It's a fungus - it exists - this tells you how it spreads
The fungus spreads underground by direct contact between the roots of infected and healthy plants and also by means of black, root-like structures called rhizomorphs (often known to gardeners as ‘bootlaces’), which can spread from infected roots through soil, usually in the top 15cm (6in) but as deep as at least 45cm (18in), at up to 1m (3¼ft) per year. It is this ability to spread long distances through soil that makes honey fungus such a destructive pathogen, often attacking plants up to 30m (100ft) away from the source of infection. extract from RHS Advice linked to above.