Whatever happened to myrtle rust? Mike McRae It was the disease plant biologists in Australia had been expecting with trepidation. In April 2010, a type of rust that had been threatening eucalypt plantations overseas was found dusting a plant leaf in a New South Wales nursery. Environmentalists predicted the rust’s spread would become a ‘mycological firestorm’. But three years on ECOS asks, has it been more of a fungal fizzle? The plant pathogen, often referred to as guava rust, has long had something of an identity crisis. In its native Brazil, the fungus was named after its original host plant. Yet this fungus, Puccinia psidii, doesn’t just attack guava – a tree that belongs to the Myrtaceae family. Melaleucas, eucalypts and other trees from the same family have proved equally suitable host plants overseas, with the pathogen infecting developing foliage, flowers and fruits and stunting growth. As a result, P. psidii has also become known as eucalyptus rust, and recognised as a threat to eucalypt plantations across the globe, not to mention native Australian flora. Apart from its variable common name (guava/eucalypt rust), the rust has had mycologists debating whether a second rust species might be involved. In the early 1980s, a reference specimen representing P. psidii was discovered to contain two different kinds of spores, one ‘prickly’ all over, while the other presented a bald patch among its protuberances. This prompted mycologists to describe the rust taxonomically as a ‘complex’ of biological forms. Three decades later, the form that produced the bald-patch spores was given the name Uredo rangelii. This was the form that was found in Australia in 2010 and referred to as myrtle rust. Today, it still remains unclear whether the complex of forms should be considered as a group of distinct species or merely varieties, in spite of no genetic differences being found between the forms. Until such questions are resolved, the pathogen continues to be technically defined as Puccinia psidii sensu lato (s.l.). How big a threat? Whatever its identity, the rust of many names has been threatening to become a pathogen of many hosts. Recent studies1 indicate that, given the right conditions, P. psidii (s.l.) has the potential to infect all members of the subfamily Myrtoideae within Australia’s native myrtle family. Given myrtles make up ten per cent of Australia’s flora, it’s no wonder the appearance of the fungus in NSW in 2010 was seen as a potential disaster for many of our wooded ecosystems. In fact, within two years of the pathogen’s arrival, scientists had identified 175 plants as potential hosts. Of these, only a third were yet to present infected specimens in the wild. But while the pathogen’s reach across the myrtle family is broad, its impact remains unclear. Dr Louise Morin from CSIRO Ecosystem Sciences has studied the rust’s host-specificity and life cycle. ‘Unfortunately, as far as I know there hasn’t been comprehensive research to assess the impact of the rust on plant populations,’ she says..... Looking for the right clues? Since 2010, the Australian Government has invested about $1.5 million into a ‘Myrtle Rust Transition to Management Program’. While this funding has led to some insight into the pathogen’s potential range, some have questioned whether enough action has been taken address the threat..... See more at: ecosmagazine/paper/EC13137.htm
Posted on: Mon, 01 Jul 2013 04:09:05 +0000
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