Ustulina deusta
Ustulina is an ascomycete- meaning it spits spores rather than drop them (a toadstool or bracket is such a shape to be able to drop spores).
It has two stages- an annual grey 'skin' with a white margin and a black, crusty perennial stage. The perennial stage is the mature fruit body, whilst the annual stage is immature and asexual.
Ustulina can be found on beech and lime, less so on oak, horse chestnut and maples.
The decay caused is a soft rot resulting in a ceramic-like failure of the stem base. The whole tree can sheer off from the roots. Failed trees often show no signs- either bio-mechanics or crown die back- of imminent failure.
Ustulina is a particularly dangerous fungus in tree hazard assessment. This is because it is very easy to overlook- the small fruit bodies are often hidden in butress flutes or beneath leaf litter and the tree may show no signs of imminent failure. It also appears to be particularly aggressive.
Ustulina has recently been re-named Kretzschmaria.
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