Honey Fungus.

Honey Fungus (Armillaria mellea) is a parasitic fungi and most obvious in autumn with the damp yet sometimes still warm daytime. The groups of honey coloured toadstools live on trees and woody shrubs. The fungus is a "White Rot" fungus and spreads between living trees, dead trees, live roots and tree stumps by means of reddish - brown to black rhizomorphs (like shoelaces) under the bark and down through root contact. Once infected a tree or plant will die once the fungus has spread fully round the girth. This can happen rapidly or over many years.
Infected plants or trees may actually show prolific flowering or fruit production shortly before death.
The presence of thin sheets of cream coloured mycelium, which smells strongly of mushrooms, beneath the bark at the base of the tree or stem (sometimes extending upwards) or a gum / resin seeping from cracks in the bark of conifers, is a sign that Honey Fungus is likely to be the problem.
Once identified, nothing can be done other than to dig it out and burn as much as possible. But even this may not be enough to protect attacks on other trees and plants, with the infection travelling through the soil from plant to plant.
It is said that spreading flour starch around the base of the plant encourages Trichoderma, which is a fungus hostile to Honey Fungus, may help.
For resistance to Honey Fungus, choose plants such as sumachs, bamboos, hebes and pittosporums. Avoid fruit trees, willows, currants, lilacs, viburnums and wisterias.