Sea spurge

Sea spurge (Euphorbia paralias) is a significant environmental weed in southern Australia. NSW coastal dune ecosystems are continuously being invaded by ocean-dispersed seed likely originating from sea spurge infestations in Victoria and Tasmania.

This sub-project focuses on the foliar blight fungal pathogen Venturia paralias (previously referred to as Passalora euphorbiae), which causes severe disease of sea spurge in its European native range.  Comprehensive host-specificity testing was performed in an Australian quarantine facility to assess any risks that the fungus could pose to non-target plant species. Results indicated that the fungus does not pose a threat to non-target species, and thus approval for its release in Australia was sought from the relevant authorities. The biocontrol agent was approved for release in November 2020 and research has since been conducted to develop effective methods for production and field releases. A release program for the agent that is specifically targeting sea spurge infestations in south-east Australia began in Spring 2021.

Gavin Hunter of CSIRO is leading this sub-project.

Background

What is the weed problem?

Sea spurge is a significant environmental weed of coastal dune ecosystems in southern Australia. Re-invasion from ocean-dispersed seed is a constant threat to achieving long-term control. Along the NSW South Coast, sea spurge keeps re-invading sand dunes and is progressively moving northward. Infestations of sea spurge can negatively affect threatened flora and fauna species and Aboriginal cultural heritage sites, alter the geomorphic processes on the coast, transform native vegetation communities and reduce public amenity in coastal recreation areas. Sea spurge can also cause skin and eye irritations in those who inadvertently touch its milky latex sap. Each plant can produce up to 5000 seeds that can survive for several years on ocean currents – the primary means of dissemination from beach to beach.

How is the weed currently managed?

Control of sea spurge infestations with manual and chemical control techniques is difficult because of the close proximity to estuaries, the marine environment and native vegetation, its persistent seed banks, and constant re-invasion from beaches within and outside of NSW. Government agencies and local community groups devote large amount of time controlling it, but ongoing efforts are required because of the established seed bank and the relentless re-invasion that occurs.

Hand pulling of sea spurge is popular with community groups, as it requires no special training, costs little, and avoids exposing community members to chemical herbicides. It can be quite effective for small populations less than a few thousand individuals. The incomplete removal of the plants’ roots however, can lead to their re-establishment. Even when plants are completely removed, recruitment of seedlings from the seed bank can occur for up to ten years. The application of herbicides is popular with land management agencies (parks, local councils or NRM organisations). While often effective in reducing large populations of sea spurge, herbicides can cause off-target damage to sensitive native coastal vegetation and aquatic and marine ecosystems.

What can biocontrol offer to the weed’s management?

The hardiness, mobility and longevity of sea spurge’s seeds in combination with the remote locations of many infestations, make management of sea spurge in southern Australia difficult using currently available control methods. Biological control would provide a sustainable, landscape scale approach for sea spurge management with no chance of off-target damage. A successful biocontrol program across the range of sea spurge in Australia would reduce the density of populations and seed production, thus reducing spread via ocean currents and infestation of new beaches. A substantial improvement of biodiversity outcomes would be expected from a progressive reduction of the number of beaches invaded by sea spurge as a result of biocontrol and the commensurate recovery of dune habitats for native flora and fauna.

Previous research

Due to its importance as an environmental weed, sea spurge was nominated and endorsed by the national Invasive Plants and Animals Committee (IPAC; now known as the Environment and Invasives Committee) as a target for biological control in Australia in August 2010.

In previous projects supported by the Tasmanian government and the Rural Industries Research and Development Corporation (RIRDC; now known as Agrifutures Australia), two damaging fungi with potential for biocontrol – the foliar blight fungus Venturia paralias (previously referred to as Passalora euphorbiae) and the rust fungus Melampsora euphorbiae – were discovered on sea spurge during field surveys in France and Spain. Initial tests, performed at the CSIRO European Laboratory in France, indicated that the two pathogens may be promising candidate biocontrol agents for sea spurge in Australia (Scott et al. 2010, 2012). Out of 15 non-target species tested with the rust fungus, only Euphorbia marginata and E. dendroides developed disease symptoms, albeit not as severe as those on sea spurge. Preliminary testing with the fungus showed that it is possibly highly specific. It was tested on eight Euphorbia species, but none were native to Australia. It was also tested on two species in the sub-family Acalyphoideae and one species in a related family to Euphorbiaceae. In all tests, disease symptoms only developed on sea spurge. The damage caused by the fungus on sea spurge in the field is impressive in the native range (M. Jourdan pers. comm.).

