The honey fungus (Armillaria) is one of the most widespread pathogens, and is found across the globe. There are a number of related species of Armillaria that parasitise the roots and trunks of a wide variety of trees; they can also feed saprophytically on the dead remains of the trees. Such dead stumps and infected tissues may act as a base for further infection.

In forests and woodlands, it has been suggested that the presence of other fungi diminish or restrain the pathogenicity (killing power) of the honey fungus, though in gardens it may spread unchecked from one tree or shrub to another. The RHS has some very useful advice and help on this subject. Whether or not a tree succumbs to infection depends to some degree on the health of the host. If the tree has been stressed, for example, by drought or water-logging, or through infection by another fungus then it is more likely to die as a result of infection.

Apart from the millions of spores produced by the fruiting bodies, the fungus spreads through the soil by root-like structures called rhizomorphs, sometimes called ‘bootlaces’; these may be on the roots or beneath the bark. The honey fungus is also seen as white / cream coloured sheet (the mycelium) spreading below the bark, which may give off a distinctive ‘mushroomy’ smell.

There are a number of different species of Armillaria whose geographical ranges overlap, and they vary in their pathogenicity i.e. their capacity to infect and damage healthy trees. Armillaria mellea and Armillaria ostoyae are two species of the honey fungus that are common in the UK.  A. mellea is often associated with lowlands, whereas A. ostoyea (sometimes referred to as the dark honey fungus) is found amongst conifers.  One specimen of A. ostoyae that was found in the Blue Mountains of eastern Oregon occupies an area of some 965 hectares (2300+ acres); its age has been variously estimated to be between 2400 years to 8000 years.  Other large specimens of the honey fungus have been found since, they are amongst the organisms that can lay claim to be the largest organism on earth though some dispute whether such specimens are single organism, viewing them rather as clones (genetically identical organisms) that have grown to together.