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A disease that has devastated eucalypt plantations in Brazil has reached Australia, where strains of myrtle rust could threaten gums, tea-trees, bottlebrushes, paperbarks and more, at great environmental and economic cost. By Tim Low.

Tree disease threatens Australian forests

Scrub turpentine infected with myrtle rust.
Credit: Tim Low

The angle-stemmed myrtle had been looking like a species saved just in time from extinction. Several hundred trees had been bred from a mere 75 survivors lingering in tattered scraps of rainforest around Logan City, just south of Brisbane. Then in 2010 a feared disease slipped past Australia’s quarantine. The new leaves that form on myrtles today are apt to sprout yellow pustules, turn black and fall off. Myrtle numbers are falling.

The disease – myrtle rust – has shown what it can do in South America, its native home. Brazil has the world’s largest plantations of Australian eucalypts – more than six million hectares tended by four million workers – and problems struck in the 1970s when seedlings began dying en masse. The survivors grew into trees too deformed to harvest. Losses of up to 40 per cent in wood production were reported. The industry survived the crisis by investing heavily in breeding eucalypt varieties that resist the disease. Some of these varieties are now at risk from a newly discovered strain of myrtle rust.

Australia could lose 14.8 per cent of its eucalypt timber output from this disease. That is a worst-case scenario produced in 2006 for the Primary Industries Ministerial Council, when the rust turned up in Hawaii and began killing trees there.  

Australia, the land of myrtles, has much to lose. Eucalypts are members of the myrtle family (Myrtaceae), Australia’s largest plant family, along with tea-trees, bottlebrushes, paperbarks, lilly pillies and others – some 2250 species altogether. Myrtle rust has been reported infecting more than 240 species, with some rainforest trees dying so quickly that scientists have voiced fears they may “become extinct in the wild within the next decade”.

The rust spread dramatically inside Australia after it was found in a large nursery near Gosford, infecting hundreds of plants. How it reached Australia is not known, though the nursery trade helped it spread rapidly along the eastern seaboard, reaching Tasmania and the northern tip of Queensland by 2015. Its spores also blow on the wind and travel on honeybees. 

Because the strain of myrtle rust that has reached Australia is not one that blights eucalypts, one view is that the risk to Australia has been overstated. But John McDonald, biosecurity manager of the Nursery and Garden Industry of Australia, rejects any talking down of the disease as “just absolute head-in-the-sand stuff”. He notes the wide range of plants infected – including many that are cultivated, a problem requiring changes in how nurseries operate – and the prospect of the Brazilian eucalyptus strain arriving. “That’s going to be an absolute nightmare across Australia, across so many levels.”  

Angus Carnegie, a New South Wales pathologist involved in the initial response to myrtle rust, believes that with strict quarantine this strain may be kept out, but conceded that all that is needed are tiny spores arriving on the clothes of a traveller. “You only really need one spore for the disease to germinate,” he said. “If it lands on a plant, you get a lesion, a pustule, then you get thousands.”

It won’t kill mature eucalypts, but by killing many seedlings and slowing growth it is expected to have profound impacts on forests, harming koalas and other wildlife, as well as hitting the forestry sector hard. The breeding programs that saved Brazil’s plantations aren’t an option inside national parks.

As it is, the lemon myrtle industry, centred in northern NSW, has taken a hit from the strain that has arrived. “It’s been an absolute disaster for us,” said Gary Mazzorana, Australia’s largest exporter of the leaves. He complained of a 50 per cent loss of output, and costs that have gone through the roof. The fungicides now in use have driven up costs and derailed plans to achieve organic status – the herbal teas produced from the native lemon myrtle tree are worth more in the United States and Europe if they can be marketed as chemical free.

The rainforest trees in trouble include native guava, which has suffered a 57 per cent death rate across its range, according to research undertaken by state government scientists. Botanist Bill MacDonald showed me dead trees close to O’Reilly’s guesthouse, the tourist resort set inside rainforest west of the Gold Coast. Surviving native guavas are producing so few seeds that this once common tree has been nominated for listing as critically endangered, along with scrub turpentine, another dwindling tree.

