Phytophthora ramorum is a fungus-like virus of plants that causes extensive damage and mortality to trees and other plants. The disease can not only effect the Oak family as the name would suggest but many more such as Beech, Spanish Chestnut, Everygreen Oak, Horse Chestnut and Larch. 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. It is believed that the disease has come to Europe from Northern America during the 1990’s. In 2011, 11 cases of the disease throughout woodlands in Ireland have been confirmed.
On trees, symptoms include lesions – sometimes known as bleeding cankers – that exude fluid from infected bark, visible as a black liquid that can dry to a crust on the trunk. The inner bark under this bleeding area is usually discoloured and dying. Trees die when the lesions become extensive on the main trunk. Infection by P. ramorum on Larch can take two forms. Shoots and foliage can be affected, visible as wilted, withered shoot tips with blackened needles. The infected shoots shed their needles prematurely. Trees with branch dieback can have numerous resinous cankers on the branches and upper trunk. On other plants, it infects the leaves and shoots of ornamental shrubs such as rhododendron, viburnum, pieris and camellia. Other typical symptoms include leaf-blackening, wilted shoots and die-back. On individual leaves, blackening of the leaf stalk usually extends into the leaf along the mid-vein, although blackening at the leaf tip can also occur. The progress of the disease can be so rapid that shoots wilt and the leaves hang down.
What to do
No cure has been found and there are no chemical treatments currently available that are effective against this disease. There are some fungicides that can suppress the symptoms, but none that will kill the disease. So, the objective of any control approach must be to prevent or minimise any further spread of the disease and the damage it causes. The best available advice is to remove and kill the living plant tissue on which the organism depends for reproduction. In the case of infected larch, this means affected trees should be felled or otherwise killed as quickly as possible after detection of the disease and before the next spring or autumn period of sporulation begins on the needles.
It is very important to report any sites which you feel are showing the symptoms of ill health listed above with photographs if possible to the Forest Service, Department of Agriculture, Food and the Marine: by email (firstname.lastname@example.org); or, by phone (01 607 2651).