Ecology and Epidemiology
Development of Nested Polymerase Chain Reaction Detection
of Mycosphaerella spp. and Its Application to the Study
of Leaf Disease in Eucalyptus Plantations
M. Glen, A. H. Smith, S. R. H. Langrell, and C. L. Mohammed
First, second, and fourth authors: Forest Biosecurity and Protection, Ensis, Private Bag 12, Hobart, Tasmania, Australia, 7001; second and
fourth authors: CRC for Sustainable Production Forestry, University of Tasmania, Private Bag 12, Hobart, Tasmania, Australia, 7001; and
third author: CSIRO Forestry and Forest Products, Private Bag 5, Wembley, Western Australia, 6913.
Current address of S. R. H. Langrell: European Commission, Directorate General Joint Research Centre, Institute for Health and Consumer
Protection, I-21020 Ispra, Italy.
Accepted for publication 9 August 2006.
ABSTRACT
Glen, M., Smith, A. H., Langrell, S. R. H., and Mohammed, C. L.
2007. Development of nested polymerase chain reaction detection of
Mycosphaerella spp. and its application to the study of leaf disease in
Eucalyptus plantations. Phytopathology 97:132-144.
Mycosphaerella leaf disease (MLD) is a serious disease of two of the
major eucalypt species grown in temperate regions worldwide, Eucalyptus globulus and E. nitens. More than 30 species of Mycosphaerella have
been reported on eucalypts worldwide. Accurate, rapid, and early discrimination of Mycosphaerella spp. causing crown damage to E. globulus
and E. nitens will assist the development of sustainable management
strategies. This study describes the development, and incorporation in a
nested polymerase chain reaction (PCR) approach, of specific primers for
the detection and identification of Mycosphaerella spp. commonly reported from leaf lesions of E. globulus and E. nitens in Australia. Primer
design was assisted by sequence alignment and phylogenetic analysis of
165 nonredundant sequences from the nuclear ribosomal DNA internal
transcribed spacer regions of Mycosphaerella and related species. Phylogenetic analysis revealed very high sequence similarity for two taxon
groups, Mycosphaerella grandis and M. parva, and M. vespa, M. ambi-
Eucalyptus spp. are a major global hardwood pulp crop.
Mycosphaerella spp. cause severe leaf diseases of temperate
Eucalyptus plantations in wetter areas of Australia and serious
disease in New Zealand, South Africa, Spain, and some regions of
Chile (19). Many species appear to have spread to other temperate
regions from Australia, and species that also occur in the Australian
subtropics appear to have spread to Indonesia, Vietnam, China,
and South America (21,58,62). Indigenous temperate Australian
forests have mixed eucalypt species where Mycosphaerella leaf
disease (MLD) generally is of minor significance, except in
coppice regrowth in certain Western Australian forests (1,14). In
southern Australia, MLD in plantations appears to be caused
principally by Mycosphaerella nubilosa and M. cryptica, although
M. vespa, M. tasmaniensis, M. parva, and M. grandis occur frequently. M. marksii and other species are encountered less often
(13,49,51,52,60). The most damaging are reported to be M. nubilosa and M. cryptica (10–12,14,25,59–62), which cause the most
severe leaf disease, resulting in significant crown damage of
E. globulus, E. nitens, and their hybrids (30,39), the most widely
Corresponding author: M. Glen; E-mail address: Morag.Glen@csiro.au
DOI: 10.1094 / PHYTO-97-2-0132
© 2007 The American Phytopathological Society
132
PHYTOPATHOLOGY
phylla, and M. molleriana, and primers were designed to differentiate
each of the two groups. Three other species, M. cryptica, M. nubilosa,
and M. tasmaniensis, were distinct and distinguished by species-specific
primers. In double-blind trials, the detection test accurately and rapidly
identified Mycosphaerella spp. in cultures and discriminated against other
pathogens that co-occur in or on Eucalyptus leaves, thereby verifying its
reliability. The detection test has an internal amplification control in the
first-round PCR with fungal-specific primers to raise confidence in test
results, particularly to highlight negative results due to PCR inhibition.
When applied to DNA extracted from leaf or stem samples either as
multiple or single lesions, it detected and identified up to five
Mycosphaerella spp. or taxon groups in both positively identified and in
young (putative) MLD lesions. The samples were 20 mm2 or larger in
surface area and were collected while undertaking disease rating assessments in an experimental investigation of Eucalyptus plantations and
regrowth forest. Using nested PCR detection, Mycosphaerella spp. were
positively identified in 2 days, 1 to 5 months earlier than by classical
methods, demonstrating the potential application of this detection test to
the early discrimination of MLD components in ecological, epidemiological, and genetic investigations.
grown temperate plantation species. Crown damage in E. globulus
in Australia has been reported to range from 10% leaf necrosis to
complete defoliation and tree death in locations strongly conducive to disease development (22,23,50,52). MLD has restricted
the use of commercially desirable eucalypt species in Australia,
New Zealand, and South Africa (28).
Currently, MLD is detected visually by lesion characteristics
that also are used in crown damage assessment of plantations
(71). Many foliar diseases that have much lower impact also
produce symptoms similar to MLD and are difficult to distinguish
without experience or training. MLD identification may be confirmed using conventional methods. Conventional methods for
positive identification of these and other leaf pathogens involve
waiting until lesions produce “typical” reproductive structures or
forms, isolating them, and identification of putative pathogens
using taxonomic criteria. A challenge for those evaluating
Mycosphaerella spp. affecting eucalypts is that reproductive structures may or may not form in leaves or cultures and, often, the
most readily available identifying characteristics, such as germinating ascospores, are also taxonomically ambiguous (10,15,49,
51,59–62). In addition, pseudothecia do not form in axenic
cultures of most species and are found only in older lesions or, for
a few species, on inoculated aseptic leaves in culture (59,60).
Identification is further confounded by the fact that more than one
Mycosphaerella spp. may be present in a single lesion (43,62).
Other limitations of conventional methods include low isolation frequencies of all or even most Mycosphaerella spp. from lesions. Detailed sampling does not always deliver all of the Mycosphaerella
spp. present in each lesion. Hence, crown damage assessment
based on lesions is not necessarily attributable to a particular
Mycosphaerella spp.
Molecular detection of plant pathogens is used in many agricultural spheres to assist decision support systems. These include
immunological and DNA and polymerase chain reaction (PCR)based methods. Immunological detection of Mycosphaerella brassicicola from spore traps has been developed to rationalize pesticide applications within crop protection programs (42). However,
pathogen discrimination is still dependent on spore production,
which may occur at a late stage in development in the disease
cycle and by which stage prophylactic intervention may not be a
management option. Furthermore, fungicide use may be financially or environmentally prohibitive in tree plantations. Detection
at an earlier stage in the fungal life-cycle would be preferable and
may permit development of silvicultural options to prevent disease or reduce disease severity (9,11,52). PCR-based methods
exploit genetic sequence differences to provide early, rapid,
specific, and sensitive detection and identification of pathogens of
concern regardless of the presence of reproductive structures.
