15-20696 (15-20354, 14-20220rev, 14-20143)
WP PR Point 7.2.1
DRAFT 2015-05-05 [presented as background for the PRA]
European and Mediterranean Plant Protection Organization
Organisation Européenne et Méditerranéenne pour la Protection des Plantes
EPPO Data sheets on pests recommended for regulation
Fiches informatives sur les organismes recommandés pour réglementation
Pityophthorus juglandis and Geosmithia morbida
INTRODUCTION
Mortality of J. nigra, now presumed to be the result of thousand cankers disease, is thought to have
occurred in the Western USA by the early 1990s (Idaho, Utah; Cranshaw and Tisserat 2008, Utley et
al., 2013). Association of dying J. nigra with P. juglandis was first noted in New Mexico in 2001 and
shortly after in Colorado. In the original reports, attacks by P. juglandis were considered secondary,
incidental to drought, with the pest attacking stressed trees. Drought stress was also originally thought
to be the cause of Juglans decline in Washington in 2008 and when such decline was first observed in
eastern North America (Tennessee) in 2010. However, it was later observed that decline did not have
any consistent association with drought, as affected trees were commonly present on well-irrigated
sites. A pathogenic fungus, subsequently described as G. morbida, was found in 2008 to be consistently
associated with P. juglandis and cankers that developed around wounds made by the insect during
feeding and gallery production. Ultimately, in 2008, mortality of J. nigra in Colorado was determined
to result from the combined action of aggressive feeding by P. juglandis and subsequent canker
development by G. morbida (Cranshaw et al., 2008; Cranshaw and Tisserat, 2008; Utley et al., 2013).
This led to the original description of the disease and suggestion of the common name "thousand
cankers".
Adults of P. juglandis carry spores of G. morbida on their body and deposit them in the wounds and
galleries they create in the bark of branches or trunks of Juglans. Other organisms are found associated
in trees affected by thousand cankers disease. However, so far, no organisms other than P. juglandis
and G. morbida acting together have been shown to cause thousand cankers disease.
Despite the finding of spores of G. morbida on the weevil Stenomimus pallidus (Coleoptera:
Curculionidae) on two girdled J. nigra in Indiana (USA), and the fact that other insects are envisaged in
various publications as "potential vectors", there is still no convincing evidence that other insects than
P. juglandis may act as effective vectors of G. morbida. Cranshaw and Tisserat (2012a) consider that
other significant vectors are unlikely because the disease requires repeated infestation by an insect that
consistently carries the fungus. No other insect in the USA appears to have the close association with
G. morbida and cambium-tunneling habit on live Juglans that would allow significant involvement in
the development of the disease (Tisserat and Cranshaw, 2012). Other species may incidentally carry
spores (such as S. pallidus), and possibly could initiate a point of infection with G. morbida, but could
not provide sustained attacks producing numerous entry points for G. morbida (Cranshaw and Tisserat,
2012a), nor do they attack healthy trees.
In the USA, F. solani was isolated from the margins of trunk cankers on Juglans trees at advanced
stages of decline, but not commonly in cankers surrounding P. juglandis galleries. A role in thousand
cankers disease was not excluded, possibly linked to canker development on the main trunk (Tisserat et
al., 2009; Cranshaw and Tisserat, 2012b). In Italy, in addition to G. morbida, strains belonging to the F.
solani species complex group 25 (FSSC25) were isolated from cankers on symptomatic J. nigra and J.
regia at early stages of thousand cankers disease (including cankers surrounding P. juglandis galleries),
as well as on P. juglandis emerging from J. nigra (Montecchio et al. 2015). FSSC25 was also isolated
in Italy from cankers on logs of J. nigra imported from the USA in the absence of both G. morbida and
P. juglandis insects or galleries (L. Montecchio, University of Padova, Italy, 03-2015; pers. comm.).
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Pathogenicity tests with F. solani were carried out both in the USA and Italy. In the USA, cankers
produced by F. solani were smaller than those produced by G. morbida (Tisserat et al., 2009), whereas
in Italy no significant size differences were observed using a FSSC 25 (Montecchio et al., 2015.
Current data are not sufficient to ascertain the exact role of F. solani (or FSSC25) in thousand cankers
disease P. juglandis was shown as a vector of F. solani in Italy (L. Montecchio, University of Padova,
Italy, 03-2015; pers. comm.). In addition, F. solani is a species complex (FSSC), known to occur in the
EPPO region on some plant species. However, it is not known where FSSC25 (found on symptomatic
Juglans and P. juglandis in Italy) occurs in the EPPO region, whether other FSSC groups are
associated to symptomatic Juglans in the USA or Italy, and their distribution.
This data sheet covers both P. juglandis and G. morbida.
A Pest Risk Analysis was conducted on P. juglandis and G. morbida and is available at: To be
completed
IDENTITY
Vector
Scientific name: Pityophthorus juglandis Blackman, 1928
Synonyms:
Taxonomic position: Animalia; Insecta; Coleoptera; Curculionidae; Scolytinae.
