Drover oats vulnerable to new crown rust pathotype
A new disease-causing pathotype of oat crown rust that overcomes the resistance gene Pc91 has been isolated from a rust-infected oat sample collected in late 2012 from Warwick, Queensland.
This is the first detection of breakdown of the Pc91 resistance gene in Australia since monitoring of this gene began in the early 1990s. Cultivars Drover and Aladdin are known to carry Pc91. While the new rust pathotype is virulent on seedlings of Drover, it is not able to attack Aladdin, presumably due to additional resistance provided by the Pc50 resistance gene.
Aladdin therefore remains resistant to all known pathotypes of crown rust in Australia. Growers of Drover oats in 2013 should monitor crops regularly for the presence of crown rust as it is vulnerable to this new rust ‘pathotype’.
Oat crown rust diversity in Australia
The rust fungi are ‘biotrophic’ – they require a living host on which to survive. Of all the cereal-attacking rusts in Australia, the two that infect oats (the stem rust Puccinia graminis f. sp. avenae and the crown rust P. coronata f. sp. avenae) have the largest populations, being maintained on ubiquitous, weedy, wild oats.
Although the two rusts that attack oats reproduce solely by clonal (asexual) means, great genetic diversity is generated by the large populations growing on wild oats. The crown rust pathogen is likely to be the most genetically diverse of all cereal rust pathogens in Australia – more than 95 unique pathotypes were detected between 1998 and 2010, in contrast to the wheat stripe rust pathogen, for which 27 pathotypes were detected during this period.
Crown rust resistance breakdown in oats
Attempts to develop crown rust resistant oat cultivars over the past 25 years have met with very limited success, with new pathotypes often being detected soon after the release of resistant cultivars (Table 1). While the change of a cultivar from resistant to susceptible is often referred to as resistance ‘breakdown’, the cause is actually a change in the pathogen.
A major difficulty in breeding oats for resistance to crown rust has been a lack of understanding of the crown resistance genes present in oat germplasm.
Detailed studies over the past 10 years have identified the genes present in many cultivars (Table 1). In most cases, it has been found that the resistance of new cultivars was based on single seedling resistance genes (also referred to as ‘all stage’ or major genes). Given the size and genetic variability in oat crown rust populations in Australia, it is therefore not surprising that these cultivars have usually succumbed to crown rust soon after release.
Adult plant resistance to crown rust in oats
Research on crown rust resistance at the Plant Breeding Institute over the past five years has targeted the identification of adult plant (minor gene) resistance (APR) in cultivated oats. Experience in wheat has shown that this type of resistance tends to be more durable than single ‘all stage’ resistance genes, especially when three or more such APR genes are combined. More than 200 oats carrying APR to crown rust have been found and studies have been initiated to understand the genetic basis of the resistance to expedite its use in future breeding efforts.
Monitoring rust variability and forwarding samples for virulence analyses
As always, monitoring rust variability is a crucial part of using genetic resistance to combat these diseases. Readers are encouraged to submit samples for confirmation of rust identity and subsequent pathotype analysis.
The Rust Bust (www.rustbust.com.au) is an initiative of the Australian Cereal Rust Control Program Consultative Committee, supported by the GRDC.