For the many wild flora facing extinction, plant banks offer the best insurance policy. Cathy Offord, the manager of Germplasm Conservation and Horticultural Research at PlantBank, part of the Australian Botanic Garden Mount Annan, in Sydney’s south-west, said most of the plants under threat are difficult to store. Rainforest seeds die when they are dried for storage. They have in any event become harder to source since the spores began drifting through forests, infecting flowers and ruining any fruits that follow. Offord said rare plants in the myrtle family have become a priority for the bank, but the best method of long-term protection – cryogenic storage of plant tissues – has to be individually tailored for each species following testing that can take years. She mentioned a PhD student spending three years devising storage methods for five species of plant.

In Logan City the endangered angle-stemmed myrtle has been singled out for attention. On a stream bank in a park, PhD student Tamara Taylor showed me a group of the trees she is monitoring. The foliage on some was meagre and mouldy because the rust has stopped them bearing new leaves since 2010. On other trees, scattered fresh leaves suggest partial resistance to the disease. Taylor is not comforted by this because, she said, a population bred from a subset of trees will be too narrow, genetically, to succeed in the wild.

Australia is highly vulnerable to foreign tree diseases because its eucalypts and wattles are grown so widely. Eucalypts were once viewed abroad as pest-free trees, but many diseases have emerged in recent years in plantations in Asia, Africa and South America. Lush groves of saplings suit pathogens spreading from nearby forests. In a 2011 survey of eucalypts planted in China, eight of the 30 fungal pathogens detected were new to science. Some of the surprises include a rust targeting eucalypts in Africa, and diseases killing wattles in Africa and Asia. Many insects have turned to eucalypts as well, including Argentinian moths, intercepted by Australian quarantine officials at Port Kembla in 2011.

The disease problems point to global forestry operating on questionable assumptions. When trees are grown somewhere new they thrive at first, then accumulate pests that hinder their performance, and end up posing risks to the countries that furnished the trees. A recent article in Science warned that unless management improves, pests “threaten the long-term sustainability of forests and forestry worldwide”. All over the world, yields are dropping and costs rising as trees of many kinds succumb and sometimes die.

Angus Carnegie nominates Ceratocystis wilt as one to fear. Known since 1890, it was not found infecting eucalypts until 1997. It has now surpassed myrtle rust as the worst eucalypt disease in Brazil. In Hawaii it has killed hundreds of thousands of native trees since it appeared there in 2010.

Of the other pathogens appearing around the world, many are doing little harm, but they shouldn’t be ignored. “Something that is trivial on eucalypts in Brazil or South Africa may be a massive issue here,” Carnegie warned.

The eucalyptus strain of myrtle rust remains a serious concern for Australia. Another possibility is that the strain already in Australia could widen its pool of victims. “There could be mutations to make it more virulent,” Carnegie said. “The jury is still out on whether that could happen.”

This strain has yet to reach Western Australia, where the many native plants at risk include Geraldton wax, Australia’s No. 1 floral export, providing 40 per cent of Australia’s wildflower production. Wax plants grown in eastern Australia suffer badly from the disease, but the farms that export the flowers are in the west, and so are the wild populations. Botanist Peri Tobias tested every wild strand of Geraldton wax and could find no hint of disease resistance anywhere. This dashes hopes that varieties can be bred to withstand myrtle rust. Tobias fears for the survival of the species in the wild. “Maybe there is a resistant plant out there and I didn’t get to it,” she said, hopefully.

Other small projects like hers are under way, but there is no national policy to guide action on the disease, and no agency leading a response. What we have is an informal working group of concerned scientists, whose voluntary chair, botanist Bob Makinson, made a comparison with feral cats. Cat eradication attracts substantial funding guided by a national framework. “Myrtle rust has the potential to cause 50 or more extinctions – more than cats – and to have far greater economic impacts,” Makinson said. “A similar long-term investment is needed for myrtle rust, starting now.”

This article was first published in the print edition of The Saturday Paper on Nov 25, 2017 as "Forestry rust belt". Subscribe here.

Tim Low
is a biologist, prize-winning author and freelance writer.