Common names: Walnut twig beetle
Notes on taxonomy: none
EPPO code: PITOJU
Phytosanitary categorization: Alert list
Fungus
Scientific name: Geosmithia morbida (M. Kolarík, E. Freeland, C. Utley & N. Tisserat, 2011)
Synonyms: none
Taxonomic position: Fungi; Ascomycota; Eurotiales; Trichomaceae.
Common names: none
Notes on taxonomy: none
EPPO code: GEOHMO
Phytosanitary categorization: Alert list
HOSTS
The hosts of P. juglandis and G. morbida all belong to the family Juglandaceae, genera Juglans and
Pterocarya. The following Juglans species are known hosts of P. juglandis and G. morbida: J.
ailantifolia, J. californica, J. cinerea, J. hindsii, J. major, J. mandshurica, J. microcarpa, J. mollis, J.
nigra, J. regia, as well as some hybrids (e.g. Paradox rootstock J. hindsii x J. regia, J. nigra x J.
hindsii, J. cinerea x J. ailantifolia, J. nigra × J. regia) (Wood and Bright, 1992; Cranshaw and Tisserat,
2008; Graves et al., 2010 & 2011; Seybold et al., 2012a; Serdani et al., 2013; Seybold et al., 2013;
Utley et al., 2013). Based on observations in the Juglans collection of the USDA-ARS National Clonal
Germplasm Repository in California, P. juglandis is considered to have the capacity to develop in all
species of Juglans that it may encounter (Hishinuma et al., 2014, in press). Regarding Pterocarya spp.,
P. fraxinifolia, P. rhoifolia and P. stenoptera are known hosts of both P. juglandis and G. morbida
(Hishinuma et al., 2014, in press).
Among Juglans hosts, J. major is considered to be a native host of P. juglandis, whose original
distribution largely coincides with that of this species (Cranshaw and Tisserat, 2008, citing others).
There is strong evidence suggesting that J. californica is also an indigenous or native host of P.
juglandis (Graves et al., 2011 citing Seybold et al., 2010; Cranshaw and Tisserat, 2012a). J.
microcarpa, whose native distribution partly overlaps that of J. major and J. nigra, was never found
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infected in its native range. However, it was found to be susceptible to G. morbida in inoculation
studies, and infested by both pests in plantings of this species in California and Colorado (Graves et al.,
2011; Cranshaw and Tisserat, 2012a; Utley et al., 2013).
Carya species (also Juglandaceae) where never found infested in the field and no cankers were
observed on Carya trees situated close to infected Juglans trees. In inoculation studies, no cankers
developed on Carya illinoiensis (pecan), C. aquatica and C. ovata (Utley et al., 2013). Carya spp. are
thus not hosts of P. juglandis and G. morbida.
Difference of susceptibility of hosts to thousand cankers disease
Susceptibility to thousand cankers disease varies between species and hybrids, and between trees in a
same species. In some trees and species, the disease is slowed or halted. Some trees may not be affected
in areas of intensive mortality. This is also the case for the most susceptible J. nigra, for which healthy
trees (free from galleries and cankers) may be found in areas severely impacted by the disease (Utley et
al., 2013, citing Tisserat et al., 2011; Freeland et al., 2012). Utley et al. (2013) makes the hypothesis
that natural hybrids of J. nigra with less susceptible species, such as J. major, may be less susceptible.
Finally, the susceptibility of the different Juglans species depends both on their susceptibility to G.
morbida and on the ability of P. juglandis to find, colonize and breed on the trees (Utley et al., 2013).
J. major and J. nigra consistently appear as the least and most susceptible host species respectively
(many publications, incl. Cranshaw and Tisserat, 2008; Utley et al., 2013). On J. major, the disease is
limited to overshaded or injured branches (Cranshaw and Tisserat, 2008). It rarely, if ever, progress to
kill trees or even major limbs (Cranshaw and Tisserat, 2012a). In inoculation studies (Utley et al.,
2013), J. major presented little canker formation. Conversely, infestation on J. nigra generally led to
death and in inoculation studies, J. nigra generally developed the largest cankers (Utley et al., 2013).
All other known Juglans hosts infested in the field or in collections, as well as hybrids, seem to fall in
an intermediate category, and this was confirmed in inoculation studies (Utley et al., 2013). Among
these species J. cinerea, J. hindsii, J. californica, J. regia, J. microcarpa have been more studied than
others. In species or hybrids in this intermediate category, the effects of the disease may be limited to
scattered dieback rather than death of the tree, and the course of disease may be substantially slower
(Cranshaw and Tisserat, 2012a). J. regia (the most important species for the EPPO region), is
susceptible but seems to present a wide intraspecific variation. Pscheidt and Ocamb (2014) notes that J.
regia is not as susceptible as J. nigra, but the disease is easily found in J. regia orchards. J. regia is
often grafted including on the rootstock Paradox (hybrid of J. hindsii and J. regia) and there may be
considerable variation in susceptibility depending on the rootstock (Utley et al., 2013). It is not clear if
J. regia that is not grafted (or grafted on J. regia) is less susceptible to P. juglandis and G. morbida
than J. regia grafted on J. nigra (or other susceptible species).