Baseline data on  sea spurge growth, phenology and reproductive output, and impact on native vegetation were gathered in 2011-12 at sites in WA and Victoria (Scott et al. 2012). A recent Honours thesis at Wollongong University (Kelly 2015), indicated that sea spurge is or has been present at 309 of the 481 beaches (64.2%) assessed in NSW based on information gathered from land managers, management documents and field surveys.

Kelly LJM (2015) Patterns of sea spurge (Euphorbia paralias) invasion in New South Wales, Australia. Honours Thesis, The University of Wollongong.

Scott JK, Jourdan M, Morin L, Thomann T, Yeoh P (2010) Exploration for potential biological control agents of Euphorbia paralias, a major environmental weed of coastal ecosystems in Australia. In: Proceedings of the 17th Australasian Weeds Conference, (ed. Zydenbos SM), pp. 223-226. New Zealand Plant Protection Society Inc., Christchurch. http://caws.nzpps.org/awc/2010/awc201012231.pdf

Scott JK, Jourdan M, Morin L, Webber BL, Yeoh PB, Ebeling SK, Cousens R (2012) Sea spurge, Euphorbia paralias, ecological assessment and testing of potential biological control agents. In: Proceedings of the 18th Australasian Weeds Conference, (ed. Eldershaw V), pp. 223-226. Weed Society of Victoria Inc., Melbourne. http://caws.nzpps.org/awc/2012/awc201213751.pdf

Trust-funded sub-project

The foliar blight fungus Venturia paralias (previously referred to as Passalora euphorbiae) was selected as the most promising candidate agent of sea spurge to pursue for the sub-project supported by the NSW Environmental Trust because it:

• has been shown to be highly host-specific in preliminary tests, although more tests have to be performed, especially on Australian native Euphorbia , to gather all information required to support an application for release in Australia,
• damages leaves and stems, which are present all-year round,
• has been observed to cause severe infections on sea spurge in the field in the native range,
• should, if approved for release in Australia, cause disease symptoms on sea spurge over most of the year, due to the high humidity present in coastal areas where the weed occurs.

In November 2020, Venturia paralias was approved for release in Australia. 

Phase 1 (July 2017 – June 2019)

The key activities of this phase of the sub-project were to:

• Undertake comprehensive host-specificity testing of the candidate biocontrol agent on closely-related plant species to sea spurge, including selected Australian native species.
• Prepare and submit to the relevant authorities an application for its release in Australia for the biocontrol of sea spurge, pending results indicate that the fungus does not pose a threat to non-target species.

Phase 2 (January – September 2021)

The key activities of this phase of the sub-project were to:

• Develop an efficient method for mass production of the agent.
• Identify sea spurge infested sites in NSW that are not under active control to test different release methods for the agent.
• Evaluate efficacy of the release methods by assessing establishment of the agent in the field.

Phase 3 (July 2021 – June 2024)

In this phase of the sub-project, releases of the approved biocontrol agent will be occurring in sea spurge infestations in regions of Tasmania and Victoria that are the source of recurrent infestations in NSW due to seed dispersal on ocean currents. Various agencies and groups, such as Coastcare, Landcare Australia, Landcare Tasmania, Parks Victoria, Tasmanian Parks and Wildlife Services, Cradle Coast Authority, Great Ocean Road Coast and Parks Authority, and the Sea Spurge Remote Area TeamS (SPRATS) volunteer group, in these states have committed to provide in-kind support to the sub-project.

The key activities of this phase of the sub-project were to:

• Mass-produce the agent.
• Gather baseline data on sea spurge population at monitoring sites in Tasmania and Victoria and release the agent.
• Release the agent at other sites, with involvement of local communities wherever appropriate.
• Evaluate establishment, spread and initial impact of the agent at monitoring sites in NSW, Tasmania and Victoria.

