News: 300km/h on the wind superhighway

A Spanish research team has confirmed what Australia's plant pathologists have long suspected: the southern hemisphere is traversed by 'wind highways' that transport spores and plant seeds between land masses thousands of kilometres apart.

By combining botanical and satellite data, the Spanish research group, led by Dr Jesus Munoz of the Madrid Royal Botanic Gardens, showed that strong similarities between the lower plant floras of sub-Antarctic land masses are best explained by strong winds wafting their spores thousands, even tens of thousands of kilometres around the globe.

But the phenomenon is almost certainly not limited to the sub-Antarctic region. At least two major epidemics of wheat stem rust in Australia in the 20th century are believed to be blow-ins from Africa.

The westerly gales of the roaring forties and raging fifties girdle the southern hemisphere, carry the spores of ferns, mosses, lichens, and liverworts between land masses many thousands of kilometres apart, according to Munoz and his colleagues.

In their paper in the May 21 issue of Science, they describe how they used data from the SeaWinds scatterometer -- a device that measures windspeed and direction from wave patterns -- aboard NASA's QuikSCAT satellite, to map windfields in the southern hemisphere.

From the windfields, they were able to estimate the frequency of 'wind highway' contact between sub-Antarctic land masses, and correlate them with similarities in their floras.

They confirmed their hypothesis that land masses distantly linked by 'wind highways' should have stronger floristic similarities than land masses much closer together, but lacking direct wind connections.

Macquarie Island, in the Southern Ocean south-west of New Zealand, lies 2465km south of Lord Howe Island, off the NSW coast. They share 18 per cent of their moss species and 11 per cent of their lichens, but share no ferns or liverworts.

In contrast, Macquarie Island shares 24 per cent of its mosses, 11 per cent of its lichens, 23 per cent of its ferns and 13 per cent of its liverworts with remote Juan Fernandez Island, 8780km across the Pacific, west of Chile.

Macquarie Island's floristic similarities with Tierra del Fuego, 7550km away on the southern tip of South America, are even higher (mosses 32 per cent, lichens 30 per cent, ferns 23 per cent and liverworts 14 per cent).

The windfield maps produced by Munoz and his colleagues show strong wind highways extending from South Africa, Zimbabwe and Kenya, across the Indian Ocean to the coast of Western Australia, between the Kimberley and Cape Leeuwin.

Out of Africa

According to Australian Cereal Rust Program director Robert Park, the two most significant outbreaks of wheat stem rust in 1925 and 1954 probably originated in South Africa.

In 1925 a new strain of wheat stem rust appeared, literally out of the blue, in Western Australia's wheatfields. By testing it against a standard array of 12 reference varieties of wheat with differing resistances to stem rust, ACRP pathologists showed it was unlike any of the six strains of stem rust detected by the ACRP's national rust survey, initiated in 1919. The contrasting infection pattern showed it was clearly not a mutant, but a strain previously unknown in Australia.

By 1929 the new Puccinia graminis ssp tritici (Pgt) 126 pathotype had completely replaced the six extant strains -- strains that Park says may already have been present in Australia, hosted by native grasses distantly related to wheat, when First Fleet farmers planted their first wheat crop at Sydney Cove in 1788.

Pgt 126 and its mutant descendants reigned in Australia's wheatfields until another new and distinctive pathotype, Pgt 21, materialised in 1945. Pgt 21 was usurping Pgt 126 when another novel pathotype, Pgt 34, emerged in northern NSW in 1957. Pgt 34 is now known to have been a Pgt 126/21 hybrid; its dynasty continues today.

Two new stem rust strains arrived simultaneously in Australia in 1969 -- again, presumptive immigrants from Africa, borne by jet streams blowing at 300kmh or more across the Indian Ocean.

The 'out-of-Africa' stem-rust invasions have thus been the major influence on the course of Australian wheat breeding throughout the past century.

From early colonial times, Australian wheat farmers were plagued by both stem and leaf rust. Park says stripe rust (Puccinia striiformis ssp. tritici) was a much later arrival -- it was 191 years from the date of European settlement before it reached Australia.

The first outbreak, detected in Victoria in 1979, was traced to southern France, and probably arrived as spores on the clothing of a recent visitor to the region.

Research has shown that stripe rust spores can remain viable for more than a week on clothing -- before the advent of high-speed air travel, spores would not have survived long enough to the spores to germinate. Stripe rust spores also require low night temperatures followed by dewy, humid conditions to germinate.

Eastward ho

Another major outbreak of stripe rust, unrelated to the original Victorian pathotype, was discovered near Newdegate in Western Australia in August 2002. It spread rapidly across the Nullarbor, and is now widespread in the Australian wheat belt.

The '79 invader diffused rapidly through the eastern states, then blew across the Tasman Sea to infect New Zealand wheat crops a year later. Western Australia was spared, protected by the arid expanse of the Nullarbor, and the dominant west-to-east movement of weather systems across the continent.

Park says the new stripe rust strain from WA has yet to vault the Tasman, but bushfire ash and red dust from Australian dust storms sometimes reach New Zealand, and it is probably only a matter of time before stripe rust spores make the trip. Wheat is not a major crop in New Zealand, except on the Canterbury Plain around Christchurch, but the region's cool, moist conditions are ideal for the fungus.

Park says Australian cereal pathologists and breeders maintain close contacts with their colleagues in South Africa and other wheat-growing nations.

"We have a good idea of what virulences are around, and if we're relying on a particular resistance gene that has succumbed to a new strain, we try to avoid it," he says.

Park says a good example is the LR24 resistance gene for leaf rust, which was widely used in North American wheat lines. Not long after Australian breeders began using it in their breeding lines, it succumbed to 'breakthrough' strains of leaf rust in North America and South Africa. "We thought its use-by date was rapidly approaching, and we were very nervous about it," he says. "Sure enough, we found a new rust strain in South Australia had become virulent for wheat varieties with the LR24 gene, and within six months the new rust reached Queensland."

Costly invaders

Park says there are numerous instances of wheat and barley rusts arriving in Australia by wind, as evidenced by initial outbreaks in Western Australia, followed within months by outbreaks in eastern Australia.

He says the arrival of the new Pgt21 strain of stem rust from South Africa in 1945 was unusual because it was virtually avirulent, yet it replaced virulent strains descended from Pgt126. But in 1973 a new stem rust strain, initially believed to be a Pgt21 mutant, had appeared in South Australia, where it attacked the widely grown Halberd variety.

In 1973 and 1974, the new pathotype caused an estimated $200-300 million in losses in SA, Victoria and southern NSW. Nearly a decade later, in 1982, another presumed P21 mutant virtually wiped out the triticale industry in northern NSW and Queensland in 1982.

Park says some experts now believe the new strains, while resembling Pgt21, were actually new arrivals from the same central African source as the original Pgt21 invader in 1945 -- the Halberd-slayer may have been present but undetected in Western Australia as early as 1969.

"We've been pushing for an Australian Research Council grant to do DNA studies of cereal rust strains in Australia," Park says. "We have a unique resource, because Australia is so isolated, and we have an enormous data set going back 80 years, and we maintain cultures of most of the isolates.

"Our monitoring suggests that when we start looking closer at these things, we might get a few surprises."