PCR amplification using species-specific primers has been widely
used for disease diagnosis in humans, other animals, and plants
(2,67). Specific PCR tests have been developed for M. graminicola (Septoria tritici) in wheat (27) and to distinguish M. fijiensis
and M. musicola from banana leaves (40). PCR detection may
target specific genes or anonymous products, such as microsatellites, random amplified polymorphic DNA, or amplified fragment
length polymorphism fragments (35,67,68,72). In order to provide
simultaneous discrimination for several Mycosphaerella spp. of
eucalypts with high reproducibility and reliability, a wellcharacterized DNA region which is likely to be highly stable must
be targeted. There are more than 30 species of Mycosphaerella on
Eucalyptus spp., and related anamorphs increase this number to
over 50 (5,6,15,17,18,49). In wheat, the M. graminicola-specific
PCR amplifies a fragment of the β-tubulin gene (27). However,
many more species occur in eucalypts than on wheat, and few βtubulin sequences are currently available for eucalypt species. The
specific primers for M. fijiensis and M. musicola (banana
pathogens) were designed to sequence divergence within internal
transcribed spacer (ITS)1 (40). In addition, molecular phylogenetic studies in the families Mycosphaerellaceae on Myrtaceae
have been based mainly on ITS1 and ITS2 sequences (19,20,
33,62,64,70). These studies have provided reliable characters for
the discrimination of species for which morphological characters
such as spore dimensions are variable within species and overlap
among species (62), leading to taxonomic confusion and frequent
misidentifications. The value of ITS sequence data is attested to
by the numerous Mycosphaerella sequences available on public
databases. Multiple sequences are available for species causing
serious diseases of industrial crops, allowing comparison of geographically diverse isolates from different host species. In addition to providing interspecific discrimination, the ribosomal RNA
genes, including the ITS regions, are multicopy, assisting in
sensitive detection from plant material. Sensitivity is essential as
lesions may have mixtures of many fungal species in addition to
several Mycosphaerella spp. A PCR test based on ITS sequences
exists for some Mycosphaerella spp. on eucalypts; however, the
published test requires a restriction digestion after PCR, with
discrimination based on fragment size (43), adding considerably
to the processing and analysis time. Analysis of restriction
fragments also may be confounded when multiple species are
present, as is often the case in MLD.
Therefore, the primary aim of this investigation was to develop
PCR-based systems for direct detection and identification of several
Mycosphaerella spp. from lesions of all ages in eucalypt species as
a research tool aimed at refining current knowledge of
epidemiology, ecology, and host resistance and to provide the
potential for certification of disease-free germ plasm. Although the
tests developed here were primarily for use on Eucalyptus spp.
leaves and stems, they also were intended to be applicable to a
wider range of woody species that are potential hosts for
Mycosphaerella spp. For example, other members of the family
Myrtaceae, members of other Myrtales families, and species of the
family Proteaceae are actual or potential hosts that co-occur
naturally throughout the landscape in Australia with Eucalyptus
spp. Plantations are grown in close proximity to natural vegetation
harboring Mycosphaerella spp. in Australia, South Africa, most
probably South America, which shares a Gondwanan origin, and
Indonesia. In addition to detecting Mycosphaerella spp. directly
from leaves, a detection system also could be used to verify
whether samples collected or isolated during earlier surveys belong
to the species or taxon group in which they had been identified
according to the taxonomic criteria at the time.
PCR, although fast, sensitive, and capable of being automated,
is not without problems. These include enzyme inhibition by
substances present in the template DNA, which may cause false
negative results. Plant material is particularly notorious for PCR
inhibitor compounds, many of which often are co-precipitated
during DNA extraction (45). Strategies to overcome PCR inhibition and increase confidence in negative results include using a
range of DNA template dilutions (40), spiking of negative
samples (40), buffer additions during the DNA extraction process
(4,34,63) magnetic DNA capture (4,45,63), post-extraction purification (44), and the use of alternative polymerases (3). In the
interests of speed, economy, and wide applicability of the tests, it
is desirable to avoid repetitious assays or lengthy and involved
DNA purification techniques. A rapid, simple DNA purification
method based on binding of DNA to silica in the presence of
chaotropic salts (31) avoids the use of toxic and volatile reagents
and has been used extensively in our laboratory for obtaining
amplifiable DNA from a wide range of material. Nested PCR also
has been used to amplify DNA containing inhibitors from plant
material (53,75). Because none of these strategies for overcoming
PCR inhibition were completely successful, detection of PCR
inhibition is important to raise confidence in negative results. This
can be achieved by the use of internal amplification controls
(IACs). Amplification of endogenous IACs using a different primer
pair has been shown to be unreliable in detecting PCR inhibition
(37); therefore, we constructed an exogenous IAC that can be added
at a controlled concentration to each sample in the first round of a
nested PCR. Specific primers were used in the second-round PCR
to detect the presence of five of the most commonly reported
Mycosphaerella spp. in southern Australian plantations. The results
of a survey of Mycosphaerella spp. in an E. globulus plantation in
northern Tasmania to establish a baseline of the species present
prior to the setting up of experimental trials also are presented here.
The increased coverage, spatial resolution, and accuracy provided
by using the detection test as a survey tool for Mycosphaerella spp.
are discussed.
MATERIALS AND METHODS
Fungal material. Cultures (Table 1) were maintained on 2%
malt extract agar. Nonviable cultures up to 10 years old also were
used for DNA extraction. Standard reference cultures for each of
the target species were nominated based on the most reliably
identified material available. These were M. cryptica (AA/1/3/10),
morphologically identified by Milgate (51), ITS sequenced here,
sequence consistent with sequences in GenBank for nine other
M. cryptica isolates; M. grandis (Q/1/1/1), morphologically identified by Milgate in collaboration with A. J. Carnegie, and ITS
Vol. 97, No. 2, 2007
133
sequenced (51), sequence identical with that of AC165, neotype material from A. J. Carnegie; M. nubilosa (Z/1/1/11),
morphologically identified by Milgate (51), ITS sequenced, and
consistent with 13 other sequences in GenBank; M. tasmaniensis
(STE-U1555), an ex-type culture from Tasmania (21); and
M. vespa (BS/3/2/1), morphologically identified by Milgate in
collaboration with A. J. Carnegie, and ITS sequenced (51).
These cultures have been deposited in the Department of
Agriculture Western Australia Plant Pathogen Collection
(WDCM77).
TABLE 1. Fungal cultures used to test species-specific primers
Species
Botryosphaeria sp.
Cryptosporiopsis eucalypti
Cylindrocladium retaudii
Mycosphaerella africana
M. cryptica
M. grandis
M. nubilosa
M. parkii
M. parva
M. pini
M. suberosa
Mycosphaerella sp. 1 (received as M. parkii)
Mycosphaerella sp. 2 (received as M. parkii)
M. tasmaniensis
M. vespa
Phaeophleospora eucalypti
Quambalaria cyanescens
Q. eucalypti
Q. pitereka
a
Superscripts identify culture source.
CSIRO FFP Canberra.
c Centraal Bureau voor Schimmelcultures..
d Type strain.
e University of Tasmania.
f CSIRO FFP Perth.
g Angus Carnegie, LaTrobe University.
h DNA was extracted from an old, nongrowing culture.
i New Zealand Forest Research.
j International Mycological Institute.
k University Viçosa, Brazil.
