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Historical context – the New Zealand biocontrol program for wandering trad

The fungus Kordyana brasiliensis was discovered on wandering trad during surveys in Brazil, as part of exploratory research that underpinned  the biocontrol program for this weed in New Zealand. The fungus causes extensive necrosis on leaves, which progressively leads to leaf senescence and an overall reduction in the foliage of wandering trad. Under optimal conditions the fungus can complete its life cycle within 2-3 weeks.

Testing of the fungus performed in Brazil demonstrated it would not pose a risk to non-target plant species present in New Zealand. Based on these results, K. brasiliensis was approved for release in New Zealand. It was first released in early 2018 and successfully established. The dew that develops on the leaf surface within shady, moist situations where wandering trad is most invasive has likely facilitated its establishment. It is predicted that native vegetation will positively respond to a reduction in the competitiveness of wandering trad as a result of fungal infection. Monitoring of the fungus impacts is on-going.

The fungus complements the three insect biocontrol agents that have also been released in New Zealand: the tradescantia leaf beetle (Neolema ogloblini), tradescantia tip beetle (Neolema abbreviata) and the tradescantia stem beetle (Lema basicostata).

Details about the New Zealand biocontrol program can be found here:

https://www.landcareresearch.co.nz/science/plants-animals-fungi/plants/weeds/biocontrol/approvals/completed/tradescantia

Projects supported by the Australian Government as part of the National Environmental Research Program — Emerging priorities (Phase 1 project: 1 July 2014 to 1 May 2015) and the National Environmental Science Program — Emerging priorities (Phase 2 project: 15 May 2016 to 22 May 2017), administered by the Department of Environment and Energy (now Department of Agriculture, Water and the Environment).

Collaborator: Agriculture Victoria Research Division, Department of Jobs, Precincts and Regions (previously Department of Economic Development, Jobs, Transport and Resources)

Research performed by CSIRO focused on assessing the risk that the fungus Kordyana brasiliensis may pose to non-target native and ornamental plants closely related to wandering trad that are present in Australia. Additional host-specificity tests, to complement testing performed as part of the New Zealand program, were required because Australia has several native species in the Commelinaceae family to which wandering trad belongs to. Tests were undertaken in a quarantine facility in Australia.

Results obtained in these tests further demonstrated that K. brasiliensis is highly host specific towards wandering trad. All Australian populations of wandering trad tested were attacked by the fungus. Non-target plant species tested developed no symptoms or a limited number of small flecks on some leaves – an indication of resistance to infection by the fungus.

An application to release K. brasiliensis into Australia was submitted in November 2016 to the Department of Agriculture and Water Resources. Following a comprehensive risk assessment performed by the Department and consultation with states and territories as well as general public, the fungus was deemed to pose very low (negligible) risk to non-target plants and was approved for release from quarantine on 14 December 2018 (https://www.agriculture.gov.au/biosecurity/risk-analysis/memos/ba2018-32).

Disease symptoms caused by the fungus Kordyana brasiliensis on leaves of wandering trad under controlled conditions. Diffuse chlorotic spots on the upper surface of leaves (A) and corresponding whitish lesions on the under surface of leaves (B) at 14 days after inoculation. Lesions become necrotic as they mature (C), eventually causing complete necrosis and death of leaves (D).

As part of these projects, scientists from Agriculture Victoria Research Division also performed host-specificity tests with the Tradescantia leaf beetle, Neolema ogloblini in a quarantine facility to assess risks to non-target plants relevant for the Australian context.

In the Phase 1 project, choice-minus-target testing using leaf beetle adults was undertaken with 16 native and ornamental plants closely related to wandering trad. Oviposition and adult feeding damage by the leaf beetle were lower on the non-target species than on wandering trad, and none of the eggs hatched on any of the non-target plants.

In the Phase 2 project, a no-choice experiment was conducted by placing equal numbers of leaf beetle eggs onto the underside of wandering trad and the non-target species tested during Phase 1. Results showed that eggs hatched on all species and developed to adults on wandering trad and to various life-stages on seven of the Australian native Commelinaceae species tested: Aneilema acuminatum, A. biflorum, Murdannia graminea, Commelina ciliata, C. cyanea, C. diffusa and C. lanceolata. Higher numbers of individuals at all life-stages and more larval feeding damage (skeletonised leaves) were recorded on wandering trad.

Based on these results, a further no-choice adult oviposition experiment was set up using all test plants on which larval development had been recorded. After 3 months, only a single pupa was recovered from the cage containing A. acuminatum, confirming oviposition and development of the beetle to at least this stage on that species. With remaining resources in the project, one further experiment was undertaken to compare utilisation of A. acuminatum and wandering trad by the leaf beetle, by exposing plants to mating adult pairs within enclosed gauze sleeves. Results confirmed that the leaf beetle can oviposit on A. acuminatum but at rates nearly 20 times lower to those recorded on wandering trad. Furthermore, oviposition was only recorded on 2 of the 15 A. acuminatum replicate plants tested. After 14 days, numerous first and second instar larvae were present on wandering trad, but none were observed on A. acuminatum plants.

These experiments showed that the leaf beetle N. ogloblini is potentially not restricted in host range to wandering trad and may be able to develop on a few related Australian native plant species. Additional no-choice adult oviposition and developmental experiments performed over a longer period to allow multiple generations of the beetle to develop are required to fully assess risks before an application for release in Australia can be considered.

