The team of scientists on the frontline of disease prevention

The team of scientists on the frontline of disease prevention

13/01/2021

On the edge of south-western Sydney, a team of dedicated scientists are working hard to protect the Australian citrus industry from disease.

The New South Wales Department of Primary Industries (NSW DPI) citrus pathology team is based at the Elizabeth Macarthur Agricultural Institute (EMAI), a 1600 ha property within the Camden Park Estate, one of the oldest farms in Australia.

EMAI was established in 1990 and is now a world-renowned biosecurity facility for plant and animal health. The citrus team has access to world class laboratories and a biosecure nursery facility. The labs include a quarantine facility for working on new diseases during an outbreak.

The scientists working with the NSW DPI Citrus Pathology Program include molecular biologists, Grant Chambers and Anna Englezou, who undertake citrus diagnostic research, and Wendy Forbes and Adrian Dando who carry out the commercial activities for the Auscitrus propagation scheme.

Grant and Anna are highly experienced after working with graft-transmissible citrus pathogens for 20 and 14 years respectively; before this Anna worked in medical research for more than ten years.

Wendy and Adrian joined the team three years ago but prior to this, Wendy spent two decades building a diverse technical capability in agricultural research and diagnostic laboratories and Adrian brings a wealth of nursery experience to the role.

Vipawee Iamsa-at (Noi) and George Haizer provide valuable nursery assistance. The team is led by Nerida Donovan, who has been working as a citrus pathologist for 22 years.

“The program aims to enhance the ability of the Australian citrus industry to combat disease threats by having the capability to test for all described citrus diseases, understanding the threats within our borders and making sure industry has access to diseasefree propagation material to give our orchards the best start,” Nerida said.

There is a strong focus on graft-transmissible diseases because they can kill trees and there is no cure.

“These diseases are spread in infected planting material, on cutting tools and a few are also spread by insects.

“They may be present in plants without symptoms, but the infected plants pose a risk to healthy plants.

“Therefore, it is essential that we can detect these diseases soon after they enter Australia or in propagation material before its use.”

Detecting graft-transmissible diseases can be difficult because field symptoms may be confused with other disorders, and the pathogen may be present below detectable levels or unevenly distributed within a tree and missed during sampling.

Nerida said it is important that diagnostic tests are specific to the target organism, sensitive (i.e. will detect even at low levels), and efficient in terms of time and cost.

“The team is working on a Hort Innovation project focussed on making sure we know how to test for any graft-transmissible diseases that the industry may face,” Nerida said.

“This includes exotic diseases that are not found here yet, such as the devastating bacterial disease huanglongbing (HLB), currently wreaking havoc in citrus orchards around the globe, plus other endemic diseases that do exist in Australian orchards and backyards.”

Past work on international aid projects in Bhutan and Lao PDR, has provided the team with infected plant material, essential for ensuring they know how to test for exotic diseases.

“When a new disease penetrates our border, early detection will be crucial to increasing the chance of a successful eradication campaign.

“The earlier we find a new disease, the greater our chance of eliminating it before it becomes a production issue.”

Detection is also critical for preventing and managing the diseases that are already here.

It is vital that disease-free and true-to type propagation material is available to industry to prevent these diseases from entering citrus nurseries and orchards.

Use of healthy planting material will avoid potential yield loss and the costly exercise of replanting infected blocks.

The Auscitrus budwood and rootstock seed source trees are tested at EMAI to ensure the supply of healthy material to industry. New diagnostic methods that were developed or validated by Grant and Anna are then used to test the Auscitrus material.

Adrian and Wendy also undertake the specialised skill of shoot tip grafting, the only way to eliminate a grafttransmissible pathogen from a new variety prior to commercialisation.

Pathogen-free trees of each variety are then included in the National Citrus Repository; an industry asset that contains disease-free and inoculated trees of more than 260 citrus varieties.

The collection is duplicated at two sites, EMAI and the Auscitrus property at Dareton. The EMAI nursery also houses a collection of plants infected with viruses and viroids, which is the only way to store these pathogens in live form, and other plants used for diagnostics or research trials; the infected plants are maintained in separate structures away from the healthy material.