b
134
PHYTOPATHOLOGY
Codea
WA36b
I00108ab
I00150b
CBS680.95c,d
AA/1/3/10e
NZ301He
E7444af
I00205b
I00207b
I00208b
I00210b
I00214b
I00216b
I00217b
I00225b
I00226b
I00234b
I00235b
I00236b
AC165g,h
I00206b
I00219b
S/1/2/3e
Q/1/1/1e
H/1/3/1e
AC106g,h
L/1/2 multisporee
StMarys/2/4e
StMarys/2/8e
Z/1/1/11e
E1 multisporee
Z/1/1/13e
S/2/1/1e
CBS 387.92c,d
AC83g,h
AC85g,h
AC89g,h
CBS110503c
DT3b
CBS 436.92c
CBS 208.94c
CBS 209.94c
CBS516.93c
I555d,e,h
BS10/2e,h
S/3/2/7e,h
A/1/3e
B/3/2/1e
A/1/7e
B/3/2/4e
BrunyIs/1/1e
NZFS85C/1i
NZFS85C/3i
NZFS85C/23i
IMI178848j
S8k
E7440f
Host
Eucalyptus wandoo
E. camaldulensis
E. urophylla
E. viminalis
E. globulus
E. nitens
E. pilularis
E. globulus
E. globulus
E. globulus
E. globulus
E. globulus
E. globulus
E. globulus
E. obliqua
E. obliqua
E. globulus
E. cypellocarpa
E. cypellocarpa
E. saligna
E. globulus
E. globulus
E. globulus/nitens
E. nitens
E. nitens
E. globulus
Not recorded
E. globulus
E. globulus
E. globulus
E. globulus
E. globulus
E. globulus
E. grandis
E. globulus
E. globulus
E. globulus
E. globulus
Pinus radiata
E. dunnii
E. grandis
E. grandis
E. globulus
Not recorded
E. nitens
E. globulus/nitens
E. globulus
E. globulus
E. globulus
E. globulus
E. globulus
E. nitens
E. nitens
E. nitens
E. pauciflora
E. globulus (bark)
Corymbia maculata
Origin
Western Australia
Queensland
Queensland
South Africa
Tasmania
New Zealand
New South Wales
Victoria
Victoria
Victoria
Victoria
Victoria
Victoria
Victoria
Victoria
Victoria
Victoria
New South Wales
New South Wales
Victoria
Victoria
Victoria
Tasmania
Tasmania
Tasmania
Victoria
Victoria
Tasmania
Tasmania
Tasmania
Tasmania
Tasmania
Tasmania
Brazil
Victoria
Victoria
Victoria
Western Australia
New South Wales
Brazil
Indonesia
Indonesia
Brazil
Tasmania
Tasmania
Tasmania
Tasmania
Tasmania
Tasmania
Tasmania
Tasmania
New Zealand
New Zealand
New Zealand
New South Wales
Uruguay
Western Australia
Sampling of leaf material from plantations and forest regrowth sites. A survey of 2-year-old E. globulus was conducted
at Smithton in northwestern Tasmania on 8 August 2003
(S40°55′21′′, E144°59′39′′). The plantation of approximately
21 ha was divided into six sections and 10 leaf samples were
taken from separate trees in each section. Juvenile leaves were
sampled from branches at a height of 1.2 m from trees that were
up to 4 m tall. These trees were selected using systematic
sampling starting from the first two trees, moving over a row and
sampling the next two trees, and so on, until the required number
of samples was taken. Leaves were taken from the fourth to fifth
leaf pair on each branch to minimize variation in leaf age,
physiological development, height in the canopy, and age of the
lesions. Lesions from lateral branch stems also were sampled
from seven randomly selected E. globulus trees in the same trial.
Additional samples for confirmation that the test was applicable
to other host species were collected at four different sites in north
and northeastern of Tasmania. In all, 16 E. nitens samples
were taken from plantations at Weldborough (S41°13′14′′,
E147°51′60′′), St. Helens (S41°20′41′′, E148°3′55′′), and Scottsdale (S41°15′34′′, E147°25′25′′). Leaf material was sampled from
branches at a height of 1.2 m and sampled from the fourth to fifth
pair on each branch from trees ≈5 to 6 m tall, selected using
a systematic sampling regime as described above. A collection
of four samples was made from natural forest regeneration
of E. regnans at a roadside site near Bicheno (S41°48′19′′,
E148°14′53′′). Leaves were taken from the fourth pair on
branches. Trees were ≈1.5 m tall and ≈12 months old.
Lesions were excised with a scalpel, using a fresh blade for
each sample to avoid DNA cross-contamination. Each sample was
1 cm2 and consisted of up to 15 lesions per leaf, pooled. The stem
samples from E. globulus were obtained by scraping the surface
of stem lesions with a scalpel to produce shavings and approximately 80 mm2 of stem surface area was taken.
DNA extraction, PCR, and sequencing. Plant material and
fungal mycelium were ground in 1.5-ml microfuge tubes with a
pellet pestle mixer (in liquid nitrogen for leaf and stem samples),
mixed with extraction buffer (65), and incubated for 1 h at 65°C.
Tubes were centrifuged at 14,000 rpm for 15 min and the supernatant removed. Purification was achieved by binding to a silica
matrix in the presence of NaI (31). PCR was carried out in an
Applied Biosystems (Foster City, CA) GeneAmp PCR System
2700 thermocycler. Reactions of 25 µl contained 0.55 U of TTH+
Polymerase (Fisher Biotec, Subiaco, Western Australia) in 1×
polymerization buffer (Fisher Biotec), 2.0 mM MgCl2, bovine
serum albumen at 0.2 mg/ml, 0.2 mM each dNTP, and 0.25 µM
each primer. For sequencing, the entire ITS1, 5.8S, and ITS2
regions were amplified using primers ITS1-F (29) and ITS4 (76)
and the following thermocycler program: 94°C for 3 min; 35
cycles of 94°C for 30 s, 55°C for 30 s, and 72°C for 30 s; followed by an extension step of 72°C for 7 min. PCR products were
electrophoresed on a 1% agarose (Fisher Biotec) gel at 4 V/cm for
1 h. Before sequencing, PCR products were purified with an
UltraClean PCR Clean-up DNA purification kit (MoBio Laboratories, Inc., West Carlsbad, CA). Sequences were determined with
an ABI Prism Dye Terminator Cycle Sequencing kit on a Bio-Rad
373 Sequencer (Hercules, CA) with stretch upgrade. Sequences
were obtained in both directions and assembled with Lasergene
software (DNAStar, Madison, WI).
Phylogenetic analysis and primer design. Sequences generated
in this project are listed in Table 2. Further sequences were
retrieved from GenBank and EMBL. Mycosphaerella and
anamorph species included in the analysis were selected on the
basis of (i) occurrence on members of Myrtaceae or (ii) high
sequence similarity to a species from Myrtaceae. All available
sequence variants for each species were included with the
exception of two related anamorph groups (Cercospora and
Trimmatostroma), with sequence similarity of <85% to any of the
target species and which previously have been shown to form
distinct clusters in phylogenetic analyses (20,33). Hortaea
werneckii was included on the basis of high sequence similarity,
but only one ITS sequence was selected from the 17 available at
the time. Alignments were performed with CLUSTALW (73) and
adjusted manually. Phylogenetic analyses were executed with
DNAPars and DNAML of the PHYLIP package (26) on the
Australian National Genome Information Service (ANGIS,
Sydney, NSW, Australia). Initial analysis was by DNAPars, with
Phaeoramularia hachijoensis (AY251086) set as the outgroup,
jumbling 10 times, and all other options as default. The set of
most parsimonious trees were subjected to maximum likelihood
analysis by DNAML with Phaeoramularia hachijoensis
(AY251086) set as the outgroup, global rearrangements on, and
all other options as default. The sequence alignment and
phylogenetic tree were deposited in TreeBase (accession number
TBA). Primers were designed following the general concepts
outlined by Dieffenbach (24), with two additional aims. To permit
future multiplexing of the five specific PCRs, primer sites were
selected to produce amplicons of different size for each species
and primer lengths were modified to result in the same annealing
temperature for all primers. BLAST searches of GenBank and
EMBL also were carried out using the taxon-specific primer
sequences.
Primer specificity testing. Primers were tested for specificity
on a broad range of template DNA, including pure cultures of
Mycosphaerella spp., other eucalypt leaf pathogens (Table 1), and
clean eucalyptus leaves. To obtain optimum specificity and
efficiency of amplification, several different annealing temperatures were tested. Primer lengths were adjusted by 1 or 2 bases
until all primer pairs amplified the target species or group at the
same annealing temperature, with no amplification from other
species.
