In the UK we have also noticed very severe external Armillaria symptoms on oak trees (Quercus robur). The causal species is Armillaria gallica.
In the gallery below the first two images show an old, blackened lesion spreading up the stem base, killing live inner bark and causing a black fluid exudate response from the tree. The third photo shows a tree killed by A. gallica with no buttress roots remaining.
However, many trees are able to withstand a severe attack by this maverick fungus and the fourth and fifth image shown how the tree has invested much energy into forming thick callus rolling in from the lesion margins to cover over the tissues killed by the pathogen. Image six shows how extensive the callus can be and that it hides the rotted interior (heartwood). The final two photos show extremely extensive and sever damage caused by A. gallica although the crown does not (yet) proportionately reflect the stem damage.
