The picture shows the surface of the trunk of a fallen ash tree (Fraxinus excelsior) that has lain on the ground for several years. The background (white) is the trunk of the tree; the black "bootstrap" structures are the rhizomorphs of the fungus. The bark of the tree is not present. A. mellea is a virulent pathogenic fungus that can infect the roots of a wide variety of woody plants as well as some herbaceous plants. It spreads through the soil by means of rhizomorphs, which can extend for several metres seeking new hosts. It can present a serious problem in gardens, as treatment is difficult and infection usually fatal to the host. It can also live as a saprotroph on decaying or fallen wood. The fungus in this picture may have killed the tree, but it may also be merely decomposing its remains.