Construction of an IAC for first-round PCR. A plasmid
containing primer-binding sites for the primers ITS1-F (29),
ITS5, and ITS4 (76) was constructed by ligation of a histone gene
fragment amplified from Phaeophleospora eucalypti DNA using
composite primers (Table 3). The forward primer sequence is a
composite of ITS1-F, ITS5, and H3-1a (32), with H3-1a at the 3′
end, and the reverse a composite of ITS4 and H3-1b (32), with
H3-1b at the 3′ end. A product of ≈1,100 bp was amplified using
conditions for the H3-1a/H3-1b primers (32). This product was
purified with an UltraClean PCR Clean-up DNA purification kit
(MoBio Laboratories, Inc.) and transformed into Escherichia coli
using a pGEM-T cloning kit (Promega Corp. Madison, WI).
Plasmid DNA was extracted using an Ultraclean plasmid extraction kit (MoBio Laboratories, Inc.).
Nested PCR. All reactions were set up in UV-irradiated
laminar flow hoods with positive (DNA from pure culture of
target species) and negative (DNA from pure cultures of nontarget
species and no added template) controls included in each set of
reactions. Aerosol barrier tips were used routinely and gloves
TABLE 2. Sequences generated in this project for primer design and species
verification
Species
Mycosphaerella africana
M. cryptica
M. grandis
M. nubilosa
M. parkii
M. parva
Mycosphaerella sp. 1
Mycosphaerella sp. 2
M. suberosa
M. tasmaniensis
M. vespa
Phaeophleospora eucalypti
Accession numbers
AY626981
AY667576, AY626989, DQ471928
AY626986
AY667577, DQ471926
AY626979 (type isolate)
AY626980
AY66982, AY626983
AY626984
AY626985
AY667578 (type isolate)
DQ471925, DQ471927
AY626987, AY626988
Vol. 97, No. 2, 2007
135
were always worn and changed frequently to minimize crosscontamination. First-round PCR used primers ITS1-F and ITS4 as
outlined above, with the addition of 25 fg of IAC in each 25-µl
reaction. Failure of first-round amplification (as occurred in a few
leaf samples) demonstrated the need to repeat the first-round PCR
using a more dilute sample. First-round product was diluted 1/10
and a 5-µl aliquot was used as template in a 25-µl nested PCR
reaction with species-specific primers. Concentrations were
0.25 µM for each primer, 2 mM MgCl2, and 0.2 mM each dNTP
in 1× polymerization buffer (Fisher Biotec), with 0.55 U of TTH+
polymerase (Fisher Biotec) per 25-µl reaction.
Cloning and sequencing of PCR products from multiple
fungal species in leaf samples. Independent confirmation of PCR
test results by isolation of corresponding species from the same
leaf samples was not possible because not all samples had pseudothecia and not all samples with pseudothecia had mature spores.
Therefore, confirmation was sought by amplification, cloning,
and sequencing of fungal PCR products from selected leaf
samples. Fungal ITS products were amplified with ascomycetespecific primer combination ITS1-F (29) and ITS4-A (46) using
the same reagent concentrations and thermocycler program as
given above for ITS1-F/ITS4. Products were purified with an
UltraClean PCR Clean-up DNA purification kit (MoBio Laboratories, Inc.) and cloned using a pGEM-T cloning kit (Promega
Corp.). These primers had not been used previously in the
laboratory and amplified a larger fragment than any primers used
previously, so that the possibility of chance contamination with
PCR product was eliminated. Individual clones were tested with
the species-specific primers, and a representative of each species
from each leaf sample was sequenced using primers ITS1-F and
ITS4 as described previously.
Plantation survey. Every set of PCR reactions included five
samples containing DNA from cultures of each of the five species
or taxa groups and one sample with no template DNA. The four
nontarget species in each of the species-specific tests were treated
as additional negative controls. The test was repeated on ≈30% of
samples to demonstrate the consistency of the analysis. Although
the ITS primers for M. vespa also are able to detect M. ambiphylla and M. molleriana, these species previously have not been
recorded in Tasmania, whereas M. vespa has been recorded over
the majority of the state (51). For simplicity, the positive results
for this group are referred to as M. vespa. The possibility of
M. molleriana and M. ambiphylla occurring in Tasmania has not
been eliminated.
RESULTS
Phylogenetic analysis and primer design. A phylogenetic
analysis of ITS sequences was carried out after sequence alignment to accommodate the following points: (i) intraspecific variation in ITS sequences of some Mycosphaerella spp., (ii) a degree
of taxonomic uncertainty for some species, and (iii) quality
control for sequences obtained from public databases (the accuracy
and reliability of which was beyond our control). Phylogenetic
analysis showed species with ITS sequences most similar to the
target species to facilitate the selection of interspecific divergent
regions most suitable for species-specific primer design. Phylogenetic analysis also assisted in the detection of misidentified
species from public sequence databases or culture collections.
In all, 165 nonredundant ITS sequences from 95 Mycosphaerella
and related anamorph species that were obtained from GenBank
or produced here were included in the phylogenetic analysis (Fig.
1). All available sequences of Mycosphaerella spp. that occur on
Eucalyptus spp. were included in the analysis, as were several
species from other hosts. Species known from eucalypts for which
ITS sequences were not available include M. delegatensis,
M. endophytica, M. gracilis, M. longibasalis, and M. swartii. Sequences AY045519 (M. suttoniae) and AY626984 (Mycosphaerella
sp.) had segments of 157 and 181 bp, respectively, removed from
ITS1 after the initial alignment revealed that these were most
probably insertions. The insertions were at different sites and did
not appear to be homologous. Analysis by DNAPARS produced
428 equally most parsimonious trees of 3,170 steps. DNAML
analysis of this set produced an ML tree with an Ln likelihood =
–13,649.6.
For each of the five target species (M. cryptica, M. nubilosa,
M. vespa, M. grandis, and M. tasmaniensis), variable regions
were visually selected from the alignment of 95 Mycosphaerella
spp. Particular attention was paid to discriminating species that
had high overall sequence similarity and that clustered closely in
the ML tree. The variable regions anchored the 3′ end of speciesspecific primers and primer length was varied at the 5′ end to give
similar annealing temperatures. Species-specific primers were
designed for M. cryptica, M. nubilosa, and M. tasmaniensis. High
interspecific sequence similarity combined with intraspecific
variation prevented the design of species-specific primers for
M. vespa and M. grandis. Therefore, primers were designed for
two taxa groups: one primer pair for M. vespa, M. ambiphylla,
and M. molleriana, and one primer pair for M. grandis and
M. parva. Regions were chosen to give a different product size for
each species with a view to future multiplexing of the tests.
Primer sequences were checked for self-annealing and BLAST
search results were checked to ensure that only the target species
matched both primers. Primer sequences and expected product
sizes are given in Table 3.