Through industry funded projects, the citrus pathology team has halved the time needed to run diagnostic tests and increased capability four-fold.

They have also developed new diagnostic methods to detect Australian strains of the pathogens because tests developed overseas often miss our variants. Efficiency of testing has also been improved by ‘multiplexing’, where they find a way to test for more than one pathogen at the same time.

Building a better understanding of the diversity, distribution and risk posed by local diseases is on ongoing component of the work that helps to determine the potential economic impact of these diseases. The team has been working on more than 30 exotic and endemic graft-transmissible organisms posing a threat to citrus.

“Collaboration and mentoring have been of vital importance to building the Citrus Pathology Program,” Nerida said.

“We have been fortunate to work with and receive guidance from local and international scientists. “Of note are Professor Paul Holford from Western Sydney University and Professor Lester Burgess from the University of Sydney, who have continued to advise and provide encouragement, even in their retirement.

“We hope that we can pay it forward to younger scientists through our current work.”

Working closely with other scientists and with industry, ensures more ‘bang for the industry dollar’ by value-adding to activities. The team has developed a strong relationship over many years with researchers and surveillance officers around the country and is strengthening national capacity with citrus diagnostic hubs at the University of Queensland and the Western Australian Department of Primary Industries and Regional Development. Interaction with industry organisations like Auscitrus and Citrus Australia also ensures the Citrus Pathology Program is providing relevant and important support for industry.

“The support given to us by sectors of the industry over the years has been fundamental, such as that provided by Auscitrus Manager Tim Herrmann, and the Auscitrus Executive Committee, and by Nathan Hancock and his team at Citrus Australia,” Nerida said.

Survey samples collected from northern Australia and off-shore by the federal government Northern Australia Quarantine Strategy, and samples collected nationally by the industry-led Citrus Watch program are tested at EMAI for the exotic diseases HLB and citrus variegated chlorosis (CVC).

“This work supports our early warning system and negative results provide evidence of absence data for these major biosecurity threats, useful for trade negotiations,” Nerida said.

“The samples are also tested for endemic diseases which checks the robustness of our test methods and increases our understanding of how widespread diseases are in Australia.”

The team plays an important role in emergency response; from diagnostics to confirm the identity of the pathogen, to genome sequencing to trace the origin of the infection, to pathogenicity testing to determine the range of hosts the new pathogen can infect, thereby informing decisions surrounding the potential economic impact and the feasibility of eradication.

The team also lends a hand with citrus samples submitted to the NSW DPI Plant Health Diagnostic Service, also based at EMAI.

Nerida works closely with the other members of NSW DPI citrus team, providing pathology input to various projects and extension activities, with the lab team providing technical support when needed.

This includes the Hort Innovation funded National Rootstock Evaluation program led by Dr Tahir Khurshid, and field trials examining interactions between commercial dwarfing viroids and new viroids as part of a Hort Innovation project led by Dr Dave Monks.

Nerida is also a technical advisor on a project starting on HLB in Indonesia, Australia and China. The project will be led by entomologist Dr Jianhua Mo and funded by the Australian Centre for International Agricultural Research and Hort Innovation.

Nerida also works closely with Citrus Industry Development Officers Steven Falivene and Andrew Creek to extend results and improve awareness of biosecurity issues.

The International Organisation of Citrus Virologists (IOCV), the organisation representing citrus pathologists worldwide, has enabled the team to link and work with experts from around the world.

This network has led to collaborations with the University of California, Riverside on citrus viroids and with Citrus Research International in South Africa on tristeza virus.

Nerida was voted by her international peers to be Chair-Elect of the society and will take over as Chair at the next conference to be held in Mildura.

The conference will largely be organised by the EMAI citrus pathology team and will provide Australian scientists and industry members access to the global leaders working on citrus diseases.

“It has taken many years to build a strong Citrus Pathology team. I feel privileged to work with skilled scientists who all understand the importance of the work we do for industry,” Nerida said.

“Diseases have been known to destroy industries. It is of crucial importance that we have an experienced and multi-skilled biosecurity team to rely upon; one that is connected globally and equipped with the knowledge and tools to protect citrus production in Australia.”