Rust sports readily adhere to clothing. This enables the spores to hitch a ride to new regions with travellers who fail to launder their clothes.
Two further changes in cereal rust pathogen populations have been detected, both in Western Australia.
These follow the identification of a new wheat leaf rust pathotype in northern New South Wales in early August (see Ground Cover, issue 107, page 37). As a result of these new WA developments, several barley and wheat varieties are likely to become more susceptible to leaf rust. However, the full extent of these changes in the rust resistance of different varieties cannot be confirmed until more extensive greenhouse seedling tests and adult plant field tests have been completed.
It is important to establish a more accurate picture of the distribution of both rust pathotypes in WA. Growers are urged to closely monitor all barley and wheat crops and any summer regrowth for rust disease, and forward infected plant samples to the University of Sydney’s Plant Breeding Institute (PBI) for pathotype analysis.
Barley leaf rust
A new pathotype of the barley leaf rust pathogen (Puccinia hordei) was detected in WA, based on samples collected in September at Boxwood Hill, Chillinup, Esperance, Kamballup and South Stirling in the state’s southern region.
A single-step mutation derived from an existing pathotype (5453 P-), the new pathotype (5457 P-) has added virulence for the rust resistance gene Rph3.
This is the first time virulence for the Rph3 gene has been detected in WA, and the third pathotype of P. hordei recorded in Australia with virulence for the gene.
The new pathotype is similar to another pathotype detected in northern NSW in late 2008, called 5457 P+, but it differs in being virulent for the Rph19 resistance gene. Because of the similarity, the new pathotype is not expected to change the rust susceptibility of barley cultivars in eastern Australia if it migrates eastwards.
The new pathotype is, however, expected to reduce resistance to leaf rust in several varieties known to carry the Rph3 resistance gene in WA.
Tests are underway to assess its impact on Bass, Fairview, Finniss, Fitzroy, Grange, Henley, Oxford, Wimmera and Yarra barley varieties, as well as advanced breeding lines carrying the Rph3 gene.
The leaf rust susceptibility of three of these varieties – Grange, Henley and Oxford – are not expected to change markedly due to the additional presence of the adult plant resistance gene Rph20.
But the other six varieties were all rated as susceptible in field tests that exposed them to the closely related Rph3-virulent pathotype 5457 P+ from 2009 to 2010 at PBI.
Wheat leaf rust
Growers, agronomists and the Department of Agriculture and Food, WA (DAFWA) reported unusually high levels of leaf rust in WA on wheat varieties such as Wyalkatchem and Cobra.
Leaf rust samples sent to PBI from five widely separated locations – Borden, Esperance, Gibson, Northampton and Southern Cross – were found to comprise the pathotype 76-1,3,5,7,9,10,12 +Lr37. This pathotype was first detected in eastern Australia in 2011 and had not previously been reported in WA. It is the first instance of virulence for the resistance genes Lr13, Lr17a, Lr17b, and Lr26 in the western grain-growing region.
Pathotype 76-1,3,5,7,9,10,12 +Lr37 is thought to have mutated in eastern Australia from a ‘parental’ pathotype called 76-3,5,7,9,10 +Lr37 – an exotic incursion first detected in Victoria in July 2006.
This parent and two mutant pathotypes have become common across eastern Australia in the past seven years, and recently, a third pathotype mutation with virulence for the Lr24 resistance gene was detected in northern NSW (see Ground Cover issue 107).
The movement of cereal rust inoculum from WA to eastern Australia has been documented many times in the past 90 years, and this is believed to be facilitated by prevailing winds.
In contrast, movement of rust inoculum from east to west is much less frequent. While six examples of west-to-east movement of cereal rusts have been documented since 1990, the latest detection is only the second example of east-to-west movement during that time.
Just how this rust pathotype moved to WA from eastern Australia is not known. Rust spores remain viable for up to two weeks under ambient conditions, and they adhere to clothing.
Normal laundering kills rust spores, and all people who travel are urged to ensure clothing is thoroughly cleaned before entering a new agricultural region.
Subject to further testing, pathotype 76-1,3,5,7,9,10,12 +Lr37 is expected to result in a more susceptible leaf rust resistance rating for several wheat cultivars, such as the widely grown Wyalkatchem variety.
The resistance rating for the Emu Rock wheat variety is likely to be revised from moderately susceptible (MS) to susceptible, and Wyalkatchem and Corack wheat varieties may be downgraded to MS.
Some varieties, such as King Rock, Fortune and Zippy, have additional leaf rust resistance that is expected to remain effective, so their resistance rating is only likely to be downgraded from moderately resistant (MR) to MS.
Cobra, Mace and Carnamah varieties have shown varying levels of response to the new pathotype, so further tests are underway to more accurately establish their responses.
Professor Robert Park,
02 9351 8806,
Reducing the stress of grain marketing
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