Hymenoscyphus fraxineus has been isolated from the roots of symptomatic trees, as well as from leaves, shoots and branch/stem lesions.
The infectious spores (sexual) of the fungus are produced by fruiting bodies (apothecia) and can be wind-blown over long distances (20-30 km). The apothecia are produced from June to October on ash leaf petioles and rachises (stalks) from the previous year in the leaf litter. The spores land on leaves or other parts of the trees. From the leaves, the fungus makes its way down the petioles, rachises and stems.
The fungus can also produce asexual spores, but these are not believed to be infectious and can only spread over short distances by water splash. When it is producing asexual spores the fungus is known as Chalara fraxinea, and the disease is therefore sometimes called Chalara dieback or just Chalara.
How does it spread?
The fungus has several pathways of spread over long distances;
It can be spread through the movement of diseased ash plants and logs or unsawn wood from infected trees. Until the ban was applied on all movement of ash trees and seeds in October 2012, high volumes of ash (F. excelsior) were imported every year either for forestry or non-forestry purposes; therefore the potential for entry of the pathogen to the UK was very high.
However, the theory that spores wind-blown from the continent are a common source of entry is now widely accepted, as cases recorded in the wider environment were initially located in the eastern parts of the country. The density of wider environment infections is still greatest in the east but there have now also been cases recorded in many other areas.
The fungus is also seed-borne.