GEOGRAPHICAL DISTRIBUTION
 P. juglandis
EPPO region:
Northern Italy (Veneto and Lombardia* regions).
North America:
Mexico*, USA (Arizona, California, Colorado, Idaho, Maryland, Nevada, New Mexico, North
Carolina, Ohio, Oregon, Pennsylvania, Tennessee, Utah, Virginia, Washington).
 G. morbida
EPPO region:
Northeastern Italy (Veneto region).
North America:
USA (Arizona, California, Colorado, Idaho, Indiana*, Nevada, New Mexico, North Carolina, Ohio,
Oregon, Pennsylvania, Tennessee, Utah, Virginia, Washington).
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* Notes on the distribution. P. juglandis and G. morbida were both recorded at the locations listed
above, except in three cases:
- Mexico. Only P. juglandis has been recorded, in the state of Chihuahua. These findings pre-date by
many decades the discovery of thousand cankers disease and description of G. morbida (as reported
in Wood and Bright, 1992). There are no known recent attempts to make dedicated collections of
either organism in Mexico.
- Indiana, USA. G. morbida was found in 2013 during surveys of trees that had been girdled and
killed. The fungus was associated with Stenomimus pallidus (Coleoptera: Curculionidae), an insect
associated with dying branches, and P. juglandis has not yet been found to date.
- Lombardia, Italy. Only P. juglandis was found so far. Seven adults were trapped at one site in July
2014.
The distributions above are based on information available at December 2014, and results of 2014
summer surveys may not all be available yet.
For references on geographical distribution see EPPO Global Database
https://gd.eppo.int/taxon/PITOJU/distribution
https://gd.eppo.int/taxon/GEOHMO/distribution
BIOLOGY
Pityophthorus juglandis
 Life cycle
P. juglandis attacks standing living trees, including healthy trees. Entry and emergence holes of P.
juglandis are present on the bark on larger diameter branches (generally above 1.5 cm on J. nigra,
Seybold et al., 2012b) or on stems. In caged experiments, entrance holes have been observed in J. nigra
seedlings with basal diameter as small as 0.55 cm (A. Mayfield, USDA Forest Service, USA, pers.
comm.).
Mating. At emergence, most P. juglandis adults fly to branches to mate and initiate new galleries for
laying eggs, while some may remain in the trunk and expand overwintering cavities (Cranshaw and
Tisserat, 2008). Males excavate a small nuptial chamber under the bark, release sex pheromones, and
are joined by a few females (1-2 according to Cranshaw and Tisserat, 2012a; 2-3 according to Graves
et al., 2010). In California, P. juglandis was observed to fly generally at dusk and little or not at night
(Seybold et al 2012a).
Egg. Females create egg galleries under the bark, which are generally across the grain of the wood
(Cranshaw and Tisserat, 2008; Tisserat and Cranshaw, 2012, Cranshaw and Tisserat, 2012a). Very few
experiments have been conducted to date on the fecundity of females. In one experiment, the average
number of egg indentions found per gallery with only one female present was 16,6 (7-25 eggs); eggs
are covered by a frass cap, which may be useful in protecting the eggs from enemies and keep them
from desiccating (Nix, 2013).
Larvae. There are three larval instars (Dallara et al., 2012). Larval galleries are meandering and
roughly perpendicular to the egg gallery (i.e. generally along the grain of the wood) (Cranshaw and
Tisserat, 2008, 2012a). In heavily-infested logs larval galleries overlapped (Nix, 2013). See Fig 1.
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Fig. 1. Larval galleries of P. juglandis. Photo by Albert Mayfield, USDA Forest Service.
Pupation and emergence. Pupation occurs at the end of the larval gallery and adults emerge through
minute, round exit holes (Cranshaw and Tisserat, 2008; Tisserat and Cranshaw, 2012). Emerging adults
either remain at the original tree or fly to other trees to mate and reproduce (Graves et al., 2010).
Emerging adults carry on their cuticule spores of G. morbida that can create new infections when they
tunnel into other branches or trunks to feed, reproduce or overwinter (Nischwitz and Murray, 2011;
Moltzan, 2011). After the tree starts showing foliar symptoms, enormous numbers of P. juglandis may
emerge from the trees. In an extreme case in Colorado, 23040 adults emerged from 2 logs of J. nigra
(circa 6/cm2) (Cranshaw, 2011).
A single generation was observed to be completed in 7 weeks in logs at room temperature (Tisserat et
al., 2009). Larval development was indicated as taking 4-6 weeks in Cranshaw and Tisserat (2008). In
laboratory conditions, Nix (2013) obtained development from egg to adult in 4-5 weeks, and eggs
hatched between 2-3 weeks after oviposition. 2-3 overlapping generations are generally mentioned,
depending on climatic conditions (Cranshaw and Tisserat, 2008, 2012a; Tisserat and Cranshaw, 2012;
Graves et al., 2010, all for Western USA).
