Armillaria mellea
Most serious disease threats to British woodland involve fungus. For conifers, the greatest threat is White Rot Fungus ("Heterobasidion annosum"). Dutch Elm Disease arises from two related species of fungi in the genus "Ophiostoma", spread by Elm Bark Beetles and acute oak decline has a bacterial cause. Another fungus, "Nectria coccinea", causes Beech bark disease, as does "Bulgaria polymorpha". Ash canker results from "Nectria galligena" or "Pseudomonas savastanoi", and most trees are vulnerable to Honey Fungus ("Armillaria mellea"). The oomycete "Phytophthora ramorum" (responsible for "Sudden oak death" in the USA) has killed large numbers of Japanese Larch trees in the UK.