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X-Men or Batman – which one is yellow spot?

X-Men: A small population of humans who are mutants, possessing superhuman powers.

Batman: A superhero without superpowers. Just a regular rich guy really.

But if superheroes were crop diseases, which one would yellow spot be?

If you guessed Batman, you would be correct. Yellow spot is kind of like Batman, in the way that we haven’t found any mutations in yellow spot’s major disease-causing gene – ToxA. Its power to kill wheat cells has stayed the same, it just keeps using the toxic proteins from its ‘utility belt’ to do the damage.

Powdery mildew and rust on the other hand are more like X-Men – they have mutated into more than 25 different pathotypes (oat crown rust has mutated into more than 100 pathotypes!). With so many versions of themselves, just as there are so many versions of X-Men – Logan, Magneto, Storm…, they’re a force to be reckoned with.

For the past three years, Dr Caroline Moffat of CCDM’s yellow spot program studied more than 350 samples of yellow spot from all regions of Australia throughout the Stop the Spot initiative, and with her team confirmed that ToxA – the major gene within yellow spot that causes necrosis (cell death), had not mutated into a different or stronger form.

“We found no evidence that ToxA is undergoing selection to become a more potent form. By not sowing ToxA sensitive varieties, we are helping to reduce the selection pressure for increased ToxA activity,” Caroline said.

Stop the Spot key findings

One of the most interesting findings from Stop the Spot over the years, was that around 50 per cent of yellow spot infected samples came from MRMS to MR varieties, such as Mace and Corack.

“These more resistant varieties are not damaged by ToxA as they do not possess the ToxA sensitivity gene,” Caroline said.

“This tells us that there are more effectors out there that are doing damage, such as ToxC and others not yet identified.  Thanks to the many samples sent in we are now working to discover what these are”

Some key results from Stop the Spot, 2014 – 2016:

Wheat on wheat – not the way to go

From three years of sample collection, 62 per cent of samples came from paddocks that had previously cropped wheat.

“This is not too surprising, but it is a concern as yellow spot fruiting bodies will survive on wheat stubble through the summer, and will release spores when the rains begin which will infect nearby wheat,” Caroline said.

“Yellow spot spores do not travel very far, so when wheat is grown in a previously infected paddock, we’re giving the pathogen the perfect opportunity to infect the next seasons’ crop.”

Still no ToxB detected

We’ve already mentioned the most damaging yellow spot effector – ToxA.

But there are two other effectors we know about besides ToxA, which are ToxB and ToxC.

ToxB has not yet been detected in Australia – Caroline and her team can confirm this from three years of Stop the Spot data.

“This is good news, as ToxB causes a lot of damage on wheat crops overseas. Even so, it is still a major biosecurity risk to Australian wheat crops, so it is extremely important that we continue to monitor the yellow spot pathogen for this effector,” she said.

“On the other hand ToxC does exist in Australia, and we are currently trying to find out more about this effector, so we can eventually find the sensitivity gene within wheat and help breed it out.”

How to choose a variety to avoid yellow spot

Caroline recommends growers stick to varieties that are insensitive to ToxA, as ToxA is still the most potent of the identified effectors, and was found in every Australian yellow spot isolate.

“Growers who choose the more resistant varieties may still get some yellow spot infection, but not as much as varieties that are sensitive to ToxA,” she said.

For any specific variety enquiries, please contact Caroline at caroline.moffat@curtin.edu.au

What three years of national sample collection has told us

For three years, Caroline and her team have been analysing yellow spot from across the country, and from the 366 yellow spot samples, they’ve been able to conclude:

  • Yellow spot is a widespread disease, infecting all wheat growing regions across Australia.
  • Even moderately resistant varieties were infected.
  • ToxA was found in 100 per cent of isolates, reiterating the importance of using ToxA insensitive varieties.
  • ToxB was not detected in any of the isolates, but still remains a biosecurity risk.
  • No mutation was detected in the ToxA gene of Australian yellow spot.
  • There are still unidentified yellow spot effectors out there.

Stop the Spot… it’s a wrap. Thanks for the samples!

CCDM would like to thank everyone who contributed samples to the Stop the Spot initiative – including grower groups, growers, agronomists and field pathologists. The initiative has been a huge success, helping our researchers obtain isolates from areas not previously sampled, saving thousands of hours if the team had driven to each location.

Although the collection of samples has finished, the isolates obtained live on in our secure laboratories. They add to our isolate collection, which is a valuable resource for ongoing work to identify new effectors and wheat molecular markers – tools for breeders to facilitate the release of more resistant varieties.
Virulence testing is now underway, in conjunction with DAFWA. Stay tuned for these results!

About Caroline Moffat

Dr Caroline Moffat studied Plant Science at Oxford University (UK) and joined Curtin in 2011, where she now leads the yellow spot program within the Centre for Crop and Disease Management.

Her team aims to minimise the cost of this disease to growers, by developing tools to help breeders produce more resistant varieties.

 

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