Arrows on branches indicate groups that have an exact match to
primer sequences (Fig. 1). Each of these branches is supported by
99% confidence limits. This includes all available sequences for
each target taxon, with two exceptions: AF173307 was very different from all other M. tasmaniensis sequences, grouping instead
with Mycovellosiella eucalypti and Phaeoramularia saururi, and
AY534227 grouped with the other M. vespa sequences but dif-
TABLE 3. Species-specific primers designed from Mycosphaerella spp. internal transcribed spacer (ITS) sequences and composite primers used to construct an
internal amplification control for ITS1-F/ITS4 and ITS5/ITS4 PCR
Species
Mycosphaerella cryptica
M. grandis/parva
M. nubilosa
M. tasmaniensis
M. vespa/ambiphylla/molleriana
Phaeophleospora eucalypti
136
PHYTOPATHOLOGY
Primer
name
Primer sequence
McrypF
McrypR
MgpF
MgpR
MnubF
MnubR
MtasF
MtasR
MvamF
MvamR
ITSPCF
ITSPCR
5′ CATCTTTGCGTCTGAGTGATAACG
5′ GGGGGTTGACGGCGCGAC
5′ CCCATTGTATTCCGACCTCTTG
5′ CGCTTAGAGACAGTTGGCTCAG
5′ CAACCCCATGTTTTCCCACCACG
5′ CGCCAGACCGGTCCCCGTC
5′ GTCACGCGGCCGACCGC
5′ CATTAGGGCACGCGGGCTG
5′ GCATCTCTGCGTCTGAGTCAC
5′ GCTCGGCCGGAGACTTCG
5′ CTTGGTCATTTAGAGGAAGTAAAAGTCGT AACAAGGACTAAGCAGACCGCCCGCCAGG
5′ TCCTCCGCTTATTGATATGCGCGGGCG AGCTGGATGTCCTT
Product
size (bp)
331
359
395
298
264
1,100
Fig. 1. Maximum likelihood tree produced by phylogenetic analysis of 165 internal transcribed spacer sequences from Mycosphaerella and related anamorph
species, Ln likelihood = –13,649.6. The outgroup is Phaeoramularia hachijoensis and the scale bar represents expected nucleotide variation of 10%. Clades that
are supported with 99% confidence are denoted by an asterisk above the branch. Arrows on branches represent the groups that have 100% similarity to the taxonspecific primer sequences as follows: G, Mycosphaerella grandis/parva group; V, M. vespa/ambiphylla/molleriana group; N, M. nubilosa; C, M. cryptica; and T,
M. tasmaniensis. Abbreviations for genera are: M., Mycosphaerella and the anamorph genera C., Cladosporium; Mv., Mycovellosiella; Pa., Passalora; Php.,
Phaeophleospora; Phr., Phaeoramularia; Ps., Pseudocercospora; and R., Readeriella. The anamorph genera are those given in the GenBank accessions, although
many of them recently have been recombined (5,6,15), appropriate formal name changes have not been made by the sequence submitters. In addition, some
sequences listed in GenBank as belonging to M. juvenis and M. ellipsoidea are included as belonging to those species, though the isolates that were sequenced
probably represent different taxa (P. W. Crous, perssonal communication). To improve readability of the tree, one branch, distant from the target taxa, has been
pruned. Group A represents two Mycosphaerella spp., M. fori and M. musicola, and 14 Pseudocercospora spp.: P. basiramifera, P. cordiana, P. cruenta,
P. eriodendri, P. eucalyptorum, P. hibbertiaeasperae, P. luzardi, P. macrospora, P. natalensis, P. paraguayensis, P. platyloba, P. pseudoeucalyptorum, P. robusta,
and P. syzygicola. These are included in the tree submitted to Treebase.
Vol. 97, No. 2, 2007
137
fered by 2.3% and had two mismatches to the forward primer for
Mycosphaerella vespa/ambiphylla/molleriana.
By contrast, M. ambiphylla and M. molleriana sequences
grouped closely with the remaining M. vespa sequences with 99%
confidence and had a 100% match to the M. vespa primers.
Percent difference among this group was only 0.2 to 1.0%. In
addition, sequences for M. grandis and M. parva were very
similar to each other (99.2%) and form a single clade with high
support. Furthermore M. bellula, derived from a Proteaceae
family host, and other unnamed isolates from eucalypts also
fall within this clade and all have a 100% match to the
M. grandis/parva primers.
IAC for first-round PCR. Plasmid p35.12 was selected for use
as an IAC because it had an insert of the correct size (≈1,100 bp)
that was amplifiable with the ITS1-F and ITS4 primers. A PCR
product was visible when 2.5 fg of IAC was the only DNA template added to a 25-µl reaction (Fig. 2); however, addition of leaf
or fungal DNA required a higher concentration of IAC (Fig. 3).
Therefore 25 fg was added to each 25-µl PCR, which equates to
6 × 104 copies per reaction. Successful first-round amplification
with ITS1-F and ITS4 primers verified that the template DNA
from plant material or fungal cultures was amplifiable, even if
fungal DNA was at too low a concentration to be detected after
one round of PCR. This increased confidence that subsequent
negative results for taxon-specific primers were not false negatives caused by PCR inhibition. If fungal DNA was present at a
much higher concentration, amplification of IAC was prevented
by competition. Thus, a visible product from either the IAC or
fungal DNA or, in some cases, both, indicated successful firstround PCR.
Specificity, sensitivity, and reliability testing of primer sets.
Species-specific primers (0.25 µM) were used in the secondround amplification of a nested PCR after first-round amplification with ITS1-F and ITS4. After specificity testing of all
primer pairs at several annealing temperatures, the optimum
thermocycler program to run the species-specific primers simul-
Fig. 2. Evaluation of sensitivity of amplification of the internal amplification
control plasmid (IAC) using primers ITS1-F/ITS4. Lane 1 contains a 200-bp
ladder showing fragments of 600 to 1,600 bp and lane 10 is no template
control. Template concentrations are lane 2, 10–18 g/µl or 60 copies per 25-µl
reaction; lane 3, 10–17 g/µl or 600 copies; lane 4, 10–16 g/µl or 6,000 copies;
lane 5, 1 fg/µl or 6 × 104 copies; lane 6, 10 fg/µl or 6 × 105 copies; lane 7,
100 fg/µl or 6 × 106 copies; lane 8, 1 pg/µl or 6 × 107 copies; and lane 9, 10 pg
or 6 × 108 copies.
Fig. 3. Sensitivity of amplification of fungal DNA with primers ITS1-F and
ITS4 with the addition of 25 fg of internal amplification control plasmid (IAC)
to each 25-µl polymerase chain reaction. Lane M contains a 200-bp ladder
showing fragments of 400 to 1,400 bp. Amplification of the IAC is competitively inhibited at the higher fungal DNA concentrations.
138
PHYTOPATHOLOGY
taneously was 94°C for 3 min; 20 cycles of 94°C for 30 s, 62°C
for 30 s, and 72°C for 30 s; followed by 72°C for 10 min.
Each individual primer pair, designed to detect Mycosphaerella
spp. or closely related species groups from plantation Eucalyptus
spp., were highly specific for their respective targets (Fig. 4B to F).
The specific primer pairs verified the taxa of 3 to 15 isolates of
each of the target species from culture collections (Table 4). These
tests were done in a series of experiments with replication of randomly chosen standard isolates and had identical results, showing
the reliability of the test. In double-blind trials, the primers unequivocally discriminated among the five species or taxon groups
and did not detect cultures of nontarget species, including possible
co-occurring fungal pathogens, verifying their reliability (Table 4;
Fig. 4B to F). With nested PCR, each primer pair was sensitive
enough to detect pathogen DNA when present at concentrations of
10 to 100 fg per 25-µl reaction (Fig. 5). Amplification at these low
levels of template DNA was not reliable in repeated trials and this
may be attributed to stochastic events involving target sequences
at such low concentrations. Amplification from 10 fg of M. grandis
DNA with no amplification from 100 fg (Fig. 5B and C) supports
this hypothesis. The addition of plant DNA to the first-round PCR,
equivalent to that in a commonly used dilution of the averagesized leaf sample, appeared to reduce final sensitivity by a factor
of ≈10 (Fig. 5B). Addition of IAC appeared to have the reverse
effect, though this also may have been a result of stochastic effects.
In 2 days in a double-blind trial, one person using the detection
test successfully differentiated the appropriate species (M. cryptica, M. grandis/parva, M. nubilosa, and Mycosphaerella sp. nov.)
(Table 1; data not shown) from 30 cultures from disease surveys
in Eucalyptus plantations that had been identified at the time of
isolation according to the taxonomic criteria available at that time.
The PCR detection test results were verified by subsequent sequencing of the complete ITS regions of these isolates (Table 2;
data not shown).
