Conifer root and butt rot (Heterobasidion annosum)
Information about conifer root and butt rot (Heterobasidion annosum), including its symptoms, distribution, threat, management and control.
Present in UK
Not notifiable – see ‘Report a Sighting’ below
Scientific name of causal agent – Heterobasidion species
Heterobasidion root and butt rot is a disease caused by the basidiomycete fungus, Heterobasidion. Heterobasidion species affect a wide range of conifer species, and cause economically significant damage to commercial forestry.
In the United Kingdom, root and butt rot is primarily caused by Heterobasidion annosum (H. annosum), previously classified under the scientific name Fomes annosus. A second species, Heterobasidion abietinum, was recently discovered on fir (Abies), western hemlock (Tsuga heterophylla) and Norway spruce (Picea abies) in England, and research is ongoing to determine its distribution and impact.
Impact
The type of damage caused by Heterobasidion infection varies depending on the host species, but can include:
- death of young seedlings
- reduced growth and carbon sequestration
- reduced timber quality and strength
- root decay resulting in increased risk of windthrow and decreased resilience to drought
The pathogen can survive for decades in infected stumps and root systems, infecting the subsequent generations of trees planted onto affected forest sites. Heterobasidion therefore has long term implications for forest sustainability and domestic supply of sustainable wood products.
Financial losses resulting from decay and growth loss were estimated at 790 million euros across Europe in the late 1990s.
Distribution
The fungus is present throughout the UK but, to date, airborne spore levels, and stand infections are generally lower than those found in Europe, because of the relatively recent establishment and intensive management of domestic conifer plantations.
Risks to the UK Forestry Sector from Heterobasidion
Increasing pathogen pressure:
Without proactive and preventative management, Heterobasidion spore levels are likely to increase within the UK’s conifer forests, approaching levels seen in southern Sweden where Heterobasidion is present in the majority of Norway spruce (Picea abies) forests, and 15-20% of harvested trees exhibit decay.
Decreased carbon sequestration:
Infection can reduce growth and increase mortality within a forest. Subsequent decay releases stored carbon and reduces the forest’s capacity to sequester CO₂, potentially affecting climate mitigation efforts.
Decreased resilience to biotic and abiotic threats:
Trees weakened by Heterobasidion are more vulnerable to extremes of climate, and secondary pests and pathogens, particularly bark beetles such as Dendroctonus and Ips species. This can amplify forest damage and contribute to pest outbreaks.
Climate Change Concerns:
Further research is needed but warmer temperatures may increase impacts of Heterobasidion in northern Europe and the UK directly by increasing pathogen growth and spore production/dispersal, and indirectly by changing soil characteristics (soil moisture and water retention) which influence fungal transmission between roots.
Introduction of non-native Heterobasidion species:
Heterobasidion parviporum is a spruce specialist, responsible for extensive losses across European spruce forests. Its establishment could have serious consequences for the domestic forestry sector, given the industry’s heavy reliance on Sitka spruce (Picea sitchensis), the backbone of UK softwood production.
Identification
Heterobasidion is a basidiomycete fungus that produces creamy white and brown perennial fruiting bodies at the base of infected living trees and on stumps created during routine forestry operations such as thinning and clear-felling (Figure 1). Fruiting bodies can vary greatly in size, ranging from popcorn size to clusters of brackets over 20 cm in diameter.
Detection can be challenging because they often develop at or below ground level around the root collar and major roots of infected trees and stumps. In many cases, they are partially or completely buried beneath soil, leaf litter, or vegetation and may only be observed following excavation around the base of the tree.
Fruiting bodies are not always produced on infected woody material, and their apparent absence should not be taken as evidence that the pathogen is not present on a forest site. Many infected trees show few or no external symptoms until the disease is well established. In some cases, significant levels of root and butt rot only become apparent after harvesting, where freshly cut stumps or felled stems exhibit characteristic staining and decay (Figure 2).
Spread
There are two principal pathways by which Heterobasidion spreads within managed UK forests (Figure 3):
- Airborne spore dispersal is responsible for the establishment of new infection centres and the long-distance spread of the pathogen. Fruiting bodies release vast numbers of spores, which can be carried considerable distances by air currents. When these spores land on freshly cut, untreated stumps or exposed wounds created during harvesting or thinning operations, they can germinate and colonise the wood. The infected stump then serves as a reservoir of inoculum, enabling the fungus to spread into the roots.
- Root-to-root transmission then allows the pathogen to spread from the initial site of infection throughout the forest stand. The fungus colonises the root systems of infected trees and stumps and spreads to neighbouring healthy trees through root contact or root grafts. This allows the pathogen to expand gradually through a stand, creating persistent centres of infection that may remain active for decades.
In unmanaged forests where thinning and clear felling is not carried out, Heterobasidion is an opportunistic pathogen, only able to colonise trees through stem wounds.
The combination of long-distance dissemination by airborne spores and local spread through root contacts makes Heterobasidion challenging to manage. Effective stump treatment following forestry operations is essential to prevent the establishment of new infection centres and limit the spread of the disease both within and between forest stands.
Management and control
The likelihood of Heterobasidion infection, establishment and transmission varies according to factors including host species, age, rotation, soil type, climate, and forest management practices. However, the principal management approach is preventative rather than curative. Once established within a stand, the fungus can persist for decades in infected stumps and root systems, making prevention the most effective and economically viable control strategy.
Preventative treatments are applied to stumps immediately following thinning or clear-felling operations (Figure 4), when freshly cut stump surfaces are most vulnerable to infection by airborne spores. Two stump treatments are available to prevent pathogen spores colonising the stump and subsequently spreading through root contacts to neighbouring trees:
PG Suspension [hyperlink here to PG page] produced by Forest Researchis a biological control agent containing a saprotrophic (non-pathogenic) fungus, Phlebiopsis gigantea. Urea [hyperlink here to urea page], is a chemical treatment. Further information on these treatment options is linked below.
Where Heterobasidion is already established, management options are more limited.
Stump removal – this process physically extracts infected woody material from a site, removing the reservoir from which the fungus can spread into the next rotation of seedlings. However, it is expensive, operationally challenging, and is impractical on most sites (Figure 5).
Species change – select less susceptible species for the following rotation on sites where Heterobasidion is well established.
As Heterobasidion infections are frequently difficult to detect before significant decay has developed and can persist for many years once established, preventing the colonisation of freshly cut stumps remains the cornerstone of effective disease management, maintaining the long-term productivity and resilience of UK conifer forests.
Report a sighting
Conifer root and butt rot is most likely to occur on commercial forestry sites, whose managers are trained to recognise and manage it. We therefore do not require reports of suspected sightings from members of the public.
Our research
Given the importance of coniferous forests to the UK’s domestic softwood timber industry, research into cost-effective and environmentally benign ways of managing conifer root and butt rot is a priority for us. Our research programme is therefore:
- Methods to increase the use of stump treatment and prevent the spread of Heterobasidion
- Exploring biological control products for use on spruce species
- Optimising molecular diagnostic methods to aid surveillance for Heterobasidion species posing a threat to UK forestry
- Investigating impacts of climate change on Heterobasidion behaviour