Drover oats vulnerable to new crown rust pathotype

A photo of plants

Crown rust of oats has become a recurring problem in grazing oats in Australia due to the occurrence of new rust sub-types capable of overcoming the resistance of many cultivars (Table 1).

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 DroverPBR logo and AladdinPBR logo are known to carry Pc91. While the new rust pathotype is virulent on seedlings of DroverPBR logo, it is not able to attack AladdinPBR logo, presumably due to additional resistance provided by the Pc50 resistance gene.

AladdinPBR logo therefore remains resistant to all known pathotypes of crown rust in Australia. Growers of DroverPBR logo 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.

 Cultivar Year of release Virulence first detected Seedling
Table 1: Tracking the breakdown of crown rust resistance in oats
 Culgoa II
 1996  PcMortlock, PcCulgoa
 Bettong  1992  2001  PCBett
 Cleanleaf  1992  1995  Pc38, Pc39, Pc52
 Barcoo  1996  2001  Pc39, Pc61, PcBett
 Graza 68
 1997  1999  Pc68
 Moola  1998  1999  Pc68
 Gwydin  1999  2001  Pc56
 Warrego  1999  1998  Pc61+
 NugenPBR logoe  2000  2005  Pc48+
 TaipaPBR logon  2001  2005  Pc48+
 VoltaPBR logo  2003  2008  Pc50, Pc68
 GeniePBR logo  2008  2010  Pc48, Pc56
 DroverPBR logo  2006  2012  Pc91
 GalileoPBR logo  2006  ?  Not tested
 Qantom  2006  2008  Pc50
 DawsonPBR logo  2009  ?  Not tested
 AladdinPBR logo  2001  Not yet detected
 Pc50, Pc91

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.

Rust Bust LogoThe Rust Bust (www.rustbust.com.au) is an initiative of the Australian Cereal Rust Control Program Consultative Committee, supported by the GRDC.