Pathogen profilempp_656 105..122
Venturia inaequalis: the causal agent of apple scab
JOANNA K. BOWEN1,*, CARL H. MESARICH1,2, V INCENT G. M. BUS3, ROBERT M. BERESFORD1,
KIM M. PLUMMER4 AND MATTHEW D. TEMPLETON1
1The New Zealand Institute for Plant & Food Research Limited, Mt. Albert Research Centre, Private Bag 92 169, Auckland 1142, New Zealand
2School of Biological Sciences, University of Auckland, Private Bag 92 019, Auckland 1142, New Zealand
3The New Zealand Institute for Plant & Food Research Limited, Hawke's Bay Research Centre, Private Bag 1401, Havelock North 4157, New Zealand
4La Trobe University, Department of Botany, Bundoora, Vic. 3086, Australia
SUMMARY
The fungus Venturia inaequalis infects members of the Mal-
oideae, and causes the disease apple scab, the most important
disease of apple worldwide. The early elucidation of the gene-
for-gene relationship between V. inaequalis and its host Malus
has intrigued plant pathologists ever since, with the identifica-
tion of 17 resistance (R)–avirulence (Avr) gene pairings. The Avr
gene products are presumably a subset of the total effector
arsenal of V. inaequalis (predominantly proteins secreted in
planta assumed to facilitate infection). The supposition that
effectors from V. inaequalis act as suppressors of plant defence
is supported by the ability of the pathogen to penetrate the
cuticle and differentiate into large pseudoparenchymatous struc-
tures, termed stromata, in the subcuticular space, without the
initiation of an effective plant defence response. If effectors can
be identified that are essential for pathogenicity, the correspond-
ing R genes will be durable and would add significant value to
breeding programmes. An R gene cluster in Malus has been
cloned, but no V. inaequalis effectors have been characterized at
the molecular level. However, the identification of effectors is
likely to be facilitated by the resolution of the whole genome
sequence of V. inaequalis.
Taxonomy: Teleomorph: Venturia inaequalis Cooke (Wint.);
Kingdom Fungi; Phylum Ascomycota; Subphylum Euascomycota;
Class Dothideomycetes; Family Venturiaceae; genus Venturia;
species inaequalis. Anamorph: Fusicladium pomi (Fr.) Lind or
Spilocaea pomi (Fr.).
Life cycle: V. inaequalis is a hemibiotroph and overwinters as
pseudothecia (sexual fruiting bodies) following a phase of sapro-
bic growth in fallen leaf tissues. The primary inoculum consists of
ascospores, which germinate and penetrate the cuticle. Stromata
are formed above the epidermal cells but do not penetrate them.
Cell wall-degrading enzymes are only produced late in the infec-
tion cycle, raising the as yet unanswered question as to how V.
inaequalis gains nutrients from the host. Conidia (secondary
inoculum) arise from the upper surface of the stromata, and are
produced throughout the growing season, initiating multiple
rounds of infection.
Venturia inaequalis as a model pathogen of a
woody host: V. inaequalis can be cultured and is amenable to
crossing in vitro, enabling map-based cloning strategies. It can
be transformed readily, and functional analyses can be con-
ducted by gene silencing. Expressed sequence tag collections are
available to aid in gene identification. These will be comple-
mented by the whole genome sequence, which, in turn, will
contribute to the comparative analysis of different races of V.
inaequalis and plant pathogens within the Dothideomycetes.
INTRODUCTION
Venturia inaequalis Cooke (Wint.) is a hemibiotrophic fungus
that is the causal agent of the apple disease scab, more com-
monly known as black spot in Australasia (MacHardy, 1996). V.
inaequalis has a wide geographical range and is found in almost
all areas in which apples are grown commercially. It is the most
important disease of apple worldwide in terms of economic cost
of control (Carisse and Bernier, 2002). However, the disease is
more severe in temperate countries with cool, moist climates
during early spring (MacHardy, 1996; Manktelow et al., 1996).
Apple scab is predominantly controlled by a combination of
sanitation and cultivation measures, and judicious fungicide
application, the correct timing of which is essential for effective
control (Beresford and Manktelow, 1994). To date, the deploy-
ment of resistant cultivars has been very limited because of their
relatively poor fruit quality. However, they are expected to play a
major role in disease control, particularly in organic fruit produc-
tion, once new cultivars with durable resistance to scab become
available.*Correspondence: Email: joanna.bowen@plantandfood.co.nz
MOLECULAR PLANT PATHOLOGY (2011) 12 (2) , 105–122 DOI: 10.1111/J .1364-3703.2010.00656.X
© 2010 THE NEW ZEALAND INSTITUTE FOR PLANT & FOOD RESEARCH LTD
MOLECULAR PLANT PATHOLOGY © 2010 BSPP AND BLACKWELL PUBLISHING LTD 105

V. inaequalis has been researched continuously for over 100
years. The resultant body of knowledge is comprehensive in its
scope, covering aspects of the basic biology and genetics of the
fungus, and the epidemiology and control of the disease
(reviewed extensively by MacHardy, 1996). This research has
revealed the uncommon parasitic strategy adopted by V.
inaequalis: no host cell penetration occurs, but fungal biomass
accumulates in the subcuticular space prior to sporulation. This
raises the question as to how the fungus remains undetected in
the plant with the absence of a co-ordinated host resistance
reaction. The fungus, in all likelihood, produces effectors that
suppress any resistance response. Effectors are proteins or sec-
ondary metabolites expressed in planta that are assumed to
facilitate infection/suppress resistance (Chisholm et al., 2006;
Dangl and Jones, 2001), some of which have been comman-
deered by resistance (R) genes for pathogen recognition and the
initiation of defence. The putative production of effectors by V.
inaequalis is supported by genetic data.
