1. Ash Dieback - Science Update
Dr Colin Fleming, SAFSD Division, Newforge Lane, Belfast

2. ASH DIEBACK
The fungus causing ash dieback (Chalara fraxinea) is an introduced pathogen
Early 1990s: disease symptoms first observed in eastern Europe
CHALARA FRAXINEA
2006: cause identified as a new fungal pathogen Chalara fraxinea (asexual stage)
2009: sexual stage identified, but thought to be a known species associated with leaf litter
2011: further study showed the species causing ash dieback is distinct - named Hymenoscyphus pseudoalbidus

3. Sexually formed apothecia
DISEASE
Only attacks ash (Fraxinus spp.)
Infects through leaves
Grows into woody tissue causing distinctive lesions
Sporulates in summer on leaf debris from previous year
Spread by air-borne spores and movement of ash plants
Sexually formed apothecia
Asexual spermatia

4. Symptoms of Chalara fraxinae can be visible on leaves, shoots and branches of infected trees
In severe cases the entire crown shows leaf loss and dieback

7. Detection and Diagnosis
Culturing Molecular
6- 24 hours
Difficult to culture
Very slow growing – could take 5 – 8 weeks to get a colony

8. Hypothetical life cycle of Hymenoscyphus pseudoalbidus (from Gross et al., 2012, Fungal Genetics and Biology 49, 977–986)

9. Sexually formed apothecia
Infection and spread within the tree
Air-borne ascospores produced on infected, fallen leaves during the summer months (June-August in mainland Europe) infect healthy ash trees through the leaves
Sexually formed apothecia
This results in the leaves withering and dying, and shoot lesions and stem lesions developing as the fungus grows into and through the leaf and into the woody tissue.

10. Leaf death results from the death of the leaf stalks (petioles and rachises) and tree death may result as the trunk and branches are killed by the fungus growing through them.
C. fraxinea grows through the woody tissue, killing it as it goes, into the heartwood of the tree. Young trees with slender stems may die quickly, but older trees may survive for several years, often succumbing to secondary organisms such as honey fungus.

11. Sporulation and spread between trees
Infected leaves fall to the ground and decay, leaving the petioles.
The fungus forms blackened structures (pseudosclerotia) in the petioles.
Pseudosclerotia are melanised hyphae (strands of the fungus) which allow it to survive over the winter and in adverse conditions.
If two mating types of the fungus are present within the infected petioles, the fungus undergoes sexual reproduction and the following summer produces spore-bearing apothecia.
In adverse conditions (e.g. drought), the fungus can delay production of apothecia and survive for at least two years, producing apothecia in the subsequent summer.

12. If an infected tree survives for more than a year and leafs out in subsequent seasons, the new leaves may not always be infective but
Infected tree producing newly infected leaves in the subsequent season
Re-infection of new sprouts after cutting an infected tree

13. Other factors
C. fraxinea has been detected in felled ash wood and can produce asexual conidia on this, but so far it has not proved possible either to germinate the conidia or to demonstrate them to be infective.
Therefore while it is prudent to consider the possibility that the disease might be spread by movement of infected logs, the risk appears to be low and can be minimised by appropriate treatment and trimming.
C. fraxinea has also been detected in ash seeds produced by trees affected by ash dieback. Seeds are clearly an infection risk.

14. Management and Chemical control of Chalara
Removal of infected / potentially infected trees
Removal and destruction of ash leaf litter
Destruction of pseudosclerotia within ash petioles
Destruction of fruiting bodies and spores
Biosecurity at infected sites

15. Times of year when symptoms are most likely to be observed
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Leaf
necrosis
Shoot
lesions
Stem
Fruiting
bodies

16. Resistance in Irish ash to the disease?
Genetic variation in Irish ash populations (PhD thesis Stephen Clarke, QUB)

17. 93% genetic variation within populations

18. Genetic variation in Chalara?
European studies on Chalara show:
2 mating types
Founder effect
High gene flow
Low geographic variation
Questions for NI outbreaks:
2 mating types?
Genetic diversity?