New warning over spread of ash dieback

According to research realized at the University of Exeter, a public research university in the United Kingdom, the ash dieback fungus (Hymenoxyphus fraxineus) could spread quickly and affect more trees than expected. H. fraxineus is a virulent fungal pathogen of ash trees first recognized in Poland, in 1992, and it has spread throughout Europe causing losses.  Ash dieback disease is characterized by leaf loss and crown dieback in infected trees. It kills the leaves, the branches, the trunk, and finally leads to the death of the tree. Even though not all trees die of the infection, once infected, trees can’t be cured.

The fungus has two reproductive phases in its life cycle. It was previously believed that the multiplication of H. fraxineus was only possible through  sexual reproduction, but Exeter scientists found that asexual spores of the fungus are infectious and can germinate on leaves or infected seedlings via soil. It was believed that the asexual spores functioned as the male in the sexual reproduction, and so they could not germinate alone. This asexual reproduction will allow it to spread more quickly.

Knowing more about the life cycle of this pathogen is important for improving the disease control and for making people conscious that when moving the soil from areas with infected trees, they could be transferring  and so spreading the fungus as well.


1. Spores arrive in the environment of a susceptible ash tree (e.g. via wind). 2. Spores germinate on leaves and on ash seed cases. 3. Leaves become infected, with the fungus growing over the surface and forming internal structures, which may be fruiting bodies, amplifying the inoculum. 4. Infected leaves are abscised along with the ripe seeds; the fungus continues to propagate on leaf litter, seed cases and other debris, as well as in the soil itself (5). 6. From the soil, mycelium/germinated spores can invade the roots of mature trees and seedlings. 7. Sporulation, including sexual sporulation if strains of opposite mating types are present, may then occur in the leaf litter, releasing inoculum to re-infect the original host tree and others nearby (8).

Original journal article and source of photo:


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