15-20693 (15-20466, 15-20340, 14-20210)
WP PR Point 7.2.2
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
Heterobasidion irregulare
IDENTITY
Scientific name: Heterobasidion irregulare (Underw.) Garbelotto & Otrosina
Synonyms: Polyporus irregularis Underwood, Torrey Bot Club. Bull. 24:85, 1897. The names "North American
H. annosum P ISG"; Fomes annosus, Fomitopsis annosa, Polyporus annosus are also mentioned in the literature.
Taxonomic position: Fungi; Basidiomycota; Russulales; Bondarzewiaceae.
Common names: Maladie du rond des pins (Québec), Annosus root and butt rot (USA)
Notes on taxonomy: H. irregulare is part of the H. annosum complex (H. annosum sensu lato, [s.l.]). It was
recognized as a distinct species by Otrosina and Garbelotto (2010). The 5 species of the complex (2 from North
America and 3 from Europe/Asia) were previously considered as 5 intersterility groups (ISGs) of H. annosum
s.l., with different host preferences (although overlapping to a certain extent). The North American S ISG
(mostly on Abies, Picea, Pseudotsuga, Tsuga, Sequoiadendron) was named as H. occidentale (Otrosina and
Garbelotto, 2010). The European and North American P ISGs (mostly on Pinus, but also on several other
genera) were shown to have nearly complete interfertility, phenotypic similarities, close levels of genetic
relatedness and similar host range and infection biology, but to be two clear sister taxa with no evidence of
recent gene flow (Stenlid and Karlsson, 1991; Otrosina et al., 1993; Linzer et al., 2008). The North American P
ISG was named H. irregulare (Otrosina and Garbelotto, 2010), while the European P ISG is now H. annosum
s.s. There is a strong genetic differentiation between the Western and Eastern (incl. Midwest) populations of H.
irregulare in North America.
EPPO code: HETEIR
Phytosanitary categorization: EPPO Alert list
HOSTS
The most important hosts of H. irregulare belong to the family Pinaceae and Cupressaceae, in particular the
genera Pinus and Juniperus and the species Calocedrus decurrens. Among Pinus spp., H. irregulare is
considered more likely to be associated to P. taeda, P. elliottii, P. ponderosa, P. jeffreyi, P. banksiana, P.
resinosa in North America and, in the infested area in Italy, to P. pinea and P. halepensis, than to other Pinus
hosts. Abies balsamea is also considered as a main host. Pseudotsuga menziensii is host but is not frequently
infested by H. irregulare. There is some uncertainty about the frequency of association with some conifer hosts
in North America, such as Larix, Picea glauca, Thuja plicata, Tsuga canadiensis. Several species of Picea are
among the hosts, as well as the three species of Larix present in North America (L. lyallii, L. laricina, L.
occidentalis).
A number of native conifer species of the EPPO region are known to be hosts according to records in Italy in the
field (Pinus pinea, P. halepensis; Gonthier et al., 2004; Scirè et al., 2008) and in North America in an arboretum
in California (P. sylvestris, P. pinaster, P. brutia; Bega, 1962 for H. annosum, later confirmed to be H.
irregulare). The susceptibility of Picea abies, P. sylvestris and P. pinaster, which are major species in the EPPO
region, has also been determined experimentally (inoculation studies – Lind et al., 2007; Garbelotto et al., 2010;
Lung-Escarmant et al., 2012). Finally, several North American tree species that are planted in the EPPO region
are hosts, such as Pinus banksiana, P. radiata, P. strobus and P. taeda, Calocedrus decurrens, Pseudotsuga
menziensii, Picea sitchensis.
Regarding angiosperms, a number of species have been identified as hosts, most notably Arbutus menziesii and
Arctostaphylos spp. for North America. For many other angiosperm hosts, reports are limited to a few sporadic
records. There are several angiosperm hosts in the family Ericaceae.
GEOGRAPHICAL DISTRIBUTION
EPPO region:
Central Italy* (Lazio region).
North America:
Canada (Ontario, Quebec, British Colombia), Mexico, USA* (Alabama, Arizona, California, Colorado, Florida,
Georgia, Illinois, Indiana, Iowa, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Mississippi, Missouri,
Montana, Nebraska, New Hampshire, New Mexico, North Carolina, Ohio, Oregon, South Carolina, Texas,
Vermont, Washington, Wisconsin).