The peak of flight activity is in summer, but there are variations in the length of the flight period. In
California, flying adults were caught every month except December, and were also trapped in January
2012 in Tennessee and Virginia (Seybold et al., 2012b; Chen and Seybold, 2014). In eastern
Tennessee, P. juglandis flight ceased (or slowed down) between late December-early March (Nix,
2013). In Colorado, adults fly in mid-April to October, overwintering starts in fall and some beetles
may complete development only in November, and limited breeding may extend well into autumn
(Cranshaw and Tisserat, 2008, 2012a; Tisserat and Cranshaw, 2012).
In areas of cold winter (e.g. Colorado), overwintering takes place as dormant adults in cavities in the
bark of the trunk, and as larvae that may continue development during warm periods (Cranshaw and
Tisserat, 2008 & 2012a; Tisserat and Cranshaw, 2012). Eggs are not present in areas with frequent
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periods of freezing (W. Cranshaw, Colorado State University, USA, 08-2014, personal
communication).
In areas with mild winter (e.g. California), overwintering stages include larvae at all stages of
development (USDA, 2011). No indication was found in the literature of whether eggs are present in
places of mild winter, which would suggest continuous breeding throughout the year at some location.
 Temperature thresholds: P. juglandis
Flight. P. juglandis flight activity peaks at temperatures between 23-24ºC and ceases below 17-18ºC
(Seybold et al., 2012a). Trapping guidelines recommend that trapping should be conducted in periods
when the temperature is greater than 18-19ºC (Seybold et al., 2012b).
Upper and lower lethal temperatures. The effect of temperatures was investigated in experimental
conditions (direct exposure). Upper lethal temperatures (LT 99) were 53°C for adults and 48 °C for
larvae (Luna et al. 2013; Peachey 2012). Lower lethal temperatures (LT99) reported in the two studies
differ (-18.1 for adults and -18.7 for larvae in Luna et al., 2013; -22.97 for adults and -25.19 for larvae
in Peachey, 2012)
Luna et al. (2013) mention P. juglandis survival in infested trees in Colorado where temperatures
reached −29°C in February 2011, suggesting that P. juglandis could survive the winter in much of the
native range of J. nigra, but may be limited in areas where temperatures regularly drop below the lower
lethal temperature. The degree-days requirements for P. juglandis are not known. Nix (2013)
conducted experiments, but those were not conclusive.
Geosmithia morbida
G. morbida is the first species of Geosmithia documented as a plant pathogen. Other Geosmithia are
common saprobes associated with bark beetles and ambrosia beetles on conifers and deciduous species
(Kolarik et al., 2011). G. morbida grows within and around the galleries, killing tissues and producing
cankers. Each canker is initiated by a wound created by P. juglandis, and multiple transmission of G.
morbida to a same tree leads to numerous cankers, which ultimately coalesce and destroy phloem
function, often leading to tree death.
Temperature thresholds: growth of G. morbida was found to be optimal at 31°C and limited at 41°C
(Kolarik et al., 2011). Higher temperatures may not enhance canker development: cankers in J. nigra
were consistently smaller at 32/20°C day/night temperatures compared to 25/20°C (Freeland et al.,
2012). Finally, in treatment experiments, G. morbida did not survive in logs exposed to treatments in
which minimum temperatures were 48 °C for 40 min (temperature measured in the sapwood, 1 cm
below the cambium) (Mayfield et al., 2014).
DETECTION AND IDENTIFICATION
Symptoms
Thousand cankers disease can result in crown symptoms and usually causes cankers on branches and
trunks. At early stages of the disease, the only indications of damage are the tiny entry holes of P.
juglandis. Small cankers are created at the feeding sites of P. juglandis, but they are generally not
visible. Individual cankers then spread inside the bark and reach the cambium between the inner bark
and the wood, turning tissue brown to black. With multiple infestations on the tree, multiple cankers
coalesce, cutting off supply of nutrients, resulting in dieback of branches (Cranshaw and Tisserat,
2012a) and later death of trees.
Crown symptoms.
Crown symptoms are not specific to P. juglandis and G. morbida, and may have other causes
(environmental stress, mechanical damage, damage by insects or other animals, storms etc.) (Cranshaw
and Tisserat, 2012a & b, Hansen et al., 2011).
The first symptoms are leaf yellowing, wilting of foliage and thinning of the crown, followed by twig
and branch dieback. Scattered small branches may show yellowing, and progressively larger branches
are affected and die. Dead leaves generally fall from declining branches (Cranshaw and Tisserat, 2008;
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Tisserat et al., 2009; Shaw et al., 2014). Pscheidt and Ocamb (2014) and Shaw et al. (2014) mention as
dominant symptom that branches fail to leaf out in spring.
Symptoms only appear after considerable canker formation. It is generally considered that it takes
several years to produce sufficient colonization to produce external symptoms according to tree species
and dimensions (age), but the exact duration is not known (5-20 years in Cranshaw and Tisserat, 2012a
& b). Windham (2012) note that as initial symptoms resemble those of drought stress, the disease may
remain unnoticed for some years.
