It is recommended WA growers and advisers closely monitor advanced cereal volunteers and emerging cereal crops for the presence of leaf rusts (Puccinia triticina and Puccinia hordei) this season – along with other rust diseases, viruses and pests.
The advice comes after autumn rain events in some areas of the wheatbelt have generated volunteer regrowth and weeds. There is potential risk that this 'green bridge' may harbour rust and other disease-inoculum.
To mid-May there have been no reports of wheat rusts received, but with Mace plantings expected to dominate the State's cropped area this year - and new strains of cereal leaf rust identified in 2013 - monitoring of wheat crops will be particularly important.
There was limited impact of the new leaf rust strains affecting last year's wheat and barley crops due to dry conditions and lack of a green bridge going into 2014.
But despite low inoculum levels and a dry summer period, autumn rain events this year have been more favourable in some regions and provided potential opportunities for rust survival into 2015.
Infection at early crop growth stages may require fungicide intervention to stem potential yield losses.
Mace and the new leaf rust strain
Department of Agriculture and Food (DAFWA) trials (with artificial inoculum) in 2014 found Mace was more susceptible to WA’s newest wheat leaf rust strain (called 76-1,3,5,7,9,10,12+Lr37) than the previous dominant strain of leaf rust in this State (called 104-1,(2),3,(6),(7),11+Lr37).
The new strain, from eastern Australia, was first identified in WA in 2013 by Professor Robert Park, head of the GRDC-funded Australian Cereal Rust Control Program (ACRCP), from samples collected right across WA’s grainbelt in conjunction with DAFWA.
DAFWA plant pathologist Geoff Thomas says Mace has been characterised as moderately susceptible (MS) to this strain.
He says in the department’s 2014 glasshouse and field trials, funded by GRDC, Mace exhibited susceptible (S) response under glasshouse conditions and MS adult response in field conditions to the new strain.
In a field site where high disease pressure was evident from late tillering stage, leaf rust was well established in Mace and yield loss was as much as 30 per cent.
However, infection severity was significantly lower - and less than 10 per cent - with later disease onset at flag leaf emergence.
Other wheat varieties and the new leaf rust strain
Geoff says susceptible (S) varieties, such as Stiletto, will continue to exhibit high levels of infection and significant yield losses to the new wheat leaf rust strain.
In the 2014 DAFWA trials, yield losses in this variety were 70 per cent when disease pressure was high from the late tillering stage and infection severity greater than 50 per cent when disease onset was from flag leaf emergence.
Disease screening nursery trials carried out by DAFWA plant pathologist Dr Manisha Shankar confirmed that several other wheat varieties maintained good resistance to the new strain 76-1,3,5,7,9,10,12+Lr37.
Her results reflected previous research findings that Magenta, remained resistant (R) to this pathotype and Carnamah and Cobra had higher resistance than Mace.
Previous research has also indicated that Sapphire and Bullaring are R to the new leaf rust and King Rock, Fortune and Zippy have other leaf rust resistance genes that are expected to remain effective against the new strain.
Varieties with the greatest potential change in resistance status to the new wheat leaf rust strain carry the resistance genes Lr13 (such as Mace, Wyalkatchem, Corack, Stiletto and Emu Rock) and Lr17a (Fortune).
Full wheat disease resistance ratings are outlined in the 2015 Wheat Variety Guide for Western Australia.
Barley leaf rust
A strain that can overcome leaf rust resistance gene Rph3 was also detected for the first time in WA in 2013.
More than half of the barley leaf rust samples from WA tested by the ACRCP in 2014 were virulent to Rph3.
This new strain will primarily affect varieties such as Bass and Compass which have resistance based primarily on Rph3. These varieties should now be considered susceptible to leaf rust.
The leaf rust responses of Grange, Henley and Oxford, which carry Rph3, have not changed markedly due to the presence of adult plant resistance genes, such as Rph20.
Professor Park suggests growers check updated resistance ratings in the Barley variety sowing guide for Western Australia 2015.
Monitoring and reporting in 2015
DAFWA researchers across the WA wheatbelt are closely monitoring volunteer cereals and emerging cereal crops for the presence of leaf rust – as well as other diseases, viruses and pests.
Geoff Thomas advises growers and advisers to monitor volunteers, particularly of S varieties such as Stiletto, and to be prepared if fungicide management of leaf rust is required this season.
Prophylactic fungicide management is not recommended in the absence of any reported rust.
Cereal plants showing rust symptoms should be sent for testing to the ACRCP headquarters at the University of Sydney Plant Breeding Institute (PBI).
These can be mailed in paper envelopes - not plastic wrapping or plastic-lined packages - to the Australia Cereal Rust Survey Plant Breeding Institute, Private Bag 4011, Narellan, NSW, 2567.
Geoff says the potential impact of new cereal rust pathotypes in WA also reinforces the importance of biosecurity awareness among growers and advisers.
He says it is vital to avoid the chances of incursions through human transfer of inoculum on clothes and equipment from other States.
Breeding for rust resistance
The ACRCP is a major investment priority for GRDC as part of its efforts to monitor, assess and develop rust management strategies for Australian growers.
Since the early 1970s, ACRCP researchers have identified new sources of rust resistance and have assisted breeders to incorporate these into new cereal varieties.
This work has included extensive testing of cereal lines with new rust strains to ensure industry-wide preparedness to the emergence of new rust threats.
This ensures that plant breeders are provided with new sources of leaf rust resistance to breed into new Australian wheat varieties.
The Rust Bust campaign, an initiative of the ACRCP Consultative Committee, encourages Australian growers to phase-out varieties that are susceptible and very susceptible to rust and to more effectively manage the disease.
Rust Bust is active on twitter and will provide reports of any rust outbreaks during the 2015 season. To follow, go to: @the_rustbust
Geoff Thomas, DAFWA
08 9368 3262
Robert Park, PBI
02 9351 8806
GRDC Project Code
DAW00229, DAW00245, US00067