Caribbean*:
Cuba, Dominican Republic.
*Notes on the distribution
The natural range of H. irregulare is generally considered to cover North America, South to Mexico, and there
are uncertainties in relation to the Caribbean and Central America. The distribution of H. irregulare is best
documented for USA and Canada. In the USA, it is considered to be widespread, but occurs predominantly in the
Eastern and Western parts of the country (and not as much in the Central part). There are additional records for
the USA, not mentioned above, which refer to H. annosum and pre-date the description of H. irregulare.
In Italy, H. irregulare is present in the Lazio region along the Tyrrhenian Coast (from Fregene Monumental
Pinewood in the north to San Felice Circeo in the south). It extends 9 km inland at Castel di Guido in the North
and 18 km at Fossanova in the South (Gonthier et al., 2014a). It was also found in the gardens of several
historical villas in Rome (Ada, Doria Pamphili, Borghese - D’Amico et al., 2007; Scirè et al., 2008, 2009).
There are uncertain records for several countries of the Caribbean (Jamaica) and Central America (Guatemala,
Honduras), as well as for Brazil. It is considered likely that H. irregulare may be present in Central America, and
possibly on Caribbean islands with endemic pine populations (M. Garbelotto, University of California, 2014-12,
pers. comm.).
For references on geographical distribution see Global Database:
https://gd.eppo.int/taxon/HETEIR/distribution
BIOLOGY
Within the H. annosum complex, H. annosum s.s. and H. irregulare have broadly similar morphology, biology
and life cycle. The morphology, biology and life cycle of H. annosum s.l. are described in details in Woodward
et al. (1998), based on a review of available literature to that date. Most elements in this data sheet are common
to several species; those specific to H. irregulare are indicated.
 Life cycle (from Greig, 1998; Korhonen and Stenlid, 1998)
As other species in the H. annosum complex, H. irregulare is pathogenic on some of its host species (mostly
conifers), and a saprobe on some others, for which the mycelium colonizes stumps, roots or dead trees. In the
former case, infection may occur through primary infection (spores spread) or secondary infection (mycelial
spread from an infected root system to a neighbouring tree via root contacts or grafts). Korhonen and Stenlid
(1998) mention that H. annosum s.l. is probably a saprobe on many of the recorded hosts.
Figure 1. Primary and secondary infections by H. annosum s.l. (from Stenlid, 1986)
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Primary infection
Primary infection by H. irregulare is caused by basidiospores released by perennial fruiting bodies
(basidiocarps) and infecting freshly exposed wood surfaces, such as stumps from recently cut trees or wounds on
stems or roots of living trees.
H. irregulare, like other Heterobasidion species, produces two types of spores: basidiospores and conidiospores.
Due to the longevity of fruiting bodies and spore-producing capacity, basidiospores are expected to constitute the
majority of inoculum (Redfern and Stenlid, 1998). Although conidiospores are also able to cause infection, their
exact importance is not known (Korhonen and Stenlid, 1998). A major difference between basidiospores and
conidiospores is that basidiospores are actively released into the air, while conidiospores are released passively
(moved by rain, wind, animals).
For H. irregulare, stump infection is more significant than wound infection, although the latter may also happen.
The success of infestation depends on many factors, including tree species as well as abiotic factors. For stump
infection, such factors include temperature (germination of spores is prevented at high or low temperatures – see
below), humidity at the surface and relative air humidity, the period of susceptibility of the stump (commonly 23 weeks after logging), and competitiveness with other microorganisms.
H. irregulare can infect roots (wounded and possibly unwounded) without first infecting stumps (Hendrix and
Kuhlman, 1964 for P. elliottii; Alexander et al., 1975 for Pinus taeda). This is not frequent (except on sandy
soils with low organic content) and is considered less important than stump infection.
Production of fruiting bodies (basidiocarps). Fruiting bodies are perennial, and therefore maintain their
reproductive potential for several years. They are typically found on dead trees or stumps and are generally
located at ground level, often partially covered by moss or leaf litter. Occasionally, they occur up to 2 m above
the ground. Fruiting bodies may also be formed on infected roots or wood debris (at a distance from a stem),
giving the impression that they emerge from the ground. In some cases, they may form on living trees in which
rot is very advanced. They can also form on the cut end and underside of logs left in the forest or on the root
system of fallen trees. Under extreme conditions (e.g. cold or dry-warm), they form only in protected places
(such as hollow stumps, underside of logs lying on forest floor) (Redfern and Stenlid, 1998).