Testing of primers on plant material. The primers successfully detected Mycosphaerella spp. directly from infected leaves
and stems (Fig. 6). At least one, and up to four, Mycosphaerella
spp. were detected from each leaf or stem sample. The robustness
of the detection systems to test for Mycosphaerella spp. in a wide
range of tissue types was further indicated by amplification of
specific products from dead, dried, herbarium collections. The
nested PCR reactions enabled detection and discrimination of the
five Mycosphaerella taxa in 30 samples with lesions at any
developmental stage by one person within 2 days.
Verification of infection with multiple Mycosphaerella spp.
by cloning of PCR products. Isolation of Mycosphaerella spp.
from leaf lesions does not produce cultures of nonsporulating
species; therefore, infection by multiple Mycosphaerella spp. was
verified by cloning PCR products from selected leaf samples, then
testing individual clones by species-specific PCR and sequencing.
A comparison of species detected by taxon-specific nested PCR
with those detected by cloning and sequencing (Table 5) shows
that, although fewer species were detected by the latter method,
the presence of multiple species in individual leaves or even
individual lesions was confirmed. One leaf sample that had given
a negative result for all the taxon-specific nested PCRs produced a
clone containing an M. grandis sequence (Table 5, sample 3).
Other species of possible leaf pathogens also were detected by the
cloning method (data not shown). No clones containing M. vespa
or M. tasmaniensis sequences were detected, even though five and
two samples, respectively, had tested positive to these species by
nested PCR. To verify that the nested PCR product was amplified
from these two species and not from an untested fungus present in
the leaf sample, the taxon-specific amplicons were sequenced.
The sequences were 99 to 100% identical to published sequences
from M. vespa and M. tasmaniensis, though single nucleotide
polymorphisms occurred among the M. vespa sequences from different geographic areas (Table 5).
Plantation survey. Most leaves contained more than one lesion
and, therefore, most 1-cm2 samples consisted of subsamples from
more than one lesion. No relationship was observed between the
number of lesions sampled per leaf and the number of species
detected. Leaves from both E. nitens and E. globulus with only
one lesion contained two to three Mycosphaerella spp. (Fig. 6).
The Mycosphaerella spp. most frequently detected on
E. globulus were M. cryptica, M. nubilosa, and M. grandis/parva
(Table 6). All E. nitens, E. globulus, and E. regnans leaves from
the plantation survey contained multiple Mycosphaerella spp.,
with three or more species occurring on most leaves (Table 7).
The analysis for 30% of samples was repeated independently and
produced consistent results. All leaf and stem lesions were
analyzed at an early developmental stage. E. globulus leaves contained M. cryptica, M. grandis/parva, and either M. vespa,
M. tasmaniensis, or M. nubilosa (Table 7), including several
Fig. 4. Specificity of each pair of species-specific primers: polymerase chain reaction products from first-round amplification with primers ITS1-F/ITS4 and
second-round amplification with specific primers.
TABLE 4. Results of taxon-specific nested polymerase chain reaction detection tests for the five Mycosphaerella taxaa
Species (no. of isolates tested)
C
G
N
T
V
Botryosphaeria sp. (1)
Cryptosporiopsis eucalypti (1)
Cylindrocladium retaudii (1)
Mycosphaerella africana (1)
M. cryptica (15)
M. grandis (7)
M. nubilosa (8)
M. parkii (1)
M. parva (4)
M. pini (1)
Mycosphaerella sp. 1 (3)
Mycosphaerella suberosa (1)
M. tasmaniensis (3)
M. vespa (5)
Phaeophleospora eucalypti (3)
Quambalaria cyanescens (1)
Q. eucalypti (2)
Q. pitereka (1)
–
–
–
–
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
+
–
–
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
+
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
+
–
–
–
–
a
Columns C, G, N, T, and V represent results for taxon-specific tests for M. cryptica, the M. grandis/parva group, M. nubilosa, M. tasmaniensis, and the
M. vespa/ambiphylla/molleriana group, respectively.
Vol. 97, No. 2, 2007
139
leaves with a single lesion. All stem samples contained
M. grandis/parva, with two testing positive for only M. grandis/
parva. Five stem samples contained M. tasmaniensis and one had
M. cryptica.
M. grandis/parva occurred on every leaf tested; however, it
always occurred with either M. cryptica or M. nubilosa and often
with M. tasmaniensis, M. vespa, or both. M. nubilosa was detected on one E. nitens sample. M. tasmaniensis was detected in
35% of E. globulus samples and 77% of E. nitens samples but
was the only Mycosphaerella sp. absent from our samples of
regrowth E. regnans. M. vespa occurred in 62% of E. globulus,
23% of E. nitens, and in samples from regrowth E. regnans.
The absence of false negatives due to PCR inhibition was
demonstrated by amplification in first-round PCR, either of IAC
or fungal ITS. Four samples that initially were negative for all
species and the first-round amplification, gave positive results
upon additional dilution of the DNA template (i.e., from 10- to
100-fold) for first-round PCR. Every positive was confirmed by
lack of amplification in all of the negative controls. Samples from
the various treatments and controls were processed randomly, and
there was no evidence of cross contamination.
amplifying the DNA of an unrelated organism is ≈4–40 (equivalent
to the probability of random occurrence of a 40-bp sequence,
assuming that ITS sequences develop over a long period of time
through a random process of unconstrained mutation and subsequent fixation), which is <10–24.
The detection system distinguished among pathogens in culture
and produced accurate, rapid diagnosis at an early stage of infection by five Mycosphaerella taxon groups, which is impossible
DISCUSSION
Five nested PCR detection tests were developed and their
specificity verified against classically identified cultures of Mycosphaerella spp. and other genera pathogenic on Eucalyptus spp.
The test identified cultures which had been given only a tentative
identification during disease surveys and whose species relationship was confirmed by subsequent ITS sequence analysis. The
sensitivity of all the nested PCR tests, at 10 to 100 fg using DNA
from pure cultures, with or without the addition of plant DNA
(Fig. 5), approximates to an ascospore or hyphal cell with two to
three nuclei, assuming a genome size of 10 to 100 Mbp. This is an
important attribute, potentially enabling the detection tests to
discriminate a few Mycosphaerella cells in leaf or stem samples
or to identify cultures nondestructively sampled 7 to 14 days after
they are initiated from germinated ascospores. The specificity of
the primers is high under the stringent conditions of the tests
because they have not amplified DNA from a wide range of
related organisms. The probability of 18 to 24 nucleotide primers
Fig. 6. Detection of Mycosphaerella spp. from leaf lesions. Polymerase chain
reaction products from A, first-round amplification with primers ITS1-F/ITS4
and B to F, second-round amplification with specific primers. B, McrypF/
McrypR; C, MgpF/MgpR; D, MnubF/MnubR; E, MtasF/MtasR; and F,
MvamF/MvamR. Lane 1, Promega 1-kb ladder showing fragments of 1,000
(A, C, and D only), 750, 500, and 250 bp; lane 2, E7417 Eucalyptus globulus,
all lesions on leaf; lane 3, E7418 E. nitens, all lesions on leaf; lane 4, E7445,
E. regnans, all lesions on leaf; lane 5, E. nitens, random sample, no disease
evident; lanes 6 to 10, E. nitens, single leaf lesions at a range of developmental stages, from small purplish spots with no reproductive structures (lane
6) to large necrotic lesions with abundant pycnidia (lane 10); lane 11, E. globulus, random sample, no disease evident; lanes 12 to 16, E. globulus, single leaf
lesion at a range of developmental stages, from small purplish spots with no
reproductive structures (lane 12) to large necrotic lesions with abundant
pycnidia (lane 16); lane 17, E. globulus, single stem lesion.
