Cereal rust strains provide clues to resistance
GroundCover™ Issue: 102 | Author: Professor Robert Park, Dr Will Cuddy
The Rust Bust
Cereal rust pathotypes – which are races or strains of the disease that vary in their ability to overcome resistance genes in cereal varieties – are common in most Australian wheat, barley, oat, rye and triticale crops.
These pathotypes were first detected in the early 1900s in the US. At that time, Professor Walter Lawry Waterhouse from the University of Sydney showed there was similar variation in isolates of stem rust and leaf rust collected from Australian wheat crops.
In 1917, Professor Waterhouse started conducting annual surveys of all cereal rust strains in Australia, including both stem and leaf pathotypes in wheat. This tradition of scientific testing is still ongoing at the University of Sydney’s Plant Breeding Institute (PBI).
These long-term studies looking at variability in rust pathotypes indicate that different strains of the disease can rapidly migrate between Australia and New Zealand. This is why genetic control of cereal rusts needs to be coordinated at a national level. For many years, both Australia and New Zealand benefited from a collaborative approach to surveying rust pathogens, but this has been discontinued due to quarantine regulations.
Annual surveys of variability in the rusts at PBI provide the basis for all rust control efforts at a genetic level.
The surveys monitor the effectiveness of rust resistance genes in commercial cereal varieties, determine responses to new endemic and exotic rust strains in current cereal cultivars, facilitate the discovery and introduction of new resistance genes into locally adapted germplasm, and allow pre-emptive resistance breeding.
Rust surveillance is increasingly being combined with extension efforts to help increase industry awareness of emerging rust threats and provide advice on choosing cropping varieties with effective resistance.
Evolution in the cereal rust pathogens has led to the demise of several resistance sources over the past five years: YrJackie (2007) and YrTobruk (2009) in triticale; Yr17 (2006) and Yr27 (2008 and 2010) in wheat; Rph3 in barley (2008 and 2011); and Pc50 (2008) and PcGenie (2010) in oats.
Knowing the distribution of these strains is important in predicting how commercial cereal cultivars will respond to rust diseases. For example, virulence for resistance gene Rph3 in the barley leaf rust pathogen occurs only in eastern Australia.
Hence, all barley cultivars carrying this resistance gene are currently resistant to leaf rust in Western Australia, while in eastern Australia, some are moderately susceptible to susceptible. Field testing at PBI using the Rph3-virulent pathotype has established that some other cultivars with this resistance gene also carry an additional source of adult plant resistance, which is likely to be the Rph20 gene.
Identification of cereal rust pathotypes involves infecting seedlings of a set of cereal varieties, which each carry a known rust-resistance gene, with a rust sample collected from the field.
The ability or inability of a particular rust sample to infect each variety allows the pathotype/s it contains to be identified. These tests take about three to four weeks to complete, but more time is often needed where a new pathotype is suspected. However, work at PBI to identify pathotypes is increasingly being supplemented by DNA profiling, which means the process may only take several days.
While DNA profiling provides important information and a rapid means of recognising exotic rust incursions, it is not yet powerful enough to identify individual pathotypes.
Professor Robert Park,
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