The formation of fruiting bodies needs a well-aerated, moist medium, moderately high relative humidity and
moderate light intensity (Korhonen and Stenlid, 1998). The duration needed for the formation of fruiting bodies
therefore depends on environmental and host conditions, and can range from a few weeks to several years.
Müller and Korhonen (2006) mention that fruiting bodies of. H. annosum s.s. can appear within 1 year after
felling, but their frequency and size is highest 3-4 years after logging. In the laboratory, the formation of fruiting
bodies can be induced on suitable substrate within 6 weeks to several months (Korhonen and Stenlid, 1998).
Fruiting bodies of H. irregulare were observed after two months in inoculation experiments on P. sylvestris (1820°C at 80% RH; Giordano et al., 2014).
Production of spores. For all species in the complex, fruiting bodies produce large number of basidiospores.
Sporulation is influenced by temperature and humidity (Redfern and Stenlid, 1998 – see abiotic factors below).
Most spores are deposited in the vicinity of the fruiting body, depending on climatic conditions and forest
structure (Redfern and Stenlid, 1998). A total of 99% of the spores released deposit within 100 m of the source
(Mökkenen et al., 1997; Korhonen and Stenlid, 1998); however, some spores have been shown to be carried
longer distances. In Italy, studies available have shown that the deposition of spores of H. irregulare is highest
below 500 m, significant up to 10 km, and minimized above 80 km from the source of sporulation (Garbelotto et
al., 2013; Gonthier et al., 2014a). Spore release from fruiting bodies of H. annosum s.l. varies diurnally and
seasonally, and a range of deposition rates of 0.59 to 1932 spores per dm2 of fertile hymenium per hour was
measured (Redfern and Stenlid, 1998). Spores may occasionally become associated to insects, but insects are
unlikely to be a major dissemination factor (Redfern and Stenlid, 1998). Basidiospores may also land on surfaces
other than stumps or wounds. If they are deposited on bark or foliage, they may survive for some months (in
ideal conditions) and be washed down by rain or fall on stumps during felling. If deposited on soil, they are may
be washed down by rainfall. Spores in soil cannot germinate in the absence of appropriate substrate, but may
survive for up to one year (see below). In order to germinate, they need contact with a root (generally wounded
roots, but healthy roots may be infested; Stenlid and Redfern, 1998).
Approximately 1-5 % of the spores deposited are viable, and viability declines rapidly in light (Redfern and
Stenlid, 1998). In experiments, basidiospores survived in soil for more than one year but lost viability in wet soil.
Extreme survival is mentioned as being 5 years in dry soil at 10°C. No information was found on survival and
viability in the field. Conidiospores survived for at least one year in artificially infested soil (Redfern and
Stenlid, 1998). The viability of spores and mycelium in soil would decrease very rapidly. The best conditions for
3
fungus survival in soil seem however to be as mycelium present in woody substrates (e.g. infected roots or
pieces of decayed wood in the soil).
Secondary infection
Secondary infection is caused by mycelium spreading from the roots of the infested stumps or trees, to
neighbouring host trees by vegetative growth through root contacts and, less frequently, root grafts. Depending
on host trees density, individual mycelia can develop to occupy an area of 50 m in diameter, but in most cases
the diameter is less than 30 m and involves only a few trees (Garbelotto and Gonthier, 2013, citing others). Big
stumps can stay infectious for decades after felling (62 years mentioned for H. annosum s.l.; Greig and Pratt,
1976). The viability of the fungus in roots or pieces of wood in the soil depends on the volume and decay rate of
the material, soil properties (see abiotic factors below) and the presence of antagonistic organisms). The rate of
spread and decay in trees vary according to parameters such as host species susceptibility, wood moisture and
competitiveness with other organisms. Wood colonization by mycelium occurs at a rate of 0.2-2 m per year
(Stenlid and Redfern, 1998; Gonthier et al., 2007 citing others).
Abiotic factors and importance of management methods
Temperature and humidity seem to be the most important abiotic factors for infection and spore release and
deposition. Wind may also influence spore dispersal and deposition. Soil properties influence primary root
infection and secondary infection through roots.
Temperature (from Greig, 1998, unless otherwise indicated) influences growth and survival of mycelium and
spores, as well as infection of stumps/trees.