Symptoms vary. In Tennessee, broad crown dieback may be accompanied with a lush growing lower
canopy, while in Western USA, dieback may be more scattered through the canopy. Top dieback
appears on some trees almost at the same time as branch dieback at various places throughout the
crown (Shaw et al., 2014). There is often very little sprouting from dying J. nigra in Colorado, and
more in Tennessee. These may be due to differences in growing conditions, initial tree health, and the
presence of other pathogens that enter affected trees (Cranshaw and Tisserat, 2012a). Foliage from
epicormic twigs growth may develop below the areas of the tree that have been killed (Cranshaw and
Tisserat, 2008; Graves et al., 2010; Conrad et al., 2013). In Colorado, basal sprouts often developed on
trees in advanced stages of decline, or from stumps of removed trees, but they also wilted and died
within 1-2 years after emergence (Tisserat et al., 2009).
Symptoms on individual branches.
Entry and emergence holes of P. juglandis can be observed on the bark on larger diameter branches
(generally above 1.5 cm on J. nigra, Seybold et al., 2012b) or on stems but these are difficult to see
when bark is deeply furrowed. Unlike many other canker diseases, the outer bark remains largely
intact, and the extent of canker and galleries development are apparent only when the outer bark is
mechanically removed (Graves et al., 2010; Tisserat and Cranshaw, 2012). Even when leaf wilting is
present, branches often show no outward appearance of bark damage except for entry and emergence
holes (Newton and Fowler, 2009 citing others). Staining of the bark, where it occurs, is most evident in
summer, and varies depending on the walnut species (Hasey and Seybold, 2010) and bark thickness. It
is more apparent in species with smooth bark (like J. regia) than furrowed (like J. nigra) (Graves et al.,
2009).
Cankers.
G. morbida causes two types of cankers (Cranshaw and Tisserat, 2008; Newton and Fowler, 2009;
Tisserat et al., 2009):
 Small, diffuse, dark brown to black cankers develop around the galleries of P. juglandis. Each
canker may originally be only a few millimeters in diameter, but ultimately measure up to 10-20
cm and often take an elongate oval form (lengthwise rather than circumferentially along the stem).
Eventually multiple cankers coalesce to produce girdling that results in branch dieback. The
number of cankers that are formed on branches and the trunk can be enormous in the late stages of
the disease. In the advanced stages of decline, galleries and associated cankers often occur every 2
to 5 cm in the bark.
 On J. nigra in advanced stages of dieback, diffuse cankers are much larger than branch cankers
(often exceeding two meters in length, extending from the ground into the scaffold branches, and
may encompass more than half the circumference of the trunk). Trunk cankers are not readily
visible without removing the outer bark, but dark staining on the bark surface or in bark cracks
often indicates the presence of a canker. The inner bark and cambium below the bark surface on
the canker face is macerated, water-soaked and stained dark brown to black.
Morphology
P. juglandis
Cranshaw and Tisserat (2008, 2012b) provide a short description of adults of P. juglandis and indicate
morphological characters that allow distinguishing adults of P. juglandis from other Pityophthorus spp.
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LaBonte and Rabaglia (2010) provide a screening aid to help differentiate P. juglandis from other bark
beetles in trap samples or specimens collected from suspect walnut trees in the USA.
Adults of P. juglandis are minute, 1.5-1.9 mm long, yellowish-brown, about three times as long as wide
(Cranshaw and Tisserat, 2008, 2012b). Larvae are white, C-shaped, typical of bark beetles, with a
reddish brown head capsule (Nix, 2013). Pupae are white, exarate with their body parts distinguishable.
Teneral adults are yellowish-brown and soft before they darken to a reddish brown and their elytra
harden (Blackman 1928). Galleries are 2-5 cm long (Graves et al., 2009).
Pictures of P. juglandis and G. morbida, as well as of signs and symptoms can be found, among others,
in Cranshaw and Tisserat (2008), LaBonte and Rabaglia (2010), Seybold et al. (2012b), Utley (2013),
as well as on the website of Forestry Images (http://www.forestryimages.org).
G. morbida
The original description of G. morbida is in Kolarik et al. (2011).
Detection and inspection methods
Visual examination of trees for symptoms
Surveys are best conducted in early to mid-summer. Earlier in the season, leaf yellowing and wilting
may not have developed; late in the season flagging may have other causes (e.g. mechanical injuries,
overshading, normal senescence), making detection of symptomatic branches more difficult (Cranshaw
and Tisserat, 2012a & b; USDA, 2014). Trees are examined for symptoms of infestation (crown
dieback or abnormal thin crown), and in closer inspection for individual branches that show flagging
(yellowing leaves, wilted leaves remaining on the branch). Special attention is given to the upper part
of the crown. Detection is very difficult in early stages of the disease, and is generally possible only if
external symptoms are expressed, which occurs several years after the tree is initially infested (Tisserat
and Cranshaw, 2012). A process for visual survey is detailed in USDA (2014).
