Ireland being an island has relatively few diseases that affect European mainland forests. The barrier created by the sea and the fact that most of our forests have been established over the past 70-80 years on bare land with introduced non-native tree species, coupled with strict phytosanitary controls has helped to keep our forests relatively disease free. While there are many fungal organisms present in Irish forest few cause economic damage. This situation may change in the future with the increased movement of people and plant materials, but perhaps more serious is the effects of climate change which may create more favourable conditions for the spread of diseases.
Today, there are only two main diseases that cause problems for foresters viz. the root and butt rot fungus Heterobasidion annosum and Phytophthora ramorum. However a third disease Chalara fraxinea or ash dieback has recently been recorded in plantations established with a batch of imported transplants. This virulent wind borne disease has devastated ash throughout northern Europe and while rigorous phytosanitary measures have been taken it still poses a serious threat to ash plantations in Ireland.
Root and Butt Rot Fungus (Heterobasidion annosum)
The principal forest disease which occurs in Ireland is Heterobasidion annosum, commonly known as ‘Fomes’. Most conifer species are susceptible to this decay fungus which colonises the recently cut stumps created in thinning or clear-cutting operations. The fungus grows down through the stump roots and across into any roots of nearby living trees which happen to be in contact, causing serious decay in the latter’s roots and lower stems. The marketable volume of timber can be greatly reduced and infected trees are liable to windthrow.
Infection of the standing crop can be prevented by treating the freshly cut stump surfaces with chemical or biological agents that inhibit infection by airborne spores of the fungus. Standard practice is to apply 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.
The presence of barrels of red or blue coloured urea solution is sometimes mistaken by visitors to forests as dangerous chemical pesticides. The urea solution is harmless and while it is a chemical fertiliser, only a small amount is applied and just to the cut stumps.
Irish conifer forests are relatively young and Fomes is not present in all crops, however, without stump treatment the incidence of the disease could be expected to eventually increase to mainland European levels, where up to 25% of a crop can be lost through decay.
Suden Oak Death (Phytophthora ramorum)
Commonly known as Sudden Oak Death this fungus was first reported in California in the 1990s as killing significant numbers of oak trees and other plant species. Since then it has been found in many European countries including Ireland, on the shrub species Rhododendron and Viburnum spp., particularly in garden centres and nurseries.
The effects of this fungus have only recently become apparent in Irish forests. In July 2010 the Forest Service detected the first findings of Phytophthora ramorum on Japanese larch, which was showing extensive dieback from the crown and down the stem. Japanese larch appears to be particularly susceptible to the disease, affecting all age classes and causing significant dieback and deaths. Noble fir (Abies procera), beech (Fagus sylvatica) and Spanish chestnut (Castanea sativa) growing in close proximity to the infected larch have also been found to be infected at a number of the sites.
To date, the disease is still mostly confined to the southern counties but it has been reported as occurring on Japanese larch in forests in Northern Ireland and also in Britain.
The occurrence of Phytophthora ramorum in Irish forests has always been associated with the presence of Rhododendron. The disease can be dispersed by the movement of infected plants and plant products. The disease can also be dispersed by rain, mists and air currents.
Symptoms of infected trees as are
- Dead and dying partially flushed trees in groups or scattered throughout the stand
- Partial or whole crown discolouration, (reddish brown or grey depending on level and stage of infection)
- Affected trees may show needle wilt, branch and shoot dieback, abnormal shoot growth. Shoot dieback from tip back along shoot.
- Resin bleeding on branches and trunk
- Excessive side shoot growth and
- Heavy cone production may be observed.
The Forestry Plant Health Contingency Plan and the EU Technical Guidelines on Control Measures for Phytophthora ramorum are being applied to control the disease. Secure & hygienic protocols have been put in place by the Forest Service in order to minimize the risk of spread of the disease through the harvesting, haulage and processing of timber from the infected sites.
Because of the level of damage to Japanese larch, the Forest Service ceased grant aiding Japanese larch in November 2010
Further information at: Phytophthora ramorum symptoms on Japanese larch
Ash Dieback (Chalara fraxinea)
Chalara fraxinea or ash dieback disease is a virulent fungal disease of ash trees that was first recorded in Poland in 1992 when ash trees were reported to be dying in large numbers. Since then the disease has rapidly spread across northern Europe causing server damage to both young and old populations of ash. The origin of the disease is unclear at present and is currently under investigation but is thought to have originated from the east.
Chalara has recently been recorded in Ireland in ash plantations that were established with transplants imported from the Netherlands. Strict phytosanitary measures, however, were, immediately employed by the Forest Service and the infected trees were destroyed along with adjacent material that may have been infected. Imports of all ash seed and transplants and also untreated timber have also been banned. The effectiveness of these measures have yet to be determined, but the fact that the disease is widespread in Britain and is spread through wind borne spores, it poses a significant risk to the ash populations in this country.
The situation regarding the spread of the disease is still evolving and it will some time before we know whether we can keep the disease at bay or whether it will indeed spread throughout the country.
Further information at: Ash dieback disease