- For H. annosum s.l., mycelium was shown to grow above 0-2°C with an optimum at 22-28°C; growth stops at
32-37°C and the mycelium dies within 2 hours at 38-45°C (Korhonen and Stenlid, 1998). The optimum for
H. irregulare from Italy was 20-25°C (Scirè et al., 2011). Smith et al. (2012) (for Wisconsin, i.e. H.
irregulare) noted seasonal variation in the development of colonies: on a medium exposed in winter during
periods of deep snow and cold temperature, there were few colonies, but colonies did develop occasionally on
medium exposed below 0°C. US Forest Service (ND) note that mycelium in wood is killed after exposure for
one hour at 40°C. The range of temperatures for germination of spores is similar (for H. annosum s.l.,
Korhonen and Stenlid, 1998); spores germinate in 20 h at 12-38°C, and do not germinate at 0-2°C or 40-42°C
within 60 h. Conidiospores and basidiospores died in 1 h at 45°C and 90% RH. Regarding lower
temperatures, mycelium tolerates low temperatures to at least -30°C and conidiospores can be frozen at very
low temperature for long period (e.g. in liquid nitrogen) without losing vitality.
- Regarding spore production and deposition, the average minimum air temperature of a four-week period has
been identified as a suitable predictor variable for modelling H. annosum s.l. spore deposition (Garbelotto and
Gonthier, 2013). Spore release and infection depends on climatic conditions (summarized in Garbelotto and
Gonthier, 2013). In Italy, high levels of spore deposition of H. annosum s.s. were detected in winter and
significantly lower levels in summer, whereas H. irregulare produced spores throughout the year. In
California, spores are also produced all year round, even with abundant snowfall (James and Cobb, 1984). In
the South-Eastern United States, high summer temperatures reduce H. irregulare sporulation and also result
in high stump temperatures that prevent infection (40°C, Gooding et al., 1966). Basidiospore production is
abundant above 5°C; in southern USA, it is limited when the daily maximum temperature reaches 32°C
(Korhonen and Stenlid, 1998). Spore release stops at 38°C (Redfern and Stenlid, 1998). It is not known if
temperature would influence H. irregulare in other parts of Europe as it influences other European species of
the H. annosum complex: in Northern Europe, infection occurs mostly during summer, and not at all in
winter, and snow prevents infection. In the UK, stump infection occurs most of the year. In the Alps and
central Europe, spores are released as early as February at most sites, but mostly in August-October
(Garbelotto and Gonthier, 2013, citing others). In Northern Europe, spore infections occur generally at
temperatures above + 5°C and infection rates increase linearly up to approximately + 25°C (Redfern and
Stenlid, 1998).
Humidity. The water content of the stump and relative humidity of the air are important for the infection. Too
low (i.e. below 20%-30%) and too high (250%) wood moisture content (in % of dry mass) (Bendz-Hellgren and
Stenlid, 1998) limit the infection rates and progression, as for other pathogens causing root rot.
Rainfall and wind. Rainfall at spore release brings the inoculum to the ground, rainfall 14-28 days earlier
promotes infection, largely due to increased activity of mycelia and spore production in fruiting bodies. Spore
deposition is also influenced by wind direction and air turbulence.
4
Radiation influences survival because UV radiation inhibits the growth of mycelium and damages spores
(basidiospores more rapidly than conidiospores) (Korhonen and Stenlid, 1998, Greig, 1998). Spores subject to
UV radiation can lose more than 95% vitality within a few days (Kallio 1974).
Characteristics of the soil. Soil properties (e.g. texture, lime content, water content) and depth may have an
impact. H. annosum s.l. does not occur on very acid soils (pH < 2,6) and may be favoured in soils with high
calcium and pH (which generally has a negative impact on antagonists) (Woodward et al., 1998). The risk is
generally considered higher on sandy soils with high pH and less on poorly drained peat (Stenlid and Redfern,
1998).
Management methods. Korhonen and Stenlid (1998) consider that these are more important than the abiotic
factors for the expression of damage of H. annosum s.l. Thinning conducted in environmental conditions
favourable to the fungus favours spread and development of H. irregulare populations. Site history is also
important. Spread and damage is higher on fertile soils, especially in even-aged and monospecific plantations
established on former agricultural land (low amount of antagonists, superficial root systems increasing root
contact), lower on heath land (similar to old forest soils), and low on old forest soils (where the risk depends on
the presence of H. irregulare in the site or in neighbouring sites and on management practices). Using non-hosts
or host with low susceptibility generally decreases the risk.