If the disease is suspected, a sample may be taken and a pheromone-baited, multiple funnel trap
installed to target P. juglandis near walnut trees on the site. A short, pheromone-baited walnut branch
section may be installed below the trap or on a nearby pole to lure live beetles to aid in the detection of
G. morbida (USDA, 2014).
Sampling
On suspicion of infestation, samples are taken and the presence of holes, galleries and cankers is
investigated. Sampling is best conducted on living branches showing recent leaf wilting or dieback (in
dead branches, galleries are numerous but cankers difficult to delineate because the walnut bark
oxidizes and turns brown at death). Small diameter branches with smooth bark are best to observe the
entry and emergence holes of P. juglandis (Cranshaw and Tisserat, 2012b). Branches should be at least
2-cm diameter, (Cranshaw and Tisserat, 2012a). Seybold et al. (2013) recommend collecting branches
5-10 cm diameter and 15-30 cm long with visible symptoms.
Galleries and cankers can be seen by carefully removing the bark (Cranshaw and Tisserat, 2012b). One
should not cut too deeply when peeling the bark because the galleries and fungus are found in the bark
(phloem) and not in the cambium or sapwood. Galleries and cankers are often more numerous on the
bottom side of branches and the west side of the trunk. P. juglandis larval tunneling is almost entirely
limited to the bark (phloem) and exit holes are often clumped due to the multiple insects developing in
close proximity (Cranshaw and Tisserat, 2012b). Exit holes, galleries and cankers may be confused
with those of other organisms, and identification is required (Cranshaw and Tisserat, 2012b).
At mill sites, logs should be examined for evidence of old sap staining on the bark, and bark should be
removed to detect galleries (Graves et al., 2009).
Trapping
Pheromone-baited traps are used to detect P. juglandis or delimit a known population. Pheromonebaited branches are used in low population areas to sample for G. morbida.
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 Pheromone-baited traps
The use of pheromone-baited traps to detect and monitor P. juglandis is detailed in Seybold et al.
(2012b) (developed and tested in orchards, urban and wildland landscape with low to high levels of P.
juglandis populations). Trapping uses a multi-funnel trap (Lindgren) baited with the male-produced
aggregation pheromone. Other types of traps may be used (sticky-coated and other barrier-type traps),
but are not as easy, convenient or consistent (Seybold et al., 2012b). The pheromone (compound X - 3methyl-2-buten-1-ol; Seybold et al., 2012c) became commercially available in 2012 (details in Seybold
et al., 2012b). Evidence of a female-produced pheromone compound has also been observed (but is not
available for trapping) (Cranshaw and Tisserat, 2012a).
Appropriate locations for trapping are sites identified to have walnuts with unexplained dieback, wood
waste utilization sites, firewood lots, and saw or veneer mill sites with walnut logs and branches. Traps
can be used near walnut trees in various types of sites, including residential areas, parklands and
roadways in urban areas, walnut plantations, orchards, riparian areas, etc. (USDA, 2014). They can also
be used on trees where P. juglandis is suspected but sampling of limbs is difficult (Cranshaw and
Tisserat, 2012b).
Traps are best placed in the proximity of healthy or declining walnut trees, near the larger branches of
these trees. Traps should be located about 4,5-6 m from the main stem of the tree, 1,5-3 m from live
branches, and about 2,5-3 m from the ground (to avoid P. juglandis infesting healthy branches or
trunks) (Seybold et al., 2012b). Traps may more easily detect beetles than a physical survey of
branches: when beetle populations are low, few branches are affected or they may be inaccessible
(Cranshaw and Tisserat, 2012b).
Pheromone-baited traps can be deployed whenever P. juglandis are active (from March through
November when ambient air temperatures are greater than 18°C). Traps are checked every 7 to 14 days,
and lures replaced approximately every 2 to 3 months (depending on temperature at the site) (USDA,
2014).
Detection of P. juglandis across a large area uses a much lower density of traps than the higher density
needed to assess the extent of a known population (USDA, 2014).
 Pheromone-baited branches.
In situations where the populations of P. juglandis are low, branches infected with G. morbida are
difficult to obtain in the field (for confirmation of G. morbida), and pheromone-baited branches are
used to lure live P. juglandis. P. juglandis inoculates G. morbida into the branch section, which can be
analyzed for G. morbida in the laboratory (USDA, 2014).
Freshly cut, healthy walnut branch sections (smooth barked; length ca. 45 cm, diameter ca. 3-6 cm) are
baited with the lure and suspended from a pole. When left in the field for 2-4 weeks during the peak
flight period, P. juglandis finds and colonizes the branch section. When 10-20 entry holes have been
observed on the trap branch section, it can be examined for galleries and cankers. In areas with low
population densities of P. juglandis, the branch sections may need to be left in the field longer to
accumulate sufficient entrance holes (method fully described in USDA, 2014).
 Other traps.
