Pathotype compromises rust resistance

A leaf rust pathotype that can overcome resistance in barley cultivars carrying the Rph3 gene has recently been detected for the first time in WA.

The pathotype (known as 5457 P-) was identified by Robert Park at the GRDC-funded Australian Cereal Rust Control Program, based at the University of Sydney's Plant Breeding Institute.

It was found during testing on seven barley crop samples collected in September 2013 from Boxwood Hill, South Stirling, Chillinup and Kamballup in conjunction with the Department of Agriculture and Food WA.

This newly detected pathotype will primarily affect the Bass variety - commercially released this year - and advanced breeding lines in WA that carry the resistance gene Rph3.

This is the first time Rph3 resistance has been compromised in barley varieties in WA, highlighting the importance of the GRDC's national rust detection and control initiative through the ACRCP and in close collaboration with DAFWA.

Research to incorporate new sources of leaf rust resistance into new Australian barley cultivars is also being undertaken through the ACRCP at the PBI.

This is targeting potentially more durable resistance sources that are effective at post-seedling growth stages, such as adult plant resistance.

Several Australian barley cultivars already contain adult plant resistance provided by the Rph20 gene, such as Flagship, Oxford, Grange, Henley, Shepherd and Westminster.

The specific new 5457 P- pathotype mutation is unique to WA, but it is expected to behave similarly to another pathotype with virulence to Rph3 that is present in eastern Australia (known as 5457 P+).

On this basis, it is likely to have the biggest impact on Bass, which has the Rph3 gene.

Grange, Henley and Oxford carry both the Rph3 gene and the Rph20 gene, which is expected to provide some residual resistance and should mean leaf rust response will not change significantly.

Further testing will assess the impact of 5457 P- on Fairview, Finniss, Fitzroy, Wimmera and Yarra varieties, which also have the Rph3 gene.

These varieties - and Bass - were rated as susceptible in PBI field tests from 2009-10 when challenged with the Eastern States' Rph3-virulent pathotype 5457 P+.

If the population of 5457 P- pathotype builds in WA, it is expected this mutant could migrate to the Eastern States, but it is not expected to have an additional significant impact on cultivars there. On-farm implications in WAGrowers of Bass, Oxford, Grange, Henley, Fairview, Finniss, Fitzroy, Wimmera and Yarra are being advised to monitor these crops closely.

For further information see www.rustbust.com.au .