DETECTION AND IDENTIFICATION
Symptoms
Symptoms are similar for species in the H. annosum complex, and the symptoms below were mostly described in
relation to H. annosum s.l. (except where indicated).
In a stand or forest, the disease spreads centrifugally from an initial infection point, and often shows as a circle
of dead and declining trees. Disease gaps correspond to the development of the fungus from an initial infection
point to neighbouring trees through secondary infection (root contacts). A single genotype may extend up to 30
m from its initial infestation point to neighbouring trees. When multiple infestations coalesce, a disease centre
may cover large areas (up to 10 times the area covered by a single infection centre. Fruiting bodies may be
observed on stumps, dead trees or other locations as described under Morphology. It is uncommon to find
fruiting bodies on living trees.
On single trees, symptoms are not always visible on infected trees and, when present, are not characteristic of the
disease. Symptoms may be: thinning and yellowing/discoloration of needles, stunting and growth reduction of
the trees (reduced height compared to neighbouring trees), shorter period of needle retention, shorter needles
(Greig, 1998; Garbelotto and Gonthier, 2013; US Forest Service, ND). They may appear late in the development
of the disease, but increase with time.
In wood, the first symptom of infection is a discoloration; the colour varies depending on the tree species. On
pine it is initially dark-purple, turning yellowish-whitish when a white rot appears. Pine wood impregnated with
resin due to initial infection response often remains undecayed for years or decades. US Forest Service (ND)
note the following symptoms on wood: bark separating easily from the wood; streaking of the wood surface with
darker brown lines; small silver to white flecks on the surface of the inner bark; commonly, heavy resin
accumulation in the wood of pines. Decay is almost always characterized as fibrous with white pitting.
Symptoms and the progression of the disease vary depending on the tree species, and are similar for all H.
annosum s.l.
- In trees with resinous heartwood (such as Pinus spp.), only the roots and the base of the stem are colonized,
leading to mortality and increased susceptibility to wind damage (Korhonen and Stenlid, 1998). Trees of all ages
can be diseased (“pencil thick seedlings” to old trees). Decay is usually confined to the lower part of the stem (up
to 1 m above the base of the trunk), rarely rising higher. There are exceptions, such as Larix in the case of H.
irregulare.
- In trees with non-resinous heartwood, it is expected that H. irregulare behaves in the same manner as other
species in the H. annosum complex, although there is no specific data on this. In such species, there may not be
external symptoms for many years. Decay may raise several meters into the stem (12 m mentioned in Stenlid and
Redfern, 1998) and for some species a hole may develop in the centre of the tree (Greig, 1998). Death occurs
only at very advanced stages of the disease, and wind damage may occur before.
- In Abies, no detailed data was found in relation to H. irregulare on its known host A. balsamea. However, the
related species H. occidentale often causes a saprot on Abies spp., which after several years can cause a decrease
in vigour of the plant, characterised by slower growth rates, discoloration of the crown and shorter needle
retention (M. Garbelotto, University of California, 2014-12, personal communication).
5
Secondary insect attacks, for example by bark beetles, may accelerate the progression of the disease. H.
irregulare has an impact on tree growth by impairing nutrient and water uptake, and because trees allocate part
of their energy to defence instead of growth (Froelich et al., 1977). Pines are more likely to be killed, but also
show growth loss (Stenlid and Redfern, 1998). Garbelotto and Gonthier (2013) note that, H. annosum s.s. and H.
irregulare are generally more effective than other species within the complex in colonizing the cambium and
sapwood, both in the root system, and at or just above trunk base, and tree mortality can occur more rapidly than
for other species in the H. annosum complex.
Mortality may occur within a few years in species whose roots are extensively attacked. Young trees of
susceptible species (e.g. pines) may die within 1 year (needles turning reddish, then brown, eventually falling),
while older trees die slower (up to 10 years or more) and may show some non-specific symptoms of decline
(such as crown thinning) (Greig, 1998). Delatour et al. (1998) reports on experiments on 1-2 years old pine
seedling, where mortality was observed 30-40 days after inoculation, but some infected plants were not killed.
Mortality of infected trees typically starts 3-8 years after a thinning operation (Wisconsin DNR, 2014, referring
to Pinus). In California, H. irregulare girdles pine trees at the trunk base within 2-6 years after infection,
resulting in tree mortality (US Forest Service, ND).
