Home' Curtin University : Curtin Edge of Tomorrow Contents SimonEllwood
Worldwide, there are eight variants of
the tan spot pathogen P. tritici-repentis.
Only half of them produce ToxA,
suggesting there are other factors that
enable the pathogen to infiltrate a plant’s
defences and take hold. To investigate
this, Moffat and her colleagues have
deleted the ToxA gene in samples of
P. tritici-repentis and are studying how
it affects the plant-pathogen interaction.
During the winter wheat-cropping
season, Moffat embarks on field
trips across Australia to sample for
P. tritici-repentis to get a ‘snapshot’ of
the pathogen’s genetic diversity and
how this is changing over time. Growers
also send her team samples as part of
a national ‘Stop the Spot’ campaign,
which was launched in June 2014 and
runs in collaboration with the GRDC.
Of particular interest is whether the
pathogen is becoming more virulent,
which could mean the decimation of
popular commercial wheat varieties.
WHEAT FUNGAL DISEASES can
regularly cause a yield loss of about
15–20%. But for legumes – such as
field pea, chickpea, lentil and faba
bean – fungal infections can be even
more devastating. The fungal disease
ascochyta blight, for example, readily
causes yield losses of about 75%
in pulses. It makes growing pulses
inherently risky, explains ascochyta blight
program leader, Dr Judith Lichtenzveig.
In 1999, Western Australia’s chickpea
industry was almost wiped out by the
disease and has never fully recovered.
With yield reliability and confidence in
pulses still low, few growers include
them in their crop rotations – to the
detriment of soil health.
Pulse crops provide significant benefit
to subsequent cereals and oilseeds in
the rotation, says Lichtenzveig, because
they add nitrogen and reduce the impact
of soil and stubble-borne diseases.
The benefits are seen immediately
in the first year after the pulse is planted.
The chickpea situation highlights the
need to develop new profitable varieties
with traits desired by growers and that
suit the Australian climate.
The CCDM also runs two programs
concerned with barley, both headed by
Dr Simon Ellwood. His research group is
looking to develop crops with genetic
The CCDM is
to plant diseases
such as powdery
mildew in barley
(main), and Septoria
(above left) in wheat,
with Dr Caroline
Moffat (right) leading
a program to tackle
the wheat tan
resistance to two diseases that account
for more than half of all yield losses in
this important Australian crop – net
blotch and powdery mildew.
Details of the barley genome were
published in the journal Nature in 2012.
The grain contains about 32,000 genes,
including ‘dominant R-genes’ that
provide mildew resistance. The dominant
R-genes allow barley plants to recognise
corresponding avirulence (Avr) genes
in mildew; if there’s a match between
a plant R-gene and pathogen Avr genes,
the plant mounts a defence response
and the pathogen is unable to establish
an infection. It’s relatively commonplace,
however, for the mildew to alter its Avr
gene so that it’s no longer recognised
by the plant R-gene.
“This is highly likely when a particular
barley variety with a given R-gene is
grown over a wide area where mildew
is prevalent, as there is a high selection
pressure on mutations to the Avr gene,”
explains Ellwood. This means the mildew
may become a form that is unrecognised
by the barley.
Many of the malting barley varieties
grown in Western Australia, with the
exception of Buloke, are susceptible to
mildew. This contrasts with spring barley
varieties being planted in Europe and
the USA that have been bred to contain
a gene called mlo, which provides
resistance to all forms of powdery mildew.
Resistance to net blotch also occurs
on two levels in barley. “As with mildew,
on the first level, barley can recognise
net blotch Avr genes early on through
the interaction with dominant R-genes.
But again, because resistance is based
on a single dominant gene interaction,
it can be readily lost,” says Ellwood.
“If the net blotch goes unrecognised,
it secretes toxins that allow the disease
to take hold.”
On the second level, these toxins
interact with certain gene products
so that the plant cells become
hypersensitised and die. By selecting
for barley lines without the sections of
genes that make these products, the crop
will have a durable form of resistance.
Indeed, Ellwood says his team has found
barley lines with these characteristics.
The next step is to determine how many
genes control this durable resistance.
“Breeding for host resistance is cheaper
and more environmentally friendly than
applying fungicides,” Ellwood adds.
NUMEROUS FUNGICIDES ARE used to
prevent and control fungal pathogens,
and they can be costly. Some have a
common mode of action, and history tells
us there’s a good chance they’ll become
less effective the more they’re used.
“The development of fungicide resistance
is one of the greatest threats to our food
biosecurity ahead of water shortage and
climate change,” says Gibberd. “It’s
a very real and current problem for us.”
Fungicides are to grain growers what
antibiotics are to doctors, explains Dr
Fran Lopez-Ruiz, head of the CCDM’s
fungicide resistance program. “The
broad-spectrum fungicides are effective
when used properly, but if the pathogens
they are meant to control start to develop
resistance, their value is lost.” Of the
three main types of leaf-based fungicides
used for cereal crops, demethylation
inhibitors (DMIs) are the oldest, cheapest
and most commonly used.
Lopez-Ruiz says that to minimise
the chance of fungi becoming resistant,
sprays should not be used year-in,
year-out without a break. The message
hasn’t completely penetrated the farming
community and DMI-resistance is
spreading in Australia. A major aim within
Lopez-Ruiz’s program is to produce a
geographical map of fungicide resistance.
“Not every disease has developed
resistance to the available fungicides yet,
which is a good thing,” says Lopez-Ruiz.
DMIs target an enzyme called CYP51,
which makes a cholesterol-like compound
called ergosterol that is essential for
fungal cell survival. Resistance develops
when the pathogens accumulate several
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