MANY OAT growers would be familiar with the diseases stem rust and leaf rust. The pathogens that cause these oat diseases do not infect wheat, barley, rye or triticale.
While both diseases occur in all cereal-growing regions of Australia, in recent times growers in Victoria and SA have experienced problems with stem rust, and growers in northern NSW, Qld and WA have experienced problems more so with leaf rust.
Wild oats harbour the oat rust pathogens, and their common occurrence supports large pathogen populations. Long-term monitoring of oat rust populations in Australia has shown that those occurring on wild oats are very simi lar to those occurring on cultivated oats.
The large rust popUlations on wild oats therefore provide the initial inoculum for epidemics in cultivated oats. They also increase the potential for the development of mutant pathotypes. Cereal rust pathogens have been estimated to undergo random mutations at the rate of about 1 per 10,000,000,000 spores, and sometimes these mutations result in new pathotypes that can infect a previously resistant cultivar. The larger the rust population, the greater the likelihood that new pathotypes will develop.
Monitoring oat rust pathogen populations has also shown that a great deal of pathogenic variability exists, with new pathotypes often being detected soon after the release of rust-resistant cultivars. For example, the cultivars Culgoa (1991), Clean leaf (1992), Warrego(1998), Graza 68 and Moola ( 1999), Nugene (1999) and Gwydirm ( 1999), although regarded as resistant to leaf rust when released, were rendered susceptible by new pathotypes not long after release.
The cultivars Bettong (1992) and Barcoo (1996), although resistant to leaf rust for some time following release, were known to share a common gene for resistance, and, as anticipated, a single mutant pathotype was detected last year that rendered both cultivars susceptible. Similar experiences with the stem rust pathogen now mean that there are no effective sources of seed ling resistance to this pathogen.
In all cases whe;e resistance has been overcome in oats, it was controlled by a single gene. Combining resistance genes has been an effective strategy in developing rust-resistant wheat cultivars, because it is less likely that a mutant pathotype will acquire virulence to two or more genes simultaneously. This approach may also work with oats, but its success will depend on a thorough knowledge of pathogenic variability in the pathogen populations.
Discovering new sources of resistance will be an important part of this process, and a newly funded GRDC initiative that will involve scientists from SARDI, Agriculture Canada and the University of Sydney will target this.