H. annosum s.l. sometimes causes root rot on certain deciduous species (Redfern and Stenlid, 1998). The
symptoms and impact of H. irregulare (and other H. annosum s.l.) on plants that are not main hosts are not clear.
On deciduous trees in Wisconsin, large fruiting bodies can be present at the base of seedlings, but dieback
symptoms have not yet been observed (Wisconsin DNR, 2014).
Morphology (from Greig, 1998; Korhonen and Stenlid, 1998; others as indicated)
Mycelium. The mycelium is white to brownish and develops only on wood. It grows at the interface between the
bark and the cambium of roots and root collar, and depending on the tree species may extend into the sapwood or
more rarely the heartwood. The mycelium produces fruiting bodies (basidiocarps, sexual stage) and
conidiophores (asexual stage).
Fruiting bodies (basidiocarps). Fruiting bodies occur singly or in groups, and may be pileate to resupinate. They
aremostly shelf-shaped and can measure up to 30 cm in length (Otrosina and Garbelotto, 2010). When pileate,
the upper surface is tan to dark brown with a whitish margin, and the lower surface is white to cream-coloured.
When resupinate, only the porous, white to cream-coloured surface is visible. Fruiting bodies can be easily
detached from their substrate. Their inner tissue is soft, corky to woody and cream-coloured. Pores are circular to
semi-elongated in section (0.3-0.6 mm) and their density is 7.3 ± 0.12 /mm2 (mean SE) (Otrosina and Garbelotto,
2010). Small fruiting bodies initials (up to circa 1 cm) appear as whitish-light brown pads of mycelium (or
“pustules”), on the bark of roots or under the bark of the stem base (Otrosina and Garbelotto, 2010).
Conidiophores. Conidiophores (for all H. annosum s.l.) are club-like, small and develop in moist atmosphere.
They are rarely observed, and generally found on moist wood surface (e.g. broken roots, insect galleries), not
commonly above the ground. They persist for a few weeks and are inconspicuous (Redfern and Stenlid, 1998).
Spores. Basidiospores and conidiospores measure 4.2-5.5 x 3.4-4.0 µm and 4.8-6 x 3.6-5 µm, respectively
(Korhonen and Stenlid, 1998, citing others). Scanning electron microscopy studies have shown that
basidiospores of H. annosum are ornamented with numerous echinulations, while conidia have a relatively
smooth surface (Shaw III and Florence, 1979)). The basidiospores of H. annosum s.l. are normally
homokaryotic. Conidia formed by heterokaryotic mycelia can be homokaryotic (representing one of the parental
genotypes), but a significant number are heterokaryotic (and enclose both parental genotypes) (Woodward et al.
1998, and references therein).
Detection and inspection methods
Detection in the field may be based on visual examination of trees for symptoms of the disease and fruiting
bodies, as well as spore trapping. In all cases, confirmation requires identification to the species level. Symptoms
are likely to be expressed after a certain level of infestation. They are not characteristic and vary between host
species. Consequently, spore trapping followed by identification is likely to be the most efficient option for
detection in an area.
Detection of fruiting bodies and symptoms on trees. The host on which symptoms or fruiting bodies are found
may provide an indication of the Heterobasidion species that is present, but identification is needed because
species in the H. annosum complex have overlapping host ranges (e.g. H. irregulare and H. annosum s.s.).
Similarly, fruiting bodies cannot be identified morphologically to the species level (see Identification below).
6
Finally, stain or rot may be observed in the wood at thinning or felling, but samples would need to be collected
for identification to species (Greig, 2008).
Sampling of trees (e.g. using increment borers or drills) followed by identification may be useful in some
circumstances, for example on symptomatic trees in areas where both H. irregulare and H. annosum s.s. occur.
Juzwik (ND) gives an example of sampling of trees: excavating two main roots of from opposite sides of a
suspected tree, removing a 6-10 inch (15-25 cm) segment, followed by identification.
Spore sampling. Spores can be trapped on sticky surfaces, plates of artificial media or wood discs (Redfern and
Stenlid, 1998; Gonthier et al., 2001; Garbelotto et al., 2010: 11-12 cm diameter wood discs of Picea abies,
exposed for 24 h (unselective for saprophytic growth of Heterobasidion). Burkhard spore samplers may also be
used. Trapping of spores with wood disc exposure method using a suitable trapping design is likely to indicate
the presence of the fungus within 500 m (Gonthier et al., 2012). Spore trapping is an easy monitoring tool, but
requires molecular identification to the species level of colonies growing from the deposited spores.
