Amongst the commoner fungal diseases affecting trees in the UK are:-
- Dutch elm disease
- honey fungus
- fomes butt rot
- oak die back
- beech bark disease
DUTCH ELM DISEASE (DED)
Elm trees are highly valued as shade and amenity trees in the UK and the US. They are highly prized for their beauty in the landscape and their ability to withstand environmental stresses in the urban environment.
Since 1970, more than 20,000,000 elms in the UK have fallen victim to this environmentally devastating disease. Over the past 70 years, more than 70% of US mature elms have succumbed to DED.
Over the past 80 years, this fungal disease has destroyed more than 300,000,000 elms throughout the Northern Hemisphere and in New Zealand.
First studied in Holland - hence its name - DED is carried by elm bark beetles. These breed under the bark of elm trees. The beetles act as a vector, carrying the fungal spores from one tree to another. After the disease is contracted, fungal growth spreads throughout the tree blocking the transport of water and minerals through the xylem plumbing system from the roots to the branches and leaves. Without an adequate water supply, the leaves wilt and die.
Scientists from the University Abertay Dundee claim to have grown the world's first genetically modified elm trees. That breakthrough could lead to the reintroduction of elms which are resistant to DED.
This is an example of environmentally-friendly biotechnology that could be adapted to tackle other tree fungal diseases decimating tree populations throughout the world.
English elms make a particularly good candidate for GM technology because they do not regenerate by seed in the UK. They spread via underground suckers.
HONEY FUNGUS & FOMES
These two root-disease fungi are common in forestry in Britain and also affect garden trees. They are both fungi that rot wood and can live in both the decaying stumps or roots of dead trees as well as attacking roots of live ones. They can both kill large trees although their impact is more notable in young ones. Both can cause both the roots and the butts of live trees to rot.
· HONEY FUNGUS - a number of species of Armillaria are found in Britain but two are particularly troublesome:- A. mellea and A. ostoyae. Like most pathogens, honey fungus benefits if the host tree is weakened through some other factor first. They are forest fungi that live in the stumps or root systems of infected trees and can spread through the soil and leaf litter via their root-like structures or rhizomorphs, nicknamed bootlaces. In conifer plantations, it may be necessary to remove the stumps from the previous conifer crop if that was infected by honey fungus to avoid re-infection.
FOMES or Heterobasidion annosum colonises freshly cut tree stumps by windborne spores that can travel large distances. It lives by decaying wood.
When cutting a conifer crop, the stumps are treated with urea mixed with a bright blue dye so foresters know which stump they have treated.
OAK DIE-BACK - recent years have seen an increase in the crowns of large oak trees loosing leaves, yellowing and the topmost trees and branches dying back, leading to the tree's death or a "stag-headed" state. One or a combination of factors may be involved and research is being carried out on this.
BEECH BARK DISEASE - is caused by a combination of an insect pest - the felted beech coccus, whose Latin name is Cryptococcus fagisuga and the fungal pathogen Nectria coccinea. The insect feeds on the beech bark and allows the fungus to invade. The insect produces what looks like flecks of white wool to protect itself from predators; with heavy infestations some trees look whitewashed. Once the fungus gets in, young lesions often produce a dark fluid or tarry spot. Beech bark disease can be troublesome in beech plantations about 15 - 30 years old.
Not all fungi damage or destroy trees. Many types of fungi and trees enjoy a mutually beneficial or symbiotic relationship. These are known as mycorrhizal fungi.
More: FC Information Note 22 - "Dieback of Pendunculate Oak" by J.N. Gibbs (1999).
FC Field Book 16 - "Diseases & Disorders of Forest Trees" by S.C. Gregory (1998).