Ash dieback is a highly destructive disease of ash trees (Fraxinus species), especially the Irish native common ash. The disease causal agent originated in Asia, and was first found in Europe in the 1990’s. In Ireland, the ash dieback was first noticed in 2012.
The disease is observed in all counties in Ireland. Symptoms of the disease include leaf death, shoot dieback, diamond shape lesions on branches/tree trunk, and crown thinning eventually resulting in the death of most trees.
Ash trees have evolved and co-exist with a native fungus – Hymenoscyphus albidus that also causes dieback of branches, but rarely affects the tree growth and kills the tree. While, Hymenoscyphus fraxineus is a more aggressive fungal pathogen from Asia, and has a more devastating effect on our Irish ash trees, killing the majority of them.
Previous research shows that around 1% of the ash population exhibit a higher level of tolerance to this more potent pathogen, and up to 10% of trees show a good level of tolerance. This resistance is controlled by several genes and is heritable from tolerant healthy trees to their offsprings. Therefore, by identifying these healthy ash trees that have a high level of tolerance, we can breed and produce ash genotypes that are tolerant to the disease.