Identification
Fruiting bodies can be identified to the genus level, but not to the species level. There are small morphological
differences between the fruiting bodies of the species in the H. annosum complex, but morphological characters
often overlap (Garbelotto and Gonthier, 2013). Some characters may point towards H. irregulare, such as
irregular elongated pores, however the chance of distinguishing Heterobasidion species from each other based
on their morphology depends on the species present in the area. In particular, morphological characters are not
sufficient to discriminate H. irregulare from H. annosum s.s., especially because the two species can hybridize
(Giordano et al., 2014). However fruiting bodies can be cultured for further study and species identified based on
morphology and barcode analysis.
In the absence of fruiting bodies, identification to the genus level may be done using traditional methods. Wood
discs or wood fragments collected from suspected trees/stumps may be incubated to produce the conidiophores,
which can then be identified at the level of H. annosum s.l.; Greig, 2008; Garbelotto and Gonthier, 2013).
Alternatively, the fungus can be cultured on selective media (Greig, 2008).
Identification to the species level requires molecular methods. There is currently no single method that allows
identification of the five species within H. annosum s.l. However, PCR methods and primers to discriminate
combinations of species are available, and summarized in Gonthier and Thor (2013). A taxon-specific PCR for
single-spores colonies is described in Gonthier et al. (2007) to distinguish H. irregulare from H. annosum s.s.
Only methods using multiple independent markers would allow detection of hybrids of H. irregulare and H.
annosum s.s.
Finally, it is not possible to use in vitro mating tests (used in some EPPO countries to distinguish, for example,
between H. annosum and H. parviporum, A. Hietala, Norwegian Forest and Landscape Institute, 2014-12,
personal communication) to identify H. irregulare, because of the significant interfertility with H. annosum s.s.
MEANS OF MOVEMENT AND DISPERSAL
H. irregulare can spread both naturally and with infested plant material. Natural spread depends on many
parameters, including the possibility for the fungus to bridge gaps in areas with discontinuous presence of a host.
In Italy, in such an area, H. irregulare has been able to cross vegetation gaps up to 20–30 km but not 50 km.
Natural spread happens through spores that infect stumps from freshly cut trees or wounds on standing live trees,
or through mycelium spreading via root contacts and grafts to roots of surrounding susceptible trees (Redfern
and Stenlid, 1998; D’Amico et al., 2007; Garbelotto et al., 2010, Garbelotto and Gonthier, 2013). Spore dispersal
is often limited to a few hundred meters at most, and is minimized at 80 km (Gonthier et al., 2007 citing others;
Garbelotto et al., 2013; Gonthier et al., 2014a). Dispersal at longer distance is considered possible, but rarer.
99% of spores deposit within 100 m of the source (Korhonen and Stenlid, 1998). Wood colonization by
mycelium occurs for H. annosum s.l. at a rate of 0.2-2 m/year (Stenlid and Redfern, 1998; Gonthier et al., 2007,
citing others). Individual mycelia can develop to occupy an area of 50 m diameter (in most cases less than 30 m
and involving only a few trees; Garbelotto and Gonthier, 2013; Gonthier and Thor, 2013).
PEST SIGNIFICANCE
Nature of the damage
H. irregulare causes root and butt rots in its host plants. It colonizes the cambial layer and sapwood of its hosts
and, in some species the heartwood. The colonization by mycelium in the wood results in staining (at initial
stages of the disease, dark, almost purple stain and later fibrous white rot). On some species, including many
7
pine species, infection of roots is extensive, and trees may die within a few years of infection). On non-resinous
tree species, infested trees may remain alive for several decades, even in the presence of extensive rot extending
high into the stem heartwood.
H. irregulare causes direct damage on wood via staining and rot. No specific data was found for H. irregulare,
but rot in the wood, even at early stages of infection, may also reduce the strength of the wood and pulping
qualities (Korhonen and Stenlid, 1998; for Picea sitchensis for H. annosum s.l.). Rot decreases the volume of
marketable timber.
The fungus leads to reduction in tree growth (Garbelotto and Gonthier, 2013) and decreases site productivity.