Fig. 5. Sensitivity of detection from cultures with and without internal amplification control (IAC) and plant DNA in the first-round amplification. Polymerase
chain reaction (PCR) products from second-round amplification with specific primers are shown. Lane 1 contains a 200-bp DNA ladder showing fragments from
200 to 1,200 bp. A, Pure fungal DNA as template for round 1 PCR; B, equal volume of Eucalyptus globulus (lanes 2 to 16) or E. grandis (lanes 17 to 26) DNA,
from a sample size and dilution equivalent to that used in testing diseased leaves; C, the IAC in round 1; and D, both plant DNA as in B and IAC as in C. Isolates
details are: Mycosphaerella cryptica AA/1/3/10 amplified with McrypF/McrypR, M. grandis S/1/2/3 amplified with MgpF/MgpR, M. nubilosa St Marys/2/8
amplified with MnubF/MnubR, M. tasmaniensis BS/10/2 amplified with MtasF/MtasR, and M. vespa A/1/7 amplified with MvamF/MvamR.
140
PHYTOPATHOLOGY
using current methodology (52,62). Data in this article also raise
concerns about the usefulness of lesion appearance as a diagnostic
tool for identifying leaf pathogens. Different necrosis symptoms
occur on Phaseolus vulgaris when co-inoculated with both common bacterial blight (Xanthomonas campestris pv. phaseoli) and
rust (Uromyces appendiculatus var. appendiculatus) than those
observed with single inoculations (78). Even within eucalypts
there is considerable host variation with necrosis symptoms for
the same Mycosphaerella spp. (8), and M. cryptica and M. nubilosa lesions often coalesce (59). Therefore, the identification of
the full complement of Mycosphaerella spp. using lesion appearance, shape, and position of pseudothecia may not be accurate
due to the presence of several species in lesions once identified as
having only one species.
The first-round amplification with general fungal-specific
primers increases sensitivity and avoids time-consuming procedures such as incubation (48). A high degree of species specificity
is not essential at this stage, though preferential amplification of
fungal DNA improves sensitivity in the nested PCR. Incorporation of an internal amplification control provides additional confidence that negative results are not due to PCR inhibition by
some component of the DNA extract, a common problem with
plant, particularly eucalypt, DNA (7). The first-round PCR is
more critical for enzyme inhibition, because plant components
will be considerably diluted by the second round of PCR. The
larger size of the IAC product was planned to ensure the IAC
amplified less efficiently than the fungal DNA target.
The inability to obtain cloned PCR products of M. tasmaniensis
and M. vespa from leaf samples that had tested positive for these
species is puzzling. M. grandis also was lacking in cloned
products from five of the samples. This discrepancy may have
been caused by preferential amplification of other fungal species
with the primers used, though amplification of these three species
with ITS1-F/ITS4A was verified from pure fungal DNA. All
possible precautions were taken to prevent cross-contamination
with PCR product. The variation among the M. vespa sequences
(Table 5), corresponding with geographic location of the leaf
samples, is evidence against cross-contamination. A more likely
explanation is that these three species were present in much
smaller amounts and testing of a larger number of clones may
have been necessary to locate clones containing DNA from these
underrepresented species. Development of species-specific realtime PCR may provide further information.
The phylogenetic analysis revealed discrepancies among isolates within species indicative of possible misidentifications.
Three isolates received as M. parkii (Table 1) were shown to be
two new species quite distinct from M. parkii (Fig. 1), although
they had produced a Stenella anamorph in culture (P. W. Crous,
personal communication). Several GenBank accessions also appeared to come from misnamed isolates (AF173298, AF173299,
AF173307, AF468879, AY251086, AY725545, and SAR244260)
(Fig. 1). These difficulties are compounded when inaccurately
identified cultures are used as reference strains; therefore, culture
verification is vital for any studies involving molecular identification.
Phylogenetic analysis showed the close relationships for
M. grandis and M. parva, and M. vespa, M. ambiphylla, and
M. molleriana, which are all species from Eucalyptus. The high
degree of interspecific ITS sequence similarity and lack of
alternative well-characterized genomic loci preclude differentiation of each currently recognized species in these groups. Additional studies based on collections from many hosts and geographic regions are needed to confirm that these species are
distinct taxa and not merely subpopulations of a single species
(22). The ecological niche occupied or pathogenicity factors are
secondary taxonomic characters for Mycosphaerella spp. from
eucalypts (62). They have been used as part of the rationale for
discriminating between M. grandis and M. parva (12), with
M. grandis regarded as pathogenic and M. parva as endophytic,
though some workers regard these as a single species (18). The
TABLE 5. Recombinant clones and sequence accession numbers of fungal ribosomal DNA internal transcribed spacer polymerase chain reaction (PCR) products
amplified from leaf lesionsa
Clones
Leaf sample
1
2
3
4
5
6
7
8
9
10
No. of lesions
Nested PCRb
M. cryptica
M. nubilosa
M. grandis
Other
Accession no.c
10
1
1
1
1
12
1
1
2
6
CGNV
C
…
G
CG
GNV
CGTV
GNTV
CGNV
C
18
4
…
…
31
…
24
…
2
1
5
…
…
…
…
8
…
1
…
…
…
…
1
7
…
…
2
…
…
…
9
4
13
21
1
8
3
15
9
13
DQ465671
…
…
…
…
DQ465671
DQ465672, DQ465674
DQ465672, DQ465675
DQ465673
…
a
A variable number of clones from each leaf sample was tested by nested PCR to determine the number of clones from each Mycosphaerella sp. At least one
representative of each species from each leaf sample was sequenced to confirm species designation, including one or more clones that were negative for all five
Mycosphaerella spp. tested.
b Results for nested PCR: positive for C = Mycosphaerella cryptica, G = M. grandis/parva, N = M. nubilosa, T = M. tasmaniensis, and V = M. vespa.
c No clones containing M. vespa or M. tasmaniensis sequences were recovered; therefore, taxon-specific amplicons for these species were sequenced directly to
confirm that the product came from the target species. Because the M. vespa sequences obtained from samples 1 and 6, and samples 7 and 8, were identical, only
one GenBank submission was made for each of these sequences.
TABLE 6. Frequency of detection for Mycosphaerella spp. on leaves of Eucalyptus globulus, E. nitens, and E. regnans and stems of E. globulus
Frequency (%)
Tree species
Sample type, number
Mycosphaerella cryptica
M. grandis
M. nubilosa
M. tasmaniensis
M. vespa
E. globulus
E. nitens
E. regnansa
E. globulus
Leaf, 60
Leaf, 13
Leaf, 4
Stem, 7
92
85
Y
14
100
100
Y
100
83
8
Y
0
35
77
N
71
62
23
Y
0
a
Due to the small sample size from the native regrowth site, only presence or absence (Y or N, respectively) of Mycosphaerella spp. has been presented.
Vol. 97, No. 2, 2007
141
of Mycosphaerella spp., including those from members of
Myrtaceae and other hosts, could be valuable in elucidating
evolutionary relationships, possible host jumps, and some of the
current taxonomic quandaries.
The Eucalyptus plantation industry is relatively new to Australia
and the study of Mycosphaerella spp. in Australia has only
recently received attention. Native Australian eucalypt forests
mainly consist of mixtures of species of uneven age, in which
MLD is usually a minor disease. The establishment of increasing
areas of even-aged monocultures provides ideal conditions for
epidemics. Crous et al. (19) have suggested that certain Mycosphaerella spp. found on eucalypts in plantations in South Africa,
South America, and Europe probably originated in Australasia
although, as yet, not all have been found in Australia. The
possibility that many species currently known only from overseas
are yet to emerge on eucalypt plantations in Australia implies that
more serious disease may yet occur.
