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Breakthrough to fast-track yellow spot research

Author: Natalie Lee | Date: 22 Jul 2014

A breakthrough achieved by Western Australian scientists will facilitate fast-tracked research into the country’s most costly wheat disease – yellow spot. Caption: CCDM program leader Caroline
Moffat says a breakthrough in yellow spot
research will accelerate the development
of resistant varieties.

The outcome of this research, supported by the Grains Research and Development Corporation (GRDC), is expected to speed up the release to growers of new wheat varieties with improved yellow spot resistance.

Researchers at Curtin University’s Centre for Crop and Disease Management (CCDM) have deleted an important gene in the yellow spot fungus (Pyrenophora tritici-repentis).

“This is an extremely powerful new tool and will accelerate the development of resistant varieties,” CCDM program leader Caroline Moffat said.

“It is a major boost that will help us identify new targets for breeders.”

Yellow spot, also known as tan spot, causes national crop losses of $212 million annually, plus control costs of an additional $463 million. In hard-hit areas, it costs wheat growers up to $30 per hectare in lost production and control costs.


ToxA is the major effector (host-selective toxin) of the yellow spot pathogen in Australia and worldwide.

It is secreted by the fungus and kills wheat cells, enabling the fungus to colonise the crop.

ToxA is only damaging on wheat varieties that are sensitive to it.

These varieties contain the tan spot necrosis 1 (Tsn1) susceptibility gene which causes the plant to produce a protein which makes it sensitive to ToxA and allows yellow spot to infect the leaves.

Major wheat varieties sensitive to ToxA include Yitpi and Stiletto.

“So far, all Australian yellow spot isolates that we have tested produce ToxA,” Dr Moffat said.

“In our research, we ‘knocked out’ the ToxA gene via ‘homologous recombination’ – a process whereby the gene encoding ToxA was replaced.

“The genetically engineered strain was unable to kill leaf cells in a wheat variety that would usually be susceptible.”

New vistas for future research

Dr Moffat said that given the yellow spot fungus was difficult to work with and not readily genetically manipulated, the gene deletion was a significant achievement. Caption: Wheat infected with yellow spot disease.

“The development of a targeted gene knockout method for such an economically significant and global wheat pathogen is a breakthrough,” she said.

“This is the first time that this has been achieved for this pathogen anywhere in the world, so we have a unique capability.”

Dr Moffat said now that scientists had this capability, it would fast-track yellow spot research to a level already established for other pathogens.

“For example, researchers have been making gene deletions in the septoria nodorum blotch wheat pathogen for more than a decade,” she said.

“Now that we have shown that gene deletions can also be achieved in yellow spot, it paves the way for deleting other genes of interest.

“By knocking out the ToxA gene, which is the main yellow spot effector, we are able to see what other effectors the pathogen produces. These have so far been masked by extensive ToxA-induced plant cell death.”

Dr Moffat said the gene deletion technique would also permit the screening of ToxA sensitive wheat ‘mapping populations’ to look for breeding targets.

“This will facilitate the identification of new molecular markers for wheat breeders, to use in breeding programs to eliminate yellow spot susceptibility,” she said.

“So far the identification of wheat chromosomal regions containing sensitivity genes has been limited to just a handful of populations.”

ToxA insensitive varieties

Dr Moffat said the results of the research reiterated the need to increase the area sown to ToxA insensitive varieties and to phase out wheat cultivars containing the Tsn1 gene.

“A recent wheat variety trial we conducted found that there was no yield penalty associated with growing ToxA insensitive varieties,” she said.

“Moreover, in the presence of yellow spot disease, ToxA insensitive lines substantially outperformed the sensitive lines.”

Effector-assisted breeding

Dr Moffat said ‘effector-assisted breeding’ had already been adopted in Australia in response to the significant losses caused by ToxA-producing pathogens.

“Semi-purified ToxA has been delivered to wheat breeders since 2009 as a selection tool towards the development of disease resistant germplasm,” she said.

“We deliver 30,000 doses of ToxA each year to wheat breeders.

“As a result, there will be a considerable decrease in the area sown to ToxA sensitive wheat varieties in the future as new varieties are released.

“This is a major step towards reducing the huge scale of crop losses caused by yellow spot.”

Stop the Spot

Growers who see yellow spot infection in their paddocks can help the CCDM yellow spot research team.

“We want growers across Australia to send us leaf samples they suspect are infected with yellow spot so that we can recover strains of the fungus,” Dr Moffat said.

“Researchers need to get a current national picture of the extent of the yellow spot problem and grower assistance will help us immensely in monitoring this pathogen and staying on top of any changes.

“Yellow spot isolates that we recover will be screened for effectors, including biosecurity risks, and tested for virulence.”

Yellow spot management

Current recommendations to reduce the risk of yellow spot (including at seedling stages) are:

Primary management decisions for yellow spot need to be made prior to and/or at sowing. Fungicides are a poor last resort for controlling yellow spot as they have reduced efficacy.

Correct identification

Yellow spot is frequently misdiagnosed so if you think you have yellow spot or have been advised that you have yellow spot and need to spray a fungicide, do some simple checks first.

If you cannot comfortably cross off each of these boxes then consider getting a second opinion from a plant pathologist.


Go to www.stopthespot.com.au for information about the Stop the Spot campaign and how you can get involved.

Information to help wheat growers manage yellow spot is available in the GRDC Yellow Spot Fact Sheet at www.grdc.com.au/GRDC-FS-YellowSpotWest.

Information about GRDC funded research into yellow spot and other fungal diseases is contained in the GRDC Cereal Foliar Fungal Diseases Supplement, available at www.grdc.com.au/GCS110.

The paper outlining the gene deletion research is published in Molecular Plant Pathology at http://onlinelibrary.wiley.com/doi/10.1111/mpp.12154/abstract


GRDC Project Code CUR00012

Region West

GRDC Project code: CUR00012

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