Last year, we wrote an article on the issue of stripe rust in barley (GroundCover™ Issue 151), making the point that there was no need for alarm. While this remains the case, a new pathotype of stripe rust with increased virulence on barley was detected in 2021. The new pathotype is not expected to cause damage in barley crops in 2022, but it will be critical to monitor this new variant in case it changes again and becomes even more virulent on barley.
Stripe rust is caused by the fungal pathogen Puccinia striiformis. Variants within this fungal species are specialised to different crops; for example, P. striiformis f. sp. tritici (WYR, so called because wheat stripe rust is called wheat yellow rust in some parts of the world) infects wheat and was widespread throughout eastern Australian wheat crops last year. Another variant, P. striiformis f. sp. hordei (BYR), does not occur in Australia and is considered a serious exotic threat due to the high susceptibility of many current barley varieties to this pathogen.
A third form of P. striiformis, first detected in Australia in 1998, is known colloquially as BGYR (barley grass stripe [yellow] rust). BGYR is very common on wild barley grass weed species such as Hordeum glaucumand Hordeum leporinum. Using whole genome sequencing, we have been able to show that BGYR also occurs in North America, where it has been isolated from triticale and from the grass species Agropyron cristatum. BGYR does not infect wheat and, while it can infect barley, all varieties other than Skiff, Tantangara and Maritime have good levels of resistance to it.
In our GroundCover™ article early last year, based on experiences in 2020, we stated that “in the 23 years that BGYR has been in Australia, our work has most fortunately failed to detect any increased adaptation to barley or wheat in this rust pathogen”. Remarkably, late in 2021 we found the very first BGYR variant in samples of stripe rust collected from wild barley grass and from several crops of RGT Planet in NSW and, subsequently, in Victoria and Queensland. The new pathotype has been designated BGYR+.
Storage of new pathotypes
New rust pathotypes that are detected in nationwide cereal rust surveillance work undertaken at the Plant Breeding Institute Cobbitty are stored in liquid nitrogen for use in genetic research, pre-breeding and post-release varietal management. We conducted high-throughput greenhouse seedling screens to compare the responses of a broad range of barley varieties and germplasm to the 1998 isolate of BGYR with the new BGYR+ isolate found in 2021.
These tests established clearly that the new BGYR+ variant has increased virulence on a significant number of barleys at seedling growth stages, with the varieties Capstan, Empress, Finniss, Keel, Ketch, Prior and Ulandra all being rendered seedling susceptible. Seedling tests also showed that 22 of the 73 barley lines in the 2021 National Variety Trials changed response (but remained resistant), and two were rendered susceptible.
Given there were no reports of serious stripe rust in barley crops in 2021, we anticipate that most, if not all, current varieties have adult plant resistance over and above the resistance seen in our seedling tests. We will establish field rust nurseries with the BYR+ pathotype this year to get a better understanding of how barley responds to it at adult plant growth stages.
The new BGYR+ pathotype was very common on wild barley grass in 2021, and we expect the same will occur in 2022. Common occurrence of this rust will provide opportunities for it to undergo further changes in virulence. Understanding the vulnerability of our current barley varieties to potential future changes in this pathogen will be vital in assessing the risk it poses to the barley industry. BYR is included as a grain High Priority Pest (HPP).
Information on cereal rusts can be found in our periodic Cereal Rust Reports which, along with a regularly updated map of the distribution of rust pathotypes, can be found at at the Sydney University website.
We encourage all stakeholders to inspect crops and nearby weeds (especially barley grass, given the current situation) for rust diseases. If rust is found, please send freshly collected samples in paper only to the Australian Cereal Rust Survey, University of Sydney, Reply Paid 88076, Narellan, NSW, 2567.