Phase 3: Progress 

October 2023

Biocontrol agent releases undertaken by community members

Packaging and provision of the biocontrol agent to community members

Since July 2022 until the end of June 2023 a total of 209 vials containing dried fungal material of V. paralias have been provided to registered community members. Eight organisations spanning community volunteer groups, State Government agencies, Landcare groups, public land managers, and a Shire Council received biocontrol agent release kits to inoculate sea spurge plants in the field. Along with the fungal biocontrol agent, all community participants received hardcopies of detailed release instructions and had access to an online video demonstrating the methodology of preparing the agent and inoculating sea spurge plants in the field.

Release of the biocontrol agent by community participants

Registered community participants released the sea spurge biocontrol agent widely across Victoria and Tasmania over the past year (July 2022-June 2023): 48 specific sites across 11 locations. Cumulatively, since the commencement of this subproject, the sea spurge biocontrol agent has been released at a total of 163 discrete release sites, across 38 locations by 24 registered community participants spanning Aboriginal Land Councils, community groups, Federal Government agencies, Landcare groups, Local Governments, private landowners, public land managers, and State Government agencies.

A prerequisite for community participants to register in the community-led releases of the agent, and to receive the agent from the CSIRO, was to supply information back to the CSIRO subproject team about infection (or not) of the inoculated sea spurge plants, as evidenced by the appearance of characteristic disease symptoms. In this regard, monitoring results for 75 discrete release sites have been submitted by community participants back to CSIRO. Data obtained from the 75 release sites indicates that the biocontrol agent has successfully infected sea spurge plants at 38 (~ 50 %) of these sites based on symptom observation and confirmation by the CSIRO project team.

Assessment of biocontrol agent establishment at the fixed monitoring sites

Assessment of agent establishment in Victoria

Comprehensive assessments of the biocontrol agent were undertaken at the six dedicated monitoring sites (Sand Island, Bushrangers Bay, Oberon Bay, London Bridge, Whites Beach, and Shelley Beach) dispersed across Victoria’s coastline.

During the September 2022 assessment (7 months post-agent release), characteristic disease symptoms (stem and leaf lesions) on sea spurge plants, associated with infection by the biocontrol agent, were observed within the monitoring plots. Stem lesions were observed at all six monitoring sites while leaf lesions were observed at five monitoring sites. In total, stem lesions were identified in 35 (39%) of the 90 experimental plots across the six monitoring sites while leaf lesions were identified in 32 (36%) of the 90 experimental plots. Average sea spurge plant health scores decreased, compared to baseline assessments (February-March 2022), at the majority of sea spurge plots across the six monitoring sites.

During the May 2023 assessments (1 year post-agent release) of the sea spurge biocontrol agent, not all of the monitoring sites could be attended. Severe weather over Port Campbell in Victoria prevented the CSIRO research team from accessing the London Bridge monitoring site in Port Campbell National Park. In addition, during assessments at Oberon Bay (Wilsons Promontory National Park) and Shelley Beach (Discovery Bay Coastal Park), one plot at each of these sites was accidentally missed by assessors and no data was recorded for these. However, data for all other plots across the five monitoring sites was recorded and this data indicated that both stem lesions and leaf lesions on sea spurge plants were observed in 36 (60%) plots across a total of 60 assessed plots for the five monitoring sites that were attended and assessed.

Surveys were also undertaken outside of the dedicated monitoring plots to determine whether the biocontrol agent had naturally spread. For this, CSIRO researchers surveyed sea spurge plants approximately 100 m in either compass direction (where possible) to search for characteristic disease symptoms of V. paralias infection. From these surveys and, based on the presence of stem and leaf lesions, the biocontrol agent has now naturally spread outside of the Bushrangers Bay and Sand Island monitoring sites.

Assessment of agent establishment in Tasmania

In Tasmania, two assessments of the three dedicated monitoring sites (Bakers Beach, Low Head, and Duck Creek) were undertaken in the past year during November 2022 and June 2023. During the November 2022 assessment, stem symptoms of the biocontrol agent were observed in 30 (67%) of the 45 plots across the three monitoring sites and leaf lesions were observed in 28 (62 %) of the plots. The assessments also indicated that the overall average health of sea spurge plots decreased. During the June 2023 assessment, stem lesions were observed on sea spurge plants in 21 (50%) plots across the three monitoring sites while leaf lesions were observed in 38 (90%) plots.

