Supported by

Same ol’ yellow spot hasn’t changed a bit

early lesions of yellow spot
Early lesions of yellow spot

We talk about mutations a lot at the CCDM, as diseases are always mutating. But there’s one disease that’s been around for a while and hasn’t changed that much, and that’s yellow spot.

The 2015 results from Stop the Spot are now in, which allowed our researchers to analyse 145 diseased-leaf samples sent from a range of locations across the Australian grain growing region.

While other diseases such as rust and powdery mildew have mutated into more than 25 pathotypes (oat crown rust has mutated into more than 100 pathotypes!), in Australia yellow spot has one predominant race that we know of.

Dr Pao Theen See of CCDM’s yellow spot program said her team studied the major disease-causing gene within 40 yellow spot strains – the gene ToxA – and did not detect any variation from previous years’ samples.

“This implies that the major gene within yellow spot that causes necrosis (cell death) has not mutated into a different or stronger form,” Pao Theen said.

“This is good news for breeding yellow spot resistant wheat, because the lesser extent of the number of pathotypes makes it easier to develop the genetic tools needed to reduce yellow spot infections on wheat.”

However, Pao Theen said with other pathotypes of yellow spot wreaking havoc in other countries, it is vital that her team continues to monitor the Australian yellow spot disease population for changes.

Yellow spot is a widespread problem

Of the 145 samples received from growers and advisors, 56 per cent were confirmed to be infected with yellow spot.

“Yellow spot can be tricky to identify, in the past it has been mistaken as the cause for leaf yellowing, and is often confused with other leaf spot diseases,” Pao Theen said.

“Nevertheless, yellow spot infections were found to be widespread across Australia with the disease found in the western, southern and northern grain growing regions.”

stop the spot map
Where the samples came from – Yellow Spot is a widespread problem


Which wheat varieties?

Interestingly, nearly half (46%) of the yellow spot samples received were from leaves of resistant varieties, including Magenta, Wyalkatchem and Mace.

pie chart
Of all yellow spot samples received, 46% came from varieties rated as MR or MRMS.


It’s as easy as ToxA, ToxB, ToxC

So far, there are three known ‘effectors’ (disease-causing toxins) within the yellow spot pathogen – we refer to them as ToxA, ToxB and ToxC.

ToxA is, by far, the most potent of the three, with the toxin gene present in every sample of yellow spot analysed from the 2015 season. From past studies, we now know that for ToxA to cause damage to wheat, wheat must possess the sensitivity gene Tsn1:

How Tox A and TSN1 wofk

Luckily, we’ve been able to help breeders breed varieties of wheat without the Tsn1 gene. We’ve done this by providing them with a large amount of ToxA to infiltrate onto different wheat variety leaves, and any damage on a leaf gives a clear sign the variety is susceptible to ToxA (and therefore not worth keeping in the breeding program).


However, removing the Tsn1 gene from wheat is not the be-all and end-all. Research at CCDM has shown that yellow spot pathogens lacking the capacity to produce ToxA were still able to cause disease, which is why moderately resistant varieties such as Magenta and Wyalkatchem that are without the Tsn1 gene still get infected.

“Although the wheat varieties that lack the Tsn1 gene are more resistant to yellow spot in comparison to varieties that still hold the gene, we know there are still other effectors that contribute to the disease,” Pao Theen said.

“We know that effector ToxC is another culprit for yellow spot damage in Australia and research is currently ongoing to identify this effector.”

“We also know there are other effectors out there too, and we are constantly on the lookout for them.

“At the CCDM we work on the model that a wheat variety insensitive to all effectors will be extremely resistant to yellow spot disease – and I look forward to the day we have a variety completely yellow spot resistant.”

What about ToxB?

For the second year, ToxB has not been detected in any Stop the Spot samples which, according to Pao Theen, is an enormous relief.

“Currently, the ToxB effector is found in overseas yellow spot pathogens and this effector is known to cause chlorosis in sensitive wheat varieties,” Pao Theen said.

“As a major biosecurity risk to Australian wheat crops, it is extremely important we continue to monitor the yellow spot pathogen, and ensure ToxB remains outside of Australia.

“And if by chance it does enter Australia, the Stop the Spot program enables us to be in the best position possible to act early to help breeders breed ToxB resistance in wheat.”

Choosing a variety insensitive to ToxA will help

ToxA remains the most potent of all effectors, so varieties that do not possess the Tsn1 gene and are therefore insensitive to ToxA are strongly recommended for better management of the disease.  Varieties in red below are best avoided in high-risk disease areas:

varieties chart
Wheat varieties in red are sensitive to ToxA and are best avoided (Note: Disease ratings may have changed since this chart was made. Please refer to the most current disease ratings)

Thanks for the samples in 2015. Please donate again in 2016!

The yellow spot team thanks everyone who contributed a sample in 2015. The full report on the 2015 Stop the Spot can be found here or, in a nutshell, our conclusions were:

  • Yellow spot is nationally widespread
  • MR and MRMS varieties were found to be vulnerable to yellow spot infection
  • Australian yellow spot has one predominant race we know of (ToxA and ToxC)
  • ToxB was not detected, but it is currently present overseas and is a major biosecurity risk
  • No mutation was detected in the ToxA gene

Stop the Spot will continue in 2016, so please get involved and help us keep track of the disease to develop solutions for improved yellow spot resistance in wheat.

For 2016 sampling envelopes, please email us at

For more information on Stop the Spot, visit or contact Pao Theen at

Stop the Spot is brought to you by the CCDM’s yellow spot team, which includes Caroline Moffat (Program Leader), Pao Theen See, Kalai Marathamuthu, Elyce Iagallo and Blake Wood.


Sorry, comments are closed for this post.