Almost half the yellow spot disease samples received in a national survey in 2015 were collected from wheat varieties which were rated as having resistance, highlighting the need for continued industry vigilance against the costly disease.
Conducted as part of the Curtin University and Grains Research and Development Corporation (GRDC) Stop the Spot initiative, the survey found that yellow leaf spot (Pyrenophora tritici-repentis) – also known as tan spot – is nationally widespread.
Researchers analysed 145 diseased-leaf samples sent from a range of locations across Australia’s grain growing regions and 56 per cent of the samples were confirmed as being infected with yellow spot.
Pao Theen See, of the yellow spot program at the Curtin University-based Centre for Crop and Disease Management (CCDM), said 46 per cent of the confirmed yellow spot samples were from varieties rated moderately resistant or moderately resistant/moderately susceptible.
“Yellow spot was found in varieties such as Mace , Magenta and Wyalkatchem , which indicates that even putatively-resistant varieties can be susceptible to the disease,” she said.
“This also highlights how vital it is that yellow spot is continually monitored so we can respond to any changes in the pathogen population.
“These varieties are not damaged by ‘Tox A’ – the most potent of known effectors (disease-causing toxins) within the yellow spot pathogen – as the wheat gene that interacts with ToxA was bred out of them.
“The reason they still get infected is because there are other damaging effectors in Australia including ToxC and others that are so far unknown.”
Dr See said it was still recommended that growers chose varieties ‘insensitive’ to ToxA such as Mace , Magenta and Wyalkatchem as ToxA was the most potent of the identified effectors.
“Growers who choose resistant varieties may get some yellow spot infection but not as much if they grow susceptible varieties,” she said.
Dr See said one of the positive outcomes from the latest Stop the Spot survey was no variation in the ToxA gene was detected compared with 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,” she said.
“This is good news for breeding yellow spot resistant wheat because fewer pathotypes makes it easier to develop the genetic tools to reduce yellow spot infections on wheat.”
Dr See said that for the second year ToxB – which is found in overseas yellow spot pathogens – was not detected in Stop the Spot samples.
“But as a major biosecurity risk to Australian wheat crops, it is extremely important that we continue to monitor the yellow spot pathogen for this effector,” she said.
“If ToxB-containing strains invaded Australia, yellow spot disease virulence would increase.”
Stop the Spot, which aims to significantly reduce the economic impact of yellow spot, will continue in 2016 and growers and advisers are encouraged to send in leaf samples of the disease to the CCDM.
“The more samples we receive, the better we can keep track of the disease to develop solutions for improved yellow spot resistance in wheat,” Dr See said.