Infested trees are also predisposed to wind damage (Georgia Forestry Commission, 2013). This has also been
shown for H. annosum s.l. in southern Sweden on Picea abies (Oliva et al, 2008). Infested trees may also present
increased susceptibility to attack by bark beetles (US Forest Service, ND).
Economic impact
H. annosum s.l. is a major pathogen in forest plantations in the Northern hemisphere. H. annosum s.l. has been
known to occur in North America for at least a century, but reports of significant damage are more recent. The
first reports of southern pine mortality came from Georgia and South Carolina in 1954 (Ostry and Juzwik, 2008).
The disease has become more prevalent in pine plantations in recent years (e.g. Wisconsin DNR, 2014,
Blanchette et al., 2015). Disease incidence is reported to increase with stand age at rates that depend on host
species and silvicultural management techniques. In the USA, although Heterobasidion is present in various
environments, it causes most problems in plantations that have been thinned (Georgia Forestry Commission
2013). Filip and Morrison (1998) report minimal mortality of seedlings in regeneration areas, while others
indicate losses. In California (where both H. irregulare and H. occidentale occurs), Heterobasidion root and butt
rot is one of the most important conifer diseases, and affects about 2 million acres (over 800 000 ha) of
commercial forest land by causing an annual volume loss of 19 million cubic feet (≈500 000 m3) (US Forest
Service, ND).
In a 6-year study on 20-years old P. elliottii in Eastern USA, in trees with more than 50% of their roots infected,
reduced diametric tree growth was observed detected three year after thinning, and reached 20% in trees with
vigorous crowns. Height growth was reduced by 40% (Froelich et al., 1977).
Mortality is observed on many pine species (most notably: P. resinosa, P. taeda, P. elliottii, P. strobus, P.
banksiana, P. jeffreyi, P. coulteri, P. radiata and P. ponderosa in North America, P. pinea in Italy) as well as on
other conifers, most notably Abies balsamea, Juniperus virginiana (Dumas and Laflamme, 2013, Wisconsin
DNR 2014, Gonthier et al., 2007, Filip and Morrison, 1998). On P. elliottii, mortality centres were observed 2-3
years after thinning and 30% mortality observed in some stands (Filip and Morrison, 1998).
In the infested area in Italy (in Lazio), H. irregulare is found in monospecific pine plantations, in urban parks
and in oak-pine mixed woodlands (Gonthier et al., 2014a). Mortality of trees was observed, and it is higher in
sites where H. irregulare has been present longer. Significant mortality of groups of trees (up to 100) was
reported for P. pinea (D’Amico et al., 2007; Gonthier et al., 2007). P. pinea is used for pine nut production in the
infested area, and losses in pine production are probably comparable to losses in cover.
Phytosanitary risk
All pine trees of the EPPO regions are at risk in the long term, but also some other conifer species. H. irregulare
in Italy has already been found on P. pinea and P. halepensis (Gonthier et al., 2004; Scirè et al., 2008). Other
major native conifer species of the EPPO region are known to be susceptible (P. sylvestris, P. pinaster, P. brutia)
(Bega, 1962 for H. annosum, later confirmed to be H. irregulare). The susceptibility of Picea abies, P. sylvestris
and P. pinaster, has also been determined experimentally through artificial inoculation. There is a large number
of other conifer species in the EPPO region that may be attacked. Finally, several North American tree species
that are widely planted in the EPPO region are known as hosts, such as Pinus radiata, P. taeda and P. strobus,
Pseudotsuga menziensii, Picea sitchensis.
It is considered that H. irregulare could add to the damage already caused by H. annosum s.s. on a number of
forest tree species, and could also have a significant impact on pine nut production (especially P. pinea). In
addition, H. irregulare could affect species that are not significantly damaged by H. annosum s.s. H. irregulare
has also shown its ability to move to different hosts and, at least on the Italian Tyrrhenian coast, is more
competitive than H. annosum s.s. H. irregulare has a much higher fruiting and saprobic ability compared to H.
annosum s.s., resulting in higher rates of primary infection by basidiospores, and possibly in higher rates of
8
secondary spread through root contacts. In addition the impact of hybridization between the two species remain
unknown, and hybridization may lead (or not) to greater damage, with increased virulence and changes in host
range (possibly of the two species) in the long-term.
PHYTOSANITARY MEASURES
(to be completed when the PRA is finalized)
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