Biotic factor that contributes to oak decline
In the first recorded episode of oak decline in Britain in the 1920s, Armillaria (honey fungus) was visible on many of the affected trees but opinions varied on whether it was the primary cause of decline or not.
One school of thought claimed that honey fungus was one of the most dangerous factors involved whilst others thought that its principal role was to kill off trees that were already ‘irretrievably damaged’.
Our current knowledge about Armillaria suggests that the latter is much more likely, and in general native oaks are not particularly susceptible to this pathogen, even the more aggressive species of Armillaria. However, there is little doubt that oaks showing symptoms of decline can be invaded by Armillaria.