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.
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.“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.”
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:
- Do not sow wheat-on-wheat;
- If you are going to sow wheat-on-wheat consider a late (autumn) stubble burn and/or;
- Select a wheat variety with some level of resistance to yellow spot (note tolerance/resistance to other diseases though).
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.
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.
- Is there wheat stubble visible in the paddock? Yellow spot is essentially a disease of wheat-on-wheat rotations as it is a stubble-borne pathogen. Burning, stubble grazing and cultivation can be effective in reducing yellow spot inoculum but depend on completeness of stubble removal. If wheat stubble is still visible then yellow spot inoculum may also still be present.
- Are black fruiting bodies (pseudothecia) visible on the stubble? By autumn/winter black, pinhead sized, raised structures with hair-like projections (which make them feel rough if rubbed with finger) will be visible on wheat stubble. These are responsible for the initial leaf infections at seedling stages and establishing lesions in lower canopy that drives secondary disease development later in the season.
- Do the lesion spots look right? Initially, the disease appears as small brown spots with discrete yellow margins. With age, these spots become more elongated and tan (dead dried leaf tissue), but still have a tight yellow (toxin production) margin. Yellow spot DOES NOT cause extensive general yellowing of leaves.
- Is the distribution on a tiller right, both on individual leaves and within leaves? Yellow spot spores (ascospores or conidia) land randomly on individual wheat leaves and just require adequate moisture for >6 hours to germinate and infect. Hence, symptoms are randomly distributed across an individual leaf. Yellow spot DOES NOT concentrate towards the leaf tip. Inoculum is generally coming from lower down in the canopy (stubble or old leaf lesions). Hence, if you pull off an individual infected tiller there will be a clear pattern of distribution on the leaves with more and larger lesions on the lowest leaves (they have been out longer so prone to more infection events and greater time for fungal growth to spread through leaf) and fewer and smaller lesions as you progressively move higher up the tiller to the next leaf.
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