The Tradescantia leaf beetle, Neolema ogloblini (left) and damage it caused on wandering trad (right) during tests performed in a quarantine facility (Photos: Raelene Kwong and Greg Lefoe, Agriculture Victoria).

Project supported by the Australian Government as part of the Improving Your Local Parks and Environment Program (2017 to 2020) administered by the Department of Industry, Innovation and Science on behalf of the Department of the Environment and Energy (now Department of Agriculture, Water and the Environment).

This project focused on releasing and monitoring progress of the fungus Kordyana brasiliensis on wandering trad in the Dandenong Ranges (2019-20) and throughout Victoria more broadly (2020), in partnership with local community stakeholders (in particular the Community Weed Alliance of the Dandenongs [CWAD], coordinated by Bill Incoll).

Developing release methods

In March 2019, after permission was granted to release the fungus from quarantine, an efficient system was developed to produce wandering trad plants infected with the fungus to have a continuous supply of material for delivery to community members. Following initial trials, two methods were selected for large-scale releases: the ‘bin-inversion’ and ‘planted-stem’ methods.

For the ‘bin-inversion’ method, between 10 and 15 wandering trad leaves infected with the fungus are carefully stuck using Vaseline to the inside-bottom of a large plastic tub (e.g. garbage bin), with the underside of each leaf facing upwards towards the opening. The bin is then inverted over a dense patch of wandering trad growing in cool, moist, shaded habitats for at least 24 hours to allow spores to be discharged from the infected leaves, land on the healthy wandering trad leaves below and infect them.

For the ‘planted-stem’ method, a single stem of wandering trad with several leaves infected with the fungus is planted into moist soil in the centre of a patch of wandering trad, thereby allowing spores to be passively discharged over several weeks and transferred to nearby healthy leaves.

The ‘bin-inversion’ method to release the leaf-smut fungus, Kordyana brasiliensis on wandering trad.

Establishing long-term monitoring plots to measure impacts of the fungus on wandering trad and associated vegetation

In May 2019, the fungus was released using the ‘bin-inversion method’ at each of 15 long-term monitoring plots (6m x 10m in size) established in moist forests of Victoria. Baseline data on the abundance of wandering trad and diversity of native vegetation were collected prior to release of the fungus are each site. Monitoring undertaken between October and December 2019 –approximately five months after the fungus was first released – recorded infection of wandering trad at 60% of the monitoring plots. The fungus was re-released at 10 monitoring plots using a combination of the ‘bin-inversion’ and ‘planted stem’ methods in October 2019 to boost infection levels.

Release of the leaf-smut fungus, Kordyana brasiliensis on wandering trad in May 2019 at a long term monitoring plot located at Birdsland Reserve, Tecoma.

Unfortunately, due to covid-19 travel restrictions, it was not possible for CSIRO field ecologists to revisit the monitoring plots to evaluate establishment and spread of the fungus in 2020. However, visits by Bill Incoll from CWAD indicated that the fungus is beginning to spread widely at several sites.

Releases of the fungus throughout Victoria in partnership with local community organisations

CSIRO held three information sessions in 2018 to showcase the project to local community stakeholders: one hosted by the CWAD (at Birdsland, 31st January 2018), one hosted by Yarra Ranges Council (Lilydale, 5th June 2018) and another by Yarra Ranges Landcare Network (Monbulk, 5th June 2018). Attendance was open to any interested local community stakeholder, weed control contractor or government officer. During these meetings, CSIRO provided attendees with information about the wandering trad fungus, a background to research underpinning its development, progress regarding the risk assessment and application to release the fungus from quarantine, its anticipated post-release impacts on wandering trad and benefits to native vegetation, and how community members would be able to participate in the release program once permission to release the fungus from quarantine was granted.

In 2019, five face-to-face workshops on the biocontrol project were delivered to local community stakeholders across the Dandenong Ranges region of Victoria. During these workshops, attendees were trained in release and monitoring techniques for the fungus. At least one bag containing approximately 10 wandering trad stems infected by the fungus were given to each of workshop participants to make releases at their trad-infested sites. Two attendees were provided with bags of at least 100 infected stems each, allowing for several releases to be made. Overall, we estimate that approximately 830 infected stems were provided to 63 unique attendees at these face-to-face workshops.

A. Members of the Community Weeds Alliance of the Dandenong attending a wandering trad biocontrol workshop on 31st May 2019. B. Members of Friends of Gembrook Park and Cardinia Shire Council attending a field-based wandering trad biocontrol workshop on 13th June 2019.

Between September 2019 and October 2020, CSIRO posted approximately 940 additional wandering trad stems infected by the fungus to 22 community participants, many of whom were unable to attend the face-to-face workshops in 2019. During 2020, CWAD set up 13 nursery sites with 990 infected stems sent by CSIRO, to provide sources of infected material in the future to make additional releases of the fungus in the region. The fungus has successfully established at these nursery sites (Bill Incoll, pers. comm.).

Symptoms caused by the fungus Kordyana brasiliensis on wandering trad in the field. A: Whitish lesions on the under surface of leaves, B: Yellowish spots on the upper surface.