The fungus is heterothallic and controlled crosses can be
performed readily in vitro. This led to in-depth genetic studies of
V. inaequalis in the 1940s and 1950s, when loci for colony
colour (presumably involved in melanin biosynthesis), mating
type, various nutritional markers and avirulence were identified,
and genetic maps were produced (Boone, 1971; Boone and
Keitt, 1957; Keitt and Langford, 1940; Shay and Hough, 1952;
Williams and Shay, 1957). These studies led to the elucidation of
the gene-for-gene nature of the relationship between V.
inaequalis and Malus at the time at which Flor (1956) reported
such interactions for the flax–Melampsora lini pathosystem.
Thus, the V. inaequalis–Malus pathosystem appears to fit the
effector paradigm.
In some pathosystems, effectors are secreted into the apo-
plast and exert their effect extracellularly, whereas, in others,
they are targeted to the host cytoplasm. Generally, effectors
that have been identified to date share little amino acid
sequence similarity with each other or with other known pro-
teins. Some have limited conserved sequence motifs, e.g. the
‘RXLR’ or crinkler motifs of many oomycete effectors (Haas
et al., 2009; Schornack et al., 2009). V. inaequalis effectors
remain elusive and have yet to be cloned (Bowen et al., 2009;
Broggini et al., 2007, 2009a; Kucheryava et al., 2008; Win
et al., 2003). Their identification, concomitant with the identi-
fication of R genes in the host (Belfanti et al., 2004; Boudichev-
skaia et al., 2009; Broggini et al., 2009b; Galli et al., 2010b;
Malnoy et al., 2008; Szankowski et al., 2009; Vinatzer et al.,
2001; Xu and Korban, 2002), is the focus of much of the
current molecular research on apple scab disease. Tools for the
dissection of the V. inaequalis–Malus interaction at the
molecular level are available. The pathogen can be cultured
easily and, as mentioned above, induced to sexually cross in
vitro (Keitt and Langford, 1941; Keitt and Palmiter, 1938).
Although slow growing, it can be transformed readily (Fitzger-
ald et al., 2003) and is amenable to functional analyses by
gene silencing (Fitzgerald et al., 2004). V. inaequalis is there-
fore a tractable, model, hemibiotrophic pathogen of a woody
host. Furthermore, assembly of the whole genome sequence
(WGS), which is currently underway, will greatly accelerate the
molecular dissection of this pathosystem.
TAXONOMY, PATHOGEN EVOLUTION AND
HOST RANGE
Debate over the taxonomic classification of many fungal species
is often fierce. The case of V. inaequalis proves to be no excep-
tion.TheVenturiaceae, including V. inaequalis, has, until recently,
been placed in the class Dothideomycetes, in the subclass
Pleosporomycetidae, order Pleosporales, on the basis of both
morphological and molecular criteria (Eriksson and Hawksworth,
1998; Goodwin, 2004; Lumbsch and Huhndorf, 2007). However,
recent comprehensive molecular phylogenetic analyses of
members of the Dothideomycetes, using data from both nuclear
and mitochondrial genes, place the Venturiaceae outside the
Pleosporales, without a subclass and order assignment (Kruys
et al., 2006; Schoch et al., 2009).
The classification of the related anamorph (asexual) stage at
the genus level has also proven to be controversial. Schubert
et al. (2003), in the most comprehensive study to date, proposed
that the anamorph be classified as Fusicladium pomi (Fr.) Lind,
replacing the previous classification of Spilocaea pomi (Fr.).
However, this has not been universally adopted, as the old name
of S. pomi is still in use (Attrassi et al., 2007; Bini et al., 2008; Jha
et al., 2009).
Members of the genus Venturia infect various fruit tree
genera: V. inaequalis infects apple (Malus); V. pirina infects
European pear (Pyrus); V. nashicola infects Asian pear (Pyrus); V.
carpophila infects peach (Prunus); and V. cerasi infects cherry
(Prunus). Venturia species can be differentiated into distinct
clades on the basis of polymorphisms in ribosomal internal tran-
scribed spacer (ITS) DNA sequences (Beck et al., 2005; Ishii and
Yanase, 2000). The topology of the Venturia species phylogram
aligns closely with that of the host genera, demonstrating a
close co-evolutionary relationship between the pathogenic Ven-
turia species and their respective fruit tree hosts.
The boundaries between the Venturia species are highly dis-
tinct and matings between the species in the laboratory have not
been achieved. There is evidence of significant variation within
each fungal species, presumably as a consequence of annual,
sexual reproduction and obligate outcrossing in these heteroth-
allic species (Langford and Keitt, 1957). The species boundaries
within a particular host genus, however, are less distinct, e.g.
various Malus species are able to hybridize, permitting gene flow
to occur between these species. It is likely that the resultant
106 J. K. BOWEN et al .
© 2010 THE NEW ZEALAND INSTITUTE FOR PLANT & FOOD RESEARCH LTD
MOLECULAR PLANT PATHOLOGY © 2010 BSPP AND BLACKWELL PUBLISHING LTDMOLECULAR PLANT PATHOLOGY (2011) 12(2 ) , 105–122