During November 2022, surveys of disease symptoms on sea spurge plants caused by the biocontrol agent were also undertaken in the vicinity surrounding the three monitoring sites in Tasmania. This was done by walking around the sites and searching for stem and leaf lesions on sea spurge plants. At the Low Head site, there was a ring of sick and dead sea spurge around the southern downwind side of the site, with signs of the biocontrol being found at 50 m north (upwind) and 250 m south (downwind). For this to have occurred the sea spurge biocontrol agent has dispersed over a 50 m wide gap in the site’s sea spurge infestation. Similarly, at the Bakers Beach site, the biocontrol agent has spread approximately 50 m to the west, 100 m to the east and 30 m inland. Whereas, at the Duck Creek monitoring site, the biocontrol agent has spread approximately 75 m to the north and the south.

Phase 2: Summary of achievements 

October 2021

Three different approaches to produce the sea spurge biocontrol agent, the fungus Venturia paralias, for field releases were tested. The most efficient method was to produce sporulating cultures of the agent on agar medium in plates and drying the cultures at room temperature for 24-48 hrs and storing at 4°C until ready for use.

Following consultation with a range of land managers (councils, national park), three sites on the NSW South Coast with an adequate number of sea spurge plants were identified to test release methods: Cudmirrah Beach, Cunjurong Beach and Brou Beach.

Up to six different methods for releasing the agent were tested across the three sites. The release methods that involved spraying spore suspensions made from fresh and dried cultures of the agent resulted in more stem lesions developing than any of the other methods tested.

Phase 1: Summary of achievements

October 2021

Cultures of the fungus Venturia paralias (previously referred to as Passalora euphorbiae) stored since 2009 at the CSIRO European Laboratory in France, were imported in the CSIRO quarantine in Canberra in October 2017. A robust and reliable methodology for host-specificity testing has been developed by optimising the culturing, inoculation and assessment protocols. Propagating material for 40 non-target plant species of the test list (76 different accessions) were sourced and successfully propagated. The test list comprised species across the families Euphorbiaceae, Picrodendraceae and Phyllanthaceae.

Results of host-specificity testing indicated that the fungus can only severely damage sea spurge. Visible necrotic lesions developed 11-12 days after inoculation of sea spurge and prolonged disease progression allowed the fungus to move from infected leaves to stems and girdle them, leading to eventual stem collapse and toppling of plants. The fungus only caused leaf lesions, albeit restricted, on one non-target species, Euphorbia segetalis, the most phylogenetically related species to sea spurge. This species is naturalised in Australia and recorded as an environmental weed, although it is very uncommon. The fungus was also capable of penetrating leaves and developing a few subcuticular amoeboid primary hyphae on Euphorbia myrsinites, Euphorbia helioscopia and Mercurialis annua, but these did not develop any further. On all other non-target species tested, conidia germinated on leaf surfaces and germ tubes formed appressoria, but the fungus was never observed to penetrate or proliferate inside leaves.

An application to release this fungus in Australia was submitted in September 2019 to the Federal Department of Agriculture, Water and the Environment (DAWE). Permission to release in Australia obtained in November 2020 at https://www.agriculture.gov.au/biosecurity/risk-analysis/biological-control-agents/risk-analyses/completed-risk-analyses/ra-release-venturia-paralias

Publications:

Crous PW et al. (2020) Fungal Planet description sheets: 1042–1111. [Hunter GC, Zeil-Rolfe I, Jourdan M, Morin L. Venturia paralias-sheet 1109]. Persoonia 44:301-459. https://www.ingentaconnect.com/content/nhn/pimj/pre-prints/content-nbc-persoonia-0550

Hunter GC, Zeil-Rolfe I, Jourdan M, Morin L (2021) Exploring the host range and infection process of Venturia paralias isolated from Euphorbia paralias in France. European Journal of Plant Pathology 159: 811-823.

Acknowledgement

The assistance of Isabel Zeil-Rolfe, Caroline Delaisse and John Lester of CSIRO is acknowledged.

Thanks to all collaborators who 1) kindly made collections of sea spurge accessions in Australia and some of the non-target plant species included in host-specificity tests, and 2) assisted with field releases of the agent in Australia.