P. juglandis has been trapped in yellow sticky panels on walnut trees and on sticky clear panels stapled
to walnut trunks, but captures may be incidental. Attempts to increase capture by use of walnut wood,
pityol and other compounds useful in trapping some other bark beetles do not increase capture of P.
juglandis in these traps (Cranshaw and Tisserat, 2012a).
Identification
P. juglandis.
P. juglandis is identified morphologically. LaBonte and Rabaglia (2010) provide a key to differentiate
P. juglandis from other bark beetles. In the EPPO region, it is not known if there is a risk of confusion
10
with European Pityophthorus species (although Duffy (1953) and Balachowsky (1949) mention that
those are associated with coniferous trees).
G. morbida.
G. morbida should be cultured for confirmation. It can be cultured from samples of wood taken from
cankers, or P. juglandis individuals, or beetle frass, on a 1/4 strength potato dextrose agar (PDA)
(Cranshaw and Tisserat, 2012b; Tisserat and Cranshaw, 2012). Details of culturing of G. morbida are
given in Cranshaw and Tisserat (2012b), as well as on the morphology of the colonies and conidia in
Kolarik et al. (2011). Isolations from trunk cankers may be more difficult if the bark is macerated.
Species-specific PCR primers have not been developed for G. morbida, because this fungus can easily
be identified based on morphological characteristics and the ease by which it can be isolated from
diseased tissue. The identity of the fungus can be confirmed by sequencing the rDNA ITS region using
the primers ITS1 or ITS 5 and ITS4 (Cranshaw and Tisserat, 2012b).
Growth and morphological characters that differentiate G. morbida from other Geosmithia are
described in Kolarik et al. (2011). Fusarium solani and other Fusarium species may also be isolated
from cankers, but their morphological features highly differ from G. morbida, as reported in Cranshaw
and Tisserat (2012b).
MEANS OF MOVEMENT AND DISPERSAL
The disease can spread naturally and with infested plant material. Natural spread will depend on many
parameters (presence of host trees, climatic conditions, wind). Even though the flight capacity of P.
juglandis is unknown, other small bark beetles of similar size are capable to fly long distances. In
Nilssen (1984), distances of 86 km was noted for Pityogenes chalcographus. G. morbida appears to
require a vector to establish well and produce the multiple, coalescing cankers required to kill a tree.
All forms of Juglans wood still carrying bark (incl. round wood, wood chips, wood waste and untreated
wood packaging material), bark, and possibly plants for planting can play a role in pest spread. Humanassisted pathways (in particular wood) are considered to have played a more important role for spread
in the USA and are critical for spread over long distances and across geographic barriers.
PEST SIGNIFICANCE
Nature of damages
Damage and death result from repeated inoculations of G. morbida by P. juglandis, and of thousands of
resulting coalescing cankers (Utley et al., 2013). The mechanism of tree death is not known, but it may
result from depletion of energy reserves by cutting off nutrients movement (Tisserat and Cranshaw,
2012). Other organisms may also contribute to tree decline during the last stages of the disease.
Once crown symptoms become visible, death may occur within a few years. In Western USA, the
disease is ultimately fatal to essentially all trees of J. nigra once infested. In Colorado, J. nigra is
usually killed within 3-4 years after initial leaf yellowing symptoms (Cranshaw and Tisserat, 2012b).
Mortality may occur within 2 years or more rapidly for smaller trees (< 10-cm diameter at breast
height) or trees growing on sites prone to drought stress (Tisserat et al., 2009). Pscheidt and Ocamb
(2014) mention that trees may die within 2-5 years, but that the majority have a slower dieback over
many years (e.g. J. nigra x J. hindsii in the Pacific Northwest).
Recent observations suggest that, even in areas where thousand cankers disease has caused extensive
damage, disease progress can vary (Cranshaw and Tisserat, 2012b). It is suggested that there can be
both “acute” and “chronic” phases of this disease, with rapid progression (to death) in the acute phase
and slower progression and even an arresting of disease progression in the chronic phase. The reason
for this is unknown.
P. juglandis readily attacks standing living trees, including healthy trees. Although it was shown to
infest logs in certain conditions (e.g. reinfesting treated logs), no information was found in the literature
on whether fallen or cut trees are preferentially colonized if there are standing trees around (e.g. in a
forest).
10
11
G. morbida is located in the phloem and cambium, and produces localized cankers in and around the
galleries of P. juglandis (Cranshaw and Tisserat, 2012a). G. morbida does not produce deep staining,
but it may reach the sapwood (superficially) at advanced stages of the disease, and may result in a
brown to black discoloration of the sapwood (Cranshaw and Tisserat, 2012a & b). In a debarking
experiment (Mayfield et al., 2014), G. morbida was recovered from the sapwood surface of one log,
suggesting that G. morbida can occur in the extreme outer sapwood and can survive, at least
temporarily, on the surface of a freshly debarked walnut log.
