Uredo rangelii


Uredo rangelii, commonly known as myrtle rust, is a fungal plant pathogen native to South America that affects plants in the family Myrtaceae. It is a member of the fungal complex called the guava rust group. The spores have a distinctive yellow to orange colour, occasionally encircled by a purple ring. They are found on lesions on new growth including shoots, leaves, buds and fruits. Leaves become twisted and may die. Infections in highly susceptible species may result in the death of the host plant.
As of late 2013, it is infecting around 179 species in New South Wales and Queensland, from 41 genera in Australia.

Taxonomy

The rust was first described scientifically in 2006 by mycologists J.A. Simpson, K. Thomas, and Cheryl Grgurinovic.

Development and symptoms

Myrtle rust is typically characterised by the appearance of urediniospores on the underside of the leaf, though urediniospores may also be found on the top of the leaf or on young stems. Initially, the disease appears as small purple or red brown flecks with a faint chlorotic halo on the leaf surface, which coalesce to form bright yellow pustules. As the rust develops, these pustules often fade to a grey brown colour. A high degree of pustule coalescence can result in distortion of the leaf. Myrtle rust also makes plants more susceptible to secondary infections, which may occur within days of the initial appearance of the pustules.
Favourable conditions that increase the infection rate include: new tissue; high humidity; free water on plant surface for more than 6 hours; moderate temperatures, around 15–25 °C. Low light conditions after spore contact can increase germination.
The main ways in which myrtle rust can be spread are by: the movement of infected plant material, the movement of contaminated equipment, wind, water and gravity, animals, humans and/or vehicles.
Myrtle rust may remain on a single host plant to complete its life cycle, which can be as short as 10–14 days.

As an invasive species

Myrtle rust was first recorded in Australia in mid-2010 and currently poses a major threat to the continent's ecosystem given that almost 80 per cent of Australian native trees are Mytraceae, most indigenous species rely on healthy trees for their survival. Additionally it poses a major threat to Australia's primary industry sector. Its current range includes much of the eastern coastal fringe of the Australian mainland.
Initial detection was in April 2010 in Gosford in the Central Coast region of New South Wales. It was initially quarantined and eradication thought viable. The New South Wales government spent $5 million attempting to eradicate the disease. However, efforts to contain it failed and it spread rapidly north and south along the eastern coast. In response to the increasing threat, a Myrtle Rust National Management Group was formed on 2 July 2010 with the aim of eradication however due to the extent of its spread at that point of time, the group conceded that it had become impossible to eradicate.
By December 2010, it had significantly spread north along the coast and recorded in South East Queensland with isolated cases in Far North Queensland cities of Cairns and Townsville. In January 2012, an isolated myrtle rust outbreak was reported in Victoria beginning in Melbourne's southern and eastern suburbs. Initial attempts to contain it were unsuccessful and by April, 2012 it had spread across much of the state via regional cities.
By late 2015 myrtle rust was widespread in Queensland, NSW and Victoria. It has reached Tasmania, where it was detected in garden plants in the north-east in February 2015, and the Northern Territory, where it was detected on Melville Island in May 2015. The Tasmanian government is seeking to contain and eradicate myrtle rust from the state while the Northern Territory government has determined it is not possible to contain or eradicate the pathogen.
In April 2017, New Zealand's Ministry for Primary Industries reported that myrtle rust had been detected on Raoul Island, off the New Zealand mainland; the following month, myrtle rust was detected within New Zealand itself, in Kerikeri.

Host genera

Species within the following plant genera have been recorded with the infection:
Since first being detected in 2010, myrtle rust has spread rapidly with entire plant species now under threat. In Australia, the family Myrtaceae - which includes eucalypts, melaleuca and lilly pilly - is diverse, widespread and important to many native ecosystems.
The impact of myrtle rust has now been seen in a range of forest ecosystems including coastal heath, coastal and river wetlands, sand island ecosystems and subtropical and tropical rainforests. A number of plant species are now at risk of becoming extinct with about 40 plant species considered highly susceptible, such as the endangered Rhodamnia angustifolia.
Native animals are also likely to suffer significant impacts. Myrtle rust grows in shoots, fruits and flowers, destroying the food relied on by some species of flying foxes, lorikeets and honey eaters. There is the strong possibility that some of these species will become regionally extinct, and their loss could have serious flow-on effects.

Management

The original plan to eradicate myrtle rust from Australia was declared to be unfeasible by the Myrtle Rust National Management Group in December 2010. The Myrtle Rust Response Plan was cancelled and focus was placed on minimising the spread and the impacts on myrtle rust. The Australian Government, through the Department of Agriculture Fisheries and Forestry, established the Myrtle Rust Coordination Group to manage the investment of $1.5 million of research funding.
In 2016, The National Environmental Science Programme hosted a national workshop on myrtle rust to discuss research findings and future management options. Participants included the Cooperative Research Centre for Plant Biosecurity, state and federal agencies, and botanical and plant conservation experts. Discussions centred around the impact on native species in Australia. A key outcome of the workshop included agreement that there is a need for a nationally coordinated approach through a long-term National Action Plan which aims to ensure that no species or ecosystems are lost to its impact.
Practical measures to minimise the risk of increasing the distribution of myrtle rust include: not moving plant matter from one site to another; minimising pathogen spread by arriving and leaving each site clean of the pathogen, and avoiding areas that may contain myrtle rust-infected plant matter.