By Professor Robert F. Park and Dr Colin R. Wellings, University of Sydney Plant Breeding Institute Cobbitty
From time to time we hear that the rust resistance of a cereal cultivar has been broken down by a new rust pathotype. What is meant by the terms "resistance breakdown" and "pathotype"?
Cereals have evolved genes that can recognise and respond to the presence of a pathogen (disease-producing agent, such as a virus, bacterium, or other microorganism).
Photo: Wheat stripe
differ in their
abilities to infect
We refer to these genes as resistance genes, and they have been used to great effect in protecting Australian cereal crops against not only fungal diseases such as rusts but also against viral diseases (for example Mackellar has a resistance gene that protects it against Barley Yellow Dwarf Virus).
Like all living organisms, the fungal pathogens that cause rust diseases undergo random mutations, and occasionally, a mutant can develop with the ability to overcome a resistance gene.
In this situation, the resistance gene is said to have "broken down", however, in reality, it is the pathogen that has changed, not the resistance gene in the cereal cultivar.
The mutant rust pathogens are referred to by rust researchers and breeders as pathotypes, races or strains. New pathotypes may also be introduced into Australia, as was the case with the new pathotype of stripe rust that was detected in WA in August 2002 and subsequently spread to eastern Australia in 2003.
The Australian Cereal Rust Control Program at the Plant Breeding Institute monitors the occurrence of cereal rust pathotypes throughout Australia each year. The pathotypes are identified by seedling tests in the greenhouse.
Surveys of rust pathotypes last year, for example, identified six pathotypes of stripe rust, nine pathotypes of wheat stem rust, and six pathotypes of wheat leaf rust.
Pathotypes differ in their abilities to infect wheat cultivars, depending on what resistance gene or genes are present. The table below illustrates the interplay between pathotypes of stripe rust and resistance genes in several wheat cultivars.
The table can be interpreted using the following principles:
With the data presented in the table, the following interpretations can be drawn:
This illustration indicates the importance of developing a comprehensive understanding of pathogen populations, and also the nature and diversity of resistances used in current commercial cereal production. With this information, we are in a better position to predict expected responses of current cereal varieties.
However, the expected changing nature of the pathogen populations and the introduction of new cereal varieties will mean that these two areas of research will need continuing support from all levels of the Australian cereals industry.
Growers in south-eastern Australia may remember the development of the Avocet attacking stripe rust pathotype in 1981 (middle photo), which was followed by the development of the Millewa/Bindawarra attacking pathotype in 1983 (right photo). The Avocet pathotype was a mutant with virulence for the resistance gene YrA, and the Bindawarra/Millewa pathotype was a mutant with virulence for the resistance gene Yr6.
For more information:
Robert Park, 02 9351 8806, firstname.lastname@example.org
(Dr Wellings is on secondment from NSW Primary Industries)
GRDC Research Code: US315, program 3
Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.