Ash Dieback - Science Update
Dr Colin Fleming, SAFSD Division, Newforge Lane, Belfast
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
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
Asexual spermatia
Sexually formed
apothecia
• 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
Detection and Diagnosis
• Culturing
Molecular
6- 24 hours
Difficult to culture
Very slow growing – could take 5 – 8
weeks to get a colony
Hypothetical life cycle of Hymenoscyphus pseudoalbidus
(from Gross et al., 2012, Fungal Genetics and Biology 49, 977–986)
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.
•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.
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.
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
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.
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
Times of year when symptoms are most likely to be observed
Jan
Leaf
necrosis
Shoot
lesions
Stem
lesions
Fruiting
bodies
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Resistance in Irish ash to the disease?
BUM
MUG
ROE
GAR
NES
BAN
REA
STR
CLB
CAI
CLW
INI
HRK
MAR
REI
ROS
DRO
CRA
GLA
WIB
Genetic variation in Irish ash populations (PhD thesis Stephen Clarke, QUB)
MAR4
STR12
MAR8
STR7
S
Strabane
Glen
STR13
STR9
STR6
STR14
STR16
MAR6
MAR11
MAR13 MAR12
MAR14
MAR15
MAR18 MAR30
MAR9
MAR2 MAR3
MAR7
STR10
MAR16
MAR5
STR4
73
STR3
Roe Valley
ROE21
STR17
MAR28
STR2
STR8
Roe
Valley
63
MAR10
STR18
ROE12
ROE2
GAR7
Glenarriff
NES7
CLW8
NES5
CLW3
CLW5
CLW4
71
MAR27
REA6
57
68
53
GAR11
GAR9
52
GAR19
BAN10
CLB12
BAN9 CLB5
CLB15
50
85
54
WIB8
51
Hanging
Rock
Rostrevor
Banagher Glen
B
DRO6
DRO25
Clandeboye
Figure 3.2.
3.1. Phylogenetic relationships
between individual ash samples constructed
by UPGMA from ISSR data. Bootstrap values
in red. Groups from the Irish Republic in blue.
Southeast in red, southwest brown, northeast
green and northwest purple.
Rea’s
Wood
Wise’s Bridge
CLB
17
CLB16
HRK13
GAR18 GAR24 BAN5
CLB13
CLB9
BAN4CLB14
CLB4
GLA7 57
BAN7
CLB7 HRK2HRK3
BAN6
BAN3
CLB11
HRK4
GLA5
BAN1
HRK7
HRK5
GLA6
CLB1
CLB18
HRK14
DRO22 GLA9
CLB10
HRK8
GLA4
HRK1
72
CLB2
GLA3
DRO23
52
HRK12
HRK17
GLA8
GLA2
DRO18
HRK16
HRK18
HRK11
DRO17 DRO26
HRK9 HRK6 HRK20 HRK19
DRO20 DRO19
GAR25
CLW6
REA4
REA9
REA2
REA7
REA8
REA3
REA13 REA10 REA18
REA17
REA12
WIB1ROS2 REA14
ROS1ROS9
ROS4 ROS8
REA11
ROS6 ROS7
BAN16WIB9
ROS5
REA15
WIB2 WIB6
REA16
HRK10
WIB4
WIB7
BAN14
BAN20REI7
WIB3WIB5
ROE23 BAN21 REI10
BAN18
REI6 BAN13
BAN12
MAR1
BAN11
CLB3
65
ROE27REI9
NES4 ROE26
56
DRO21
ROE28 ROE24REI1
BAN19
ROE25
REI5
NES8
BAN15
DRO4
BAN17
NES9
DRO1
CLB6
GLA1
DRO3
DRO2
DRO7DRO5
74
DRO8
65
69
GAR17 GAR28
GAR16 GAR27 GAR29
GAR1
58
GAR4
GAR3
GAR6
GAR20
GAR23
GAR5GAR2 GAR26
GAR22 GAR21
Clare
Wood
CLW2
55 CLW7
REA1
REA5
GAR13
GAR8
MAR33
MAR36
NES1
MAR35 NES6 NES3
CLW10
ROE4
ROE5
MAR32
STR15
67
GAR15
MAR31
CLW1
MAR25
MAR29
MUG6
ROE7
ROE1
Marble
Arch
CLW9
MUG3 ROE16
ROE15
MUG2
ROE14
MUG4
ROE22
ROE17
65 ROE13
MUG1
ROE18
ROE11
ROE6
ROE10
MUG5
ROE3
ROE8
MAR24
MAR21
MAR20
MAR19 MAR34
MAR22
MAR26
MAR23
STR11
ROE20
Muff Glen
MAR17
STR5
STR1
93% genetic
variation within
populations
DRO16
DRO12
DRO13
DRO15
DRO14
Glashaboy River
Dromore
Wood
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?