Cereal rust pathotypes (aka races or strains) are isolates of rust that differ in their ability to overcome resistance genes in cereal varieties. Pathotypes are identified by using a sample of rust from a cereal crop to infect a set of cereal varieties (“differentials”) – each carrying a known resistance gene – and determining which resistance genes are overcome and which are not.

The process of identifying rust pathotypes takes about three weeks. A week after sowing and raising the differentials, they are sprayed with a suspension of rust spores made from the crop sample and given an artificial dew treatment for about 24 hours so that the spores will germinate and infect. The rust then needs to grow on the plants for about 10 to 12 days.

Identifying pathotypes provides insight into pathogen population structure. For example, we know that the epidemics of wheat stripe rust in eastern Australia in 2020, 2021 and 2022 were caused almost entirely by two pathotypes that found their way into Australia, probably from Europe and South America, in 2017 (pathotype 239 E237 A-17+ 33+; “239”) and in 2018 (198 E16 A+ J+ T+ 17+; “198”).

Impact on current varieties

However, identifying pathotypes is of limited use in rust control in the absence of information on their impact on current cereal varieties and on advanced breeding lines. This can only be achieved by knowing which resistance genes are present in varieties and breeding lines, and by conducting research in the greenhouse and field with new pathotypes to determine their impact, if any.

While the greenhouse component of this research is usually completed within a month or so, the field-based research can take about a year to generate accurate information on varietal response.

To illustrate this, our surveys identified a new pathotype of wheat stripe rust in September 2021, 238 E191 A+ 17+ 33+ (“238”). Following detection, about one month was spent generating pure inoculum (spores) of the new pathotype in our greenhouse system by infecting a susceptible wheat.

Threat from new pathotype

The spores thus produced were then used in greenhouse experiments that compared the responses of wheat varieties and important genetic stocks to pathotype “238” with those to other important existing pathotypes maintained in long-term liquid nitrogen storage at the University of Sydney. These tests told us that the new pathotype did not pose a threat beyond that posed by the two most common pathotypes of stripe rust, “198” and “239”.

Despite these results, our surveillance work in 2022 unexpectedly revealed a significant increase in the frequency of pathotype “238” in 2022, suggesting that this pathotype may be more “aggressive” – for example, faster-growing and producing more spores. Our research is now looking at this to try to determine why this pathotype became more frequent in the eastern Australian stripe rust population in 2021.

Identifying rust pathotypes and determining their impact on current cereal varieties provides the basis of all rust control efforts, be they genetic-based or chemical-based. Please forward freshly collected rust samples in paper only to the Australian Cereal Rust Survey, University of Sydney, Australian Rust Survey, Reply Paid 88076, Narellan, New South Wales, 2567.

Information on the cereal rusts can be found in periodic Cereal Rust Reports which, along with a regularly updated map of the distribution of rust pathotypes, can be found at the “Rust Laboratory” website.