In Italy, cankers and holes were observed on 1-1,5 cm diameter twigs in the field; G. morbida can be
artificially inoculated and originate cankers in 5-10 mm diameter plants (2-year old), both J. nigra and
J. regia (L. Montecchio, pers. comm., 2014).
Fusarium solani has been isolated from cankers; its role in tree mortality is not known, although this
fungus is known to be pathogenic to J. nigra and may contribute to canker development on the main
trunk and tree mortality (Tisserat et al., 2009; Cranshaw and Tisserat, 2012a,b ; Tisserat and Cranshaw,
2012; Utley et al., 2013; Sbrizza, 2014).
Economic impact
In the USA, to date amenity trees have been most affected. However, the greatest potential impacts of
thousand cankers disease are considered on timber production (primarily J. nigra) with additional
losses to nut production (primarily J. regia).
It is difficult to estimate the value of Juglans as an amenity plant. These include walnuts that occur in
gardens, parks, along streets and fence rows, and in woodlots in the urban-forest interface. In one area
of the USA, Boulder Colorado, where thousand cankers diseases has been present for over a decade,
the value of affected plants is estimated at approximately $3 million and over 60% of trees died within
6 years of its original detection (Newton and Fowler, 2009 citing others). Many municipalities and
homeowners in the USA have already incurred costs associated with loss of Juglans amenity plants due
to tree removal and replacement costs, indirect effects on shade, heating/cooling, and added landscape
value to property.
Potential impact on timber and nut production has been estimated in the USA. In Eastern USA, where
the potential damage and losses are ultimately expected to be greatest, it is estimated there are 306
million live J. nigra trees, with a live volume of 112.76 million m3 (Missouri with 57.9 million trees,
followed by Ohio with 24.6 million trees and Kentucky 24.5 million trees) (Randolph et al 2013). The
estimated value for J. nigra growing stock in the USA is estimated to be over $500 billion (Newton and
Fowler 2009). USA exports of walnut logs and wood products are currently estimated at $325 million,
annually. The potential impact of thousand cankers disease on J. nigra in its native range (Eastern
USA) is still unclear as it has only recently been discovered there and does not yet affect the primary
areas of USA Juglans timber production. Phytosanitary requirements imposed by import markets may
directly impact the value of export markets for certain Juglans materials.
California is the major producer of nuts in the USA (supplying ca. 99% of the walnuts consumed in the
USA and 33% of those traded worldwide) with an annual value (2006) estimated at $563 million. P.
juglandis has been present in Southern California, but historically has not been an issue in cultivated J.
regia until recent years. Pscheidt and Ocamb (2014) note that J. regia is not as susceptible as J. nigra,
but the disease is easily found in J. regia orchards. Potential losses related to thousand cankers are
likely to be due to decreased nut production (from loss of twigs and branches) and decline or death of
producing trees (Newton and Fowler, 2009). Certain rootstocks on which nut-producing J. regia may
be grown (e.g. ‘Paradox’ a hybrid of J. hindsii x J. regia) are susceptible to thousand cankers disease.
J. regia is considered to be less susceptible to thousand cankers disease than are some other Juglans
species (e.g. J. nigra, J. hindsii), however, mortality, although not extensive, has been observed.
There are also costs incurred by government and universities associated with survey and detection,
monitoring, public outreach, and development and implementation of interstate quarantines.
Social damage
12
Social damage in the USA is currently due to death of amenity and garden trees, but losses of jobs are
anticipated for the future. In some cities of Colorado, mature J. nigra have been nearly eliminated
(Tisserat and Cranshaw, 2012). For example 60% of the population of J. nigra trees in Boulder (over
1300 trees) were removed in 2004-2012, and fewer than 300 asymptomatic trees with breast-height
diameter over 25 cm remain. Similar mortality was observed in Colorado Springs and Denver (Tisserat
et al., 2009; Tisserat and Cranshaw, 2012). In some areas of the Midwestern USA, there is cultural
value given to the collection and use of J. nigra nuts by families and small communities. To the extent
that thousand cankers impacts nut production, these values may be reduced.
Control
No control methods are currently available to effectively protect individual trees from developing
thousand cankers disease or to cure diseased trees. Research is actively conducted on control methods
(chemical, biological control, semiochemical, resistant cultivars) of this recently-recognized pest
complex.
Phytosanitary risk
Among the species known to be present in the EPPO region, at least J. ailantifolia, J. californica, J.
cinerea, J. hindsii, J. major, J. mandshurica, J. microcarpa, J. nigra, J. regia and several hybrids are
known to be susceptible, to various degrees, to thousand cankers disease. The native Juglans regia and
the exotic J. nigra are the two main species used for wood and nut production, as well as ornamental
species in the EPPO region and are also important in the environment. Other species of Juglans and
Pterocarya are also used as ornamental species. All walnut trees of the EPPO regions are at risk in the
long term. The greatest risk is to J. regia nut production, with secondary losses to timber (J. regia, J.
nigra) and amenity plants. Although there are differences in the occurrence of Juglans and their uses
between the USA and the EPPO region, similar impacts from thousand cankers disease can be expected
(even if there is still an uncertainty on the potential impact on J. regia).
PHYTOSANITARY MEASURES
(to be completed when the PRA is finalized)
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