Heterobasidion annosum is a fungus that can infect and decay the roots and lower stems of coniferous and broadleaved trees. It is one of the most damaging diseases of coniferous trees in Europe, and can cause significant losses in timber production and quality. It can also affect the stability and safety of the infected trees, as they can become more prone to windthrow or breakage. It can spread through root contact, soil movement, or spores.
Heterobasidion annosum is present in the UK and continental Europe, but the problem is less severe in Ireland. This is because Irish conifer forests are relatively young and have been intensively managed for only a relatively short period (fewer than 200 years) compared with those in continental Europe. Heterobasidion annosum spores are therefore relatively infrequent, so infection levels remain lower than in Europe.
However, there is a risk that spore levels in Ireland’s conifer forests might increase over time, causing more trees to become affected. Experience in continental Europe has shown that the fungus can cause serious economic losses to timber growers and the businesses which depend on them. The Irish softwood forests support investment of hundreds of millions of pounds in hundreds of businesses employing thousands of people in planting, managing and harvesting the trees, and hauling and processing the timber. These benefits would be put at risk if there were not effective control of the fungus. The landscape and biodiversity benefits of Irish conifer forests would also be put at risk, particularly those of native Scots pine (Pinus sylvestris).
The most common and susceptible conifer species in Ireland are Norway spruce (Picea abies), Sitka spruce (Picea sitchensis), and larch (Larix spp.). The symptoms of Heterobasidion annosum infection vary depending on the tree species and the site conditions. In general, infected trees show signs of reduced growth, crown thinning, branch dieback, and resin bleeding from the stem. The fungus also produces fruiting bodies, which are woody brackets that grow on the infected stumps or stems. The fruiting bodies are brown or black on the upper surface and white or cream on the lower surface, where the spores are produced.
There is no chemical or biological treatment available for Heterobasidion annosum. The best option is to prevent the infection by applying a stump treatment product on the freshly cut stump surfaces soon after the trees have been felled. This kills any developing spores and prevents them from spreading the infection to nearby standing trees. The standard treatment product in Ireland is a solution of 1kg fertilizer grade urea dissolved in 5 litres of water coloured with a harmless food dye. This is normally applied by the harvesting machine heads at the time of felling, or by brush and bucket on trees felled by chainsaw.
If the infection is already established, the infected trees should be removed and destroyed, preferably by burning or burying them on site. The infected trees should not be used for firewood, timber, or hurleys, as this can spread the disease to other areas. The infected trees should not be pruned or cut, as this can create wounds that can facilitate the infection or spread of the fungus. The site should be monitored for signs of further infection and replanted with resistant or tolerant tree species.