Background
What is the weed problem?
Wandering trad (Tradescantia fluminensis) is an herbaceous groundcover plant native to South America that has become a significant environmental weed of temperate Australia, with hotspots in moist forests of eastern NSW, south-east Queensland and Dandenong Ranges region of Victoria. It forms dense swathes along the forest floor, particularly in moist riparian zones, where it reduces the diversity of native vegetation and alters the movement and behaviour of native animals. Wandering trad can also cause skin irritations to domestic pets, thereby reducing the recreational amenity of hikers and dog walkers using public pathways through infested forests.
Wandering trad primarily spreads via stem sections by water movement along creeks, soil disturbance and in dumped garden waste, meaning that it does not rely upon pollination and seed production for spread.
Close-up of wandering trad flowers.
How is the weed currently managed?
For small infestations, manual removal of wandering trad is the most suitable option for control, but it may cause disturbance to the soil profile and stimulate germination of other weed species. Wandering trad can regenerate rapidly from broken stem and root fragments, meaning that hand-pulling is ineffective in the long term unless all material is removed from a site. For large infestations, chemical control is considered more practical, although there is potential for non-target damage to native plants growing alongside wandering trad. Herbicide application is particularly problematic along creek lines in the presence of threatened native vegetation. There is increasing evidence that wandering trad can regenerate after primary chemical control, leading to the need for multiple follow-up herbicide treatments, which is considerably costly.
Dense infestation of wandering trad in Kangaroo Valley, NSW.
For more information see:
http://weeds.dpi.nsw.gov.au/Weeds/Details/141
What can biocontrol offer to the weed’s management?
Classical biological control is the only recognised sustainable solution to the suppression (but not eradication) of environmental weeds to reduce their impacts at a landscape scale in Australia. Use of mechanical or chemical control for wandering trad in sensitive rainforest settings, especially along creek lines comprising endangered plant species, is unviable over large spatial scales, because wandering trad can routinely regenerate unless long term intervention is undertaken. A biocontrol approach may offer a longer term, sustainable solution to controlling wandering trad in sensitive ecological communities with lower risks to native vegetation.
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: