Australia - Barley grass stripe rust variants characterised by increased virulence

The stripe rust pathogen Puccinia striiformis f. sp. tritici (WYR) has been prevalent in Australian wheat crops since its introduction in 1979 from Europe. Another variant, Puccinia striiformis f. sp. hordei (BYR), remains absent in Australia; however, it is considered a serious exotic threat due to the high susceptibility of many current barley varieties to this international pathogen.

A third form, colloquially known as BGYR (barley grass stripe [yellow] rust), was first detected in Australia in 1998, predominantly affecting wild barley grass weed species such as Hordeum glaucum and Hordeum leporinum. The Plant Breeding Institute’s comparative pathogenomics work identified this stripe rust variant as P. striiformis f. sp. pseudo-hordei; it is also found in North America, isolated from triticale and the grass species Agropyron cristatum.

The BGYR pathogen, examined annually since 1998, exhibited no pathogenic variants until late 2021. A new variant, designated BGYR+, was detected in stripe rust samples from wild barley grass and RGT Planet crops in New South Wales, subsequently spreading to Victoria and Queensland.

The stripe rust pathogen Puccinia striiformis has been prevalent in Australian wheat crops since its introduction from Europe in 1979.    Photo: GRDC

Increased virulence on seedlings

Through comparative greenhouse studies, the Plant Breeding Institute established that the BGYR+ variant has shown increased virulence on several barley varieties at seedling growth stages, with the varieties Capstan, Empress, Finniss, Keel, Ketch, Prior and Ulandra rendering susceptible.

Preliminary genetic research suggests that this new pathotype has rendered three to four resistance genes in Australian barley varieties ineffective. Although it has not resulted in yield losses in barley crops to date, stripe rust samples have been received from barley crops every year since its detection.

The pathogen has been widespread and severe on weedy barley grass, causing severe natural infections in nurseries at Horsham, Victoria and Wagga Wagga, NSW, in 2023, raising concerns about the potential spread of BGYR+ from weedy barley grass.

In 2022, a significant shift in virulence for BGYR+ pathotype occurred, with a variant assigned BGYR+ A+ gaining virulence for a resistance gene in the Avocet wheat variety. This raises concerns about potential further changes in virulence. In 2023, extensive research was allocated to investigate these variants and understand their full impact. Further research is essential to fully understand their effects and implications.

Additionally, recent fungicide insensitivity tests on 2022 surveyed samples revealed that the BGYR+ and BGYR+ A+ mutant pathotypes both detected from NSW displayed insensitivity to fungicides.

Despite the absence of registered fungicides specifically for BGYR control, none of the four DMIs fungicidetested (tebuconazole, prothioconazole, propiconazole and triadimenol) were effective at the high field rates recommended for other diseases against the two BGYR+ fungicide insensitive pathotypes.

Potential impact of variants

The detection of BGYR+ variants, characterised by increased virulence on several Australian barley varieties and their display of insensitivity to DMI fungicides, raises concerns regarding their potential impact on barley crops.

It is crucial to explore alternative fungicides through research to comprehensively understand their implications and to formulate effective management strategies. Undertaking this research is essential to ensure the long-term sustainability of the Australian barley industry.

Growers can access insights into cereal rusts through the university’s periodic Cereal Rust Reports and regularly updated map, available at the Australian Cereal Rust Survey website.