By Graeme O"Neill
Late in August, 2002, an alert agronomist inspecting a wheat crop north of Newdegate, Western Australia, noticed the plants’ leaves were streaked with bright yellow pustules. The infection was unfamiliar, but unmistakable.
A day later, at the WA Department of Agriculture laboratories in Perth, plant pathologist Dominie Wright confirmed the agronomist’s suspicions – stripe rust, Puccinia striiformis, had finally arrived in WA. But was it a new strain or a vagrant from eastern Australia, where stripe rust is endemic? If so, how had it reached WA?
Within 10 days of receiving specimens of the WA rust, researchers with the Australian Cereal Rust Control Program (ACRCP) at the University of Sydney’s Plant Breeding Institute at Cobbitty, near Camden in NSW, had tentatively identified the intruder as alien and quite unrelated to strains in eastern Australia. Since then, the appearance of this new strain has become another intriguing chapter in the ongoing saga of struggle between agriculturalists and this most persistent and damaging of crop diseases.
The University of Sydney has been keeping watch on emerging cereal pathogens since the end of the First World War. In 1925, its vigilance confirmed the first appearance of an exotic cereal rust isolate. It was a new strain of the stem rust pathogen (P. graminis) and was also found in specimens from WA. But while rusts generally have been a constant headache ever since, the first stripe rust outbreak did not occur until 1979, on a farm in central Victoria.
ACRCP cereal pathologist Dr Colin Wellings identified the strain as originating from the south of France, probably imported as spores on the clothing of a recent visitor to the region.
The 1979 invader diffused rapidly through the eastern states, then blew across the Tasman Sea to infect New Zealand wheat crops a year later. But WA was spared, protected by the arid expanse of the Nullarbor, and the dominant west-to-east movement of weather systems.
ACRCP director Professor Robert Park says the WA stripe rust detected in 2003 produced a pattern of infection (in greenhouse-grown indicator lines) markedly different to eastern strains descended from the 1979 invader, confirming it was a new introduction and not a mutant.
The initial suspicion was that the new stripe rust was a blow-in from Africa – this was how stem rust (P. graminis) most likely reached WA in 1925, again in 1945 and twice more in 1969. High-altitude jet streams, blowing at up to 300kmh, can transport fine particles like bacteria and fungal spores thousands of kilometres in 24 hours.
But Professor Park believes a jet passenger, not a jet stream, was the likely mode of the new rust’s arrival in Australia. Jet travel has brought the wheat farms of Europe, Asia and North America – and their resident pathogens – within a day’s flight of Australia.
Dr Wellings demonstrated with his own unintended laundry experiment that the once-held belief that rust spores could only survive off a host plant for a few days, was not true: “I once came out of a rust infected plot with my trousers streaked with spores. I left them, unwashed, on a shelf at home, at room temperature, for a week. The spores were still able to infect susceptible plants”.
Dr Wellings’ subsequent letter to an international plant pathology journal led the Australian Quarantine Inspection Service to begin warning travellers to decontaminate clothing and shoes if they had visited cereal growing regions overseas.
Dr Robert Loughman, principal plant pathologist with the WA Department of Agriculture, says it is possible the new stripe rust strain reached WA late in the 2001 season, but went unnoticed because of drought. Stripe rust spores require 8- 12°C temperatures, and high humidity to germinate and infect wheat plants.
However, within a week of its discovery, it was found across a 150- kilometre front between Newdegate and Gnowangerup. By season’s end, the outbreak had reached Esperance in the south-east, caused moderate, patchy damage in the central wheat belt, and extended as far north as Geraldton.
In season 2003, WA outbreaks were confined to areas east of Merredin, around Gnowangerup and Esperance, and the disease looked to have been contained – until it was identified in crops on South Australia’s York Peninsula. The new stripe rust had vaulted the Nullarbor, probably as wind-blown spores. By November, patchy outbreaks had been detected throughout the eastern Australian wheat belt.
Photo: Raising the barrier: Professor Robert Park with on the left, plants infected with stripe rust, and on the right, genetically similar wheat plants that carry a stripe rust resistance gene.
Professor Park says researchers had anticipated the rust would reach the east sooner rather than later, but were surprised by the higher-than expected responses of some eastern Australian wheat cultivars. While resistant to eastern pathotypes of stripe rust descended from the 1979 invader, several key varieties like Krichauff, Babbler, Annuello, Wyalkatchem, Janz and Chara proved susceptible to the WA stripe rust.
The WA strain is able to overcome a resistance gene that these cultivars have in common. Fortunately, they have more than one gene for resistance to stripe rust, which is why they have not been rendered completely susceptible.
South Australian Research and Development Institute (SARDI) cereal pathologist Dr Hugh Wallwork says, however, that their resistance has been ratcheted down a notch.
For example, Krichauff, prominent in SA breeding programs, has moved from a rating of ‘intermediate’ to ‘susceptible’. Dr Wallwork says its progeny will need better resistance incorporated before they can be considered for release.
Dr Wallwork says the good news for SA growers is that Frame and Yitpi, the state’s most widely grown varieties, remain unchanged in their partial resistance to the new rust. Frame is also widely grown in the Victorian Mallee. Kukri, the only variety in SA with good resistance to stem, leaf and stripe rust, also retains its previous resistance.
The news is not so good for growers still keen on the popular, high-yielding H45 variety, which is very susceptible to the new rust.
Professor Park says the arrival of the new rust will force breeders to discard H45, along with advanced breeding lines sired by it – including some nearing commercial release. He says it could be five to 10 years before commercial and publicly funded breeding programs recover fully.
Professor Park is worried that some growers may persist with H45 because of its yield potential and the availability of cheap, out-of-patent fungicides from countries like China. But he says volunteer plants from susceptible varieties can provide a ‘green bridge’ that allows the fungus to persist between crops.
He says that growing susceptible varieties will only increase the size of the pathogen’s reservoir, and amplify the risk that new mutants will break through resistant wheat varieties, rendering valuable fungicides – even whole classes of fungicides – useless.
For more information:
Professor Robert Park, 02 9351 8806, firstname.lastname@example.org
Dr Hugh Wallwork, 08 8303 9382, email@example.com
GRDC Program: 3
Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994.