By Dr Colin R. Wellings and Professor Robert F. Park University of Sydney Plant Breeding Institute Cobbitty. (Dr Wellings is on secondment from the NSW Department of Primary Industries.)
Resistance in wheat, barley, oats and triticale to their respective rust diseases can be conveniently divided into two broad categories, depending on the growth stages at which they are effective.
Seedling resistances can be detected from the emergence of the seedling leaf, and remain effective throughout the life cycle of the plant. These resistances have historically been of value to wheat breeders as they can be easily selected in large breeding populations in greenhouse tests.
However, many seedling resistances have become rapidly ineffective due to change in the pathogen that develops the ability to cause disease (ie virulence) on a resistance previously known to be effective when the variety was released. One example of a seedling resistance that was recently overcome by a rust pathogen is Lr24, a resistance gene present in a range of varieties including Anlace, Cunningham, Krichauff, Janz and Mitre.
Adult plant resistance (APR) is expressed at various growth stages, ranging from the second leaf through to flowering. APR has been generally effective in protecting Australian wheats, and is often based on one or more resistance genes. However, APR is not the "silver bullet" for resistance breeding, and several examples can be given where a single gene APR has become suddenly ineffective (ie "broken down") in commercial agriculture.
A problem with APR is the variable onset of expression. For example, APR to stripe rust in varieties such as the Cook types (for example, Sunco) is expressed quite early in the growth cycle, possibly as early as the third and fourth leaf. Other varieties have stripe rust APR expression delayed as late as full flag leaf emergence, and this was a feature of certain former long season wheats in the Condor group (for example, Osprey). In general, APR will be evident at or before flag and flag-1 leaves in most varieties.
Both resistance types are present in cereal varieties grown in Australia. Wherever possible, breeders will choose to release varieties with more than one effective resistance gene in order to avoid the dramatic effects of "resistance breakdown". It is therefore important to catalogue resistance genes in new varieties when they are released, and monitor rust pathogens to detect any new changes that may impact on particular resistance genes.
Reaction of adult plant wheat leaves to stripe rust infection, ranging from moderately susceptible (left) to highly susceptible (right). Note that striping occurs on adult plant leaves and not seedling leaves in response to stripe rust infections.
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