A special mention for Armillaria mellea, better known as honey fungus. Native to the UK (and many other countries), it is profiting mightily from non-native diseases.
Honey fungus is widespread throughout the UK. Armillaria melleaaffects a wide range of broad-leaved species, also coniferous species (A. soldipes), often following other diseases and killing off disease-weakened trees.
The impact of honey fungus is increasing as non-native diseases weaken many species of trees. Because this fungus commonly inhabit roots, detection is difficult unless characteristic mushrooms are produced around the base of the tree or symptoms become obvious in the crown or the lower stem.
Crown symptoms on conifers and broad-leaved trees vary somewhat. Generally, however, the foliage thins and discolours, turning yellow, then brown, branches die back, and shoot and foliar growth are reduced.
On large, lightly infected or vigorous trees, crown symptoms develop over a number of years until the trees die. Conifers frequently produce a larger-than-normal crop of cones, known as stress cones, shortly before they die.