The capacity to detect Mycosphaerella spp. or taxon groups in
samples from plantations and natural forests and to provide
detailed and spatially explicit information on the species present
within and between trees in a plantation clearly was demonstrated. In our case, this information provided baseline information prior to establishing a rotational length trial for the longterm monitoring of disease impact and the efficacy of different
control options. Using standard techniques based on lesion
attributes, ascospore germination, and cultural characteristics,
rapid determination of the Mycosphaerella spp. associated with
the large number of lesions sampled in this study would be
impractical. Considering that all the stems and many of the leaves
analyzed in the present study had lesions without reproductive
structures, the use of classical techniques to rapidly identify
species also would be impossible. Spore release from mature
fruiting bodies is highly variable, with clusters of spores released
by only one or two sporulating species per lesion. Therefore, in
the past, Mycosphaerella spp. damage usually has been attributed
to these one or two sporulating species (11,14,25,38,47,50,51,61).
Although the species detected at the trial site and elsewhere in
Tasmania support the findings of a seasonally and geographically
more comprehensive survey (51), the detection frequency for all
species was much higher in this study, presumably due to the
sensitivity and specificity of the molecular detection methods, the
ability of the test to detect species in tissue without reproductive
structures and to test large numbers of samples. It is evident from
this survey that, in Tasmania, where E. globulus is indigenous in
native forests and E. nitens, which is indigenous to mainland
Australia, has been grown in plantations for over 30 years, species
compositions are far more complex than original surveys in
eucalypt plantations using classical techniques have suggested.
There is substantial evidence from this study to indicate that the
co-occurrence of several pathogenic and saprophytic species in a
Mycosphaerella leaf lesion is typical for Eucalyptus spp. in
Tasmania. Several Mycosphaerella spp. previously have been
identified on the same lesions on Myrtaceous and Proteaceous
hosts (16). The co-occurrence of several Mycosphaerella spp. in a
small leaf volume raises the issue of species–species interaction
and their roles as primary or secondary pathogens or saprophytes.
In other systems, the co-occurrence of pathogenic species is
ubiquity of M. grandis/parva in plantation samples reinforces the
need for taxonomic clarification. Additional unnamed isolates and
another species from a non-Myrtaceae host (M. bellula) also have
high ITS sequence similarity to M. grandis and M. parva (Fig. 1).
Whether this is a single, variable species with a broad host range
or many species with highly conserved ITS sequences is unclear.
The anamorphs of M. grandis and M. parva are unknown and
there is uncertainty regarding their taxonomic differentiation (15).
There is need for a simplified, workable taxonomy (20); however,
in the interim, analysis of Mycosphaerella spp. on eucalypts and
other woody hosts must accommodate an evolving taxonomic
framework.
There is potential to increase the range of tests by developing
further primers for more taxa, though additional groups, such as
M. aurantia and M. africana (Fig. 1), also may be difficult to
distinguish using ITS sequences. The failure of most Mycosphaerella spp. from Eucalyptus spp. to produce teleomorphs in
culture complicates taxonomic investigations (62). Comparative
analysis involving cross inoculations under a range of conditions
onto various hosts may indicate variation in teleomorph characteristics of individual isolates. These investigations also could
confirm anamorph relationships to teleomorphs, especially for
isolates that do not form anamorphs in culture. If further taxonomic studies confirm the validity of currently recognized species
that are not differentiated by ITS sequences, another genomic
region may be able to provide greater differentiation among those
species.
Increasingly, β-tubulin, elongation factor, actin, calmodulin,
histone, and DNA-dependent RNA polymerase II genes are being
used in phylogenetic studies of fungi. Phylogenetic analysis based
on multiple genes was able to differentiate several species previously all identified as Botryosphaeria dothidea (69). Analysis of
elongation factor 1-α and β-tubulin genes supported the synonymy
of M. juvenis with M. nubilosa (18). Any region that provides
phylogenetic discrimination is a suitable candidate for speciesspecific detection. For example, the calmodulin gene was included in a multiple gene phylogeny that provided greater phylogenetic discrimination than ITS sequences for Fusarium spp. in
the Gibberella fujikuroi species complex (57). Subsequently, the
calmodulin gene was exploited successfully for species-specific
detection of two toxigenic Fusarium spp. in asparagus plants (54).
As yet, none of these regions have been examined in a large
number of Mycosphaerella spp.
The occurrence of highly similar ITS sequences in isolates of
distinct species from different host genera raises questions about
past evolutionary relationships or recent ones involving new host
relationships. This is highlighted by the high similarity among
M. aurantia, M. africana (Eucalyptus spp. hosts), M. confusa
(Rubus spp.), and M. pini (Pinus spp.). However, the group is
distinct from other species infecting Eucalyptus spp. and M. dearnessii, another species infecting Pinus, is very different from
M. pini. In addition, there is significant support for a group containing M. intermedia, M marksii (both from eucalypts), and
M. musae (from banana) (Fig. 1). In contrast, three poplar-infecting species, M. populi, M. populicola, and M. populorum, form a
distinct group. A multiple gene molecular phylogenetic study on
carefully isolated and identified isolates of a broad cross-section
TABLE 7. Coexistence of multiple Mycosphaerella spp. in single leaves of Eucalyptus nitens, E. regnans, and E. globulus
Percentage of samples with the corresponding number of Mycosphaerella spp.
Tree species
Sample type, number
1
2
3
4
5
E. nitens
E. globulus
E. regnansa
E. globulus
Leaf, 13
Leaf, 60
Leaf, 4
Stem, 7
0
0
N
29
22
10
Y
57
67
37
Y
14
11
33
N
0
0
20
N
0
a
Due to the small sample size from the native regrowth site, only presence or absence (Y or N, respectively) of Mycosphaerella spp. has been presented.
142
PHYTOPATHOLOGY
common. These highly characterized disease systems include
Fusarium head blight in cereals (55,56), Melampsora rust on
willow and poplar (66), sooty blotch in apple trees (41), and the
Dutch elm disease complex of Ophiostoma ulmi, O. novo-ulmi,
and O. himal-ulmi (36). There is limited knowledge of the pathogenicity of most Mycosphaerella spp., on eucalypts, including
M. tasmaniensis and M. vespa, which have been isolated frequently in past surveys in Tasmania (51,77). The ubiquity of
M. grandis/parva in the leaf samples tested raises questions about
its possible role in pathogenicity. M. parva is considered to be
endophytic and there are doubts about the taxonomic distinction
between M. parva and M. grandis (15). M. grandis was described
as a leaf pathogen by Carnegie and Keane (12). The detection of
only M. grandis/parva in two stem lesions indicates their potential role as a pathogen in stems.
This study has illustrated that nested PCR is a suitable technique for differentiating Mycosphaerella spp. in prenecrotic and
prereproductive lesions and has provided a rapid and accurate
method for Mycosphaerella spp. identification to validate disease
surveys and research trials. Unlike classical methods, the detection technique enables rapid processing of large sample numbers
and provides more comprehensive and reliable data. The nested
PCR is a reliable and cost-effective method, useful for studies on
the developmental ecology and epidemiology of MLD. It also
may provide valuable information for the advance of management
strategies for MLD on eucalypt plantations. The species-specific
primer pairs described here also meet the requirements of an
effective quarantine tool, providing rapid, direct detection of
pathogens of concern from asymptomatic tissue, including germ
plasm (74). This capability also may assist in nursery quality
assurance that Mycosphaerella pathogens are eliminated and not
merely suppressed prior to certification of disease-free seedlings
and planting stocks. Such a move may help minimize the risk to
industry of unwittingly introducing disease, particularly cryptic
infections, into plantations at their most vulnerable stage.
ACKNOWLEDGMENTS
We thank M. Dudzinski and R. Gibbs (CSIRO, Australia), M. Dick
(SCION), A. Alfenas (Federal University of Viçosa, Brazil), and A.
Carnegie (State Forests New South Wales, Australia) for providing
isolates.
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