Still Tussling with Tussocks
Dying nassella tussock in Argentina affected by crown rot fungus. Photo credit: Bill Pettit
Many New Zealanders would be very happy if we could successfully biocontrol two closely related weedy grasses: Chilean needle grass (Nassella neesiana) and nassella tussock (Nassella trichotoma). We have found a suitable agent for Chilean needle grass, a rust fungus (Uromyces pencanus), which was approved for release in 2011. Unfortunately, Argentinian authorities have not granted an export permit yet, so we have not been able to import and release the rust.
Less progress has been made towards developing biocontrol for nassella tussock, but recently the pros and cons of reactivating a biocontrol project have been reconsidered. Current management, which largely involves the hand-grubbing of tussocks, costs millions annually and is required in perpetuity to keep populations at their current low levels and prevent an economic loss estimated at $417 million (present value) in Canterbury alone.
Freda Anderson (CERZOS-CONICET, Argentina) was employed in the 1990s to look for potential biocontrol agents for Chilean needle grass and nassella tussock for Australia, with New Zealand joining the project later. The project was challenging, with many obstacles to overcome. Freda found three pathogens that initially looked like promising biocontrol agents for nassella tussock: a rust fungus (Puccinia nassellae), a smut fungus (Tranzscheliella sp.), and an unidentified fungus originally thought to belong in the Corticiaceae family, which we call ‘the crown rot fungus’. “We had high hopes with the rust fungus, but it disappointed us,” reported Jane Barton, who assisted with the project. “We couldn’t get it to complete its life cycle; it needed several hours of free water for infection (a rarity in habitats favoured by nassella tussock); it was attacked by another fungus (a hyperparasite); and preliminary hosts range testing showed it could infect a non-target Stipa species native to Australia. We gave up on it, and were frankly happy to see the last of it!”
The smut fungus can replace nassella tussock flower heads and seeds with black fungal spores. “A fungus which reduces the ability of nassella tussock to form seed would be a very useful biocontrol agent,” explained Freda. Unfortunately the disease proved to be consistently rare in the field, and at sites where the smut was present it usually infected only a few isolated plants. Laboratory work soon revealed why. “The smut only infects germinating seed, and so there is only a short window of opportunity for infection,” said Freda. Given that nassella tussock seeds prolifically, the levels of infection seen in the field in Argentina (about 1%) would be unlikely to have a significant impact on populations in New Zealand, and therefore the smut was ruled out too.
The crown rot fungus was impressive in the field, associated with dead and dying tussocks and at times quite common. The root systems of affected tussocks were far less developed and plants were much easier to pull out of the ground. Other tussock species that occurred in the same area were not affected, giving hope that it might be host specific. However, once again there were difficulties. “We couldn’t isolate the fungus causing the crown rot on artificial media,” said Freda. At the time this made it impossible to identify it to the species or genus level, or to understand its lifecycle. “We tried to test its host range by planting non-target plants and disease-free nassella tussock plants next to diseased ones in both the field and the laboratory, but after 6 months none of the healthy plants had developed disease symptoms.” At this point a decision was made to focus the limited resources for the work on the much more promising Chilean needle grass rust.
Fast forward to 2016, and the ongoing challenge of nassella tussock meant it was worth taking another look at whether biocontrol would be feasible for this target. AgResearch, in collaboration with key stakeholders, obtained a grant from the Sustainable Farming Fund (SFF) to do this. The project involved reviewing information known about potential agents, modelling the likely impacts of agents on nassella tussock populations, and determining the likely costs and benefits of a biocontrol programme.
“Freda and I prepared a report on the potential for pathogens to be used as classical or inundative biocontrol agents for nassella tussock,” explained Jane. While the rust and the smut remain rejected as potential agents, Jane and Freda believe the crown rot fungus deserves further attention. “These days molecular techniques are available that will enable us to quickly identify it at least to the genus level,” explained Jane. “Once we have a name, it should be much easier to work out its biology and life cycle, and whether it would make a suitable biocontrol agent.”
When Freda and Jane compiled all the information available about this fungus (including unpublished reports from South African researchers who noted dying tussocks in Argentina in the 1970s), it became clear that the crown rot fungus is rarely found alone in the field. “When we tried to isolate the crown rot fungus, Fusarium species often grew out of the tissue onto the artificial media,” revealed Freda. Fusarium species were also found in diseased nassella tussocks in Australia by researchers looking for fungi with potential for inundative biocontrol (i.e. to be developed into bioherbicides). However, when they applied the Fusarium species to tussocks alone they weren’t damaging. Further research revealed that the Fusarium species appeared to only infect plants suffering from feeding damage inflicted by soil nematodes. This is consistent with observations made by South African researchers that both fungi and invertebrates seemed to be involved in nassella tussock death in Argentina. This suggests that a biocontrol programme should focus on developing a combined approach, involving the crown rot fungus and other fungi (such as Fusarium species) plus invertebrates, if needed. As a bonus, Fusarium species are already commonly present on nassella tussock in New Zealand. Other pathogens of nassella tussock leaves (e.g. Septoria and Pseudoseptoria species), found occasionally in surveys in Argentina, could also be investigated, and work should be done to explore the largely unknown invertebrate options.
Once Freda and Jane confirmed that there are biocontrol options worth pursuing, AgResearch worked on the next step. A population model previously developed by Shona Lamoureaux and colleagues was used to simulate the consequences on nassella tussock populations of biocontrol agents that could reduce seed production annually by 10%, plus reduce the plants’ growth rate by 5, 10 or 15%. The model predicted that these levels of impact could result in nassella tussock population reductions of 47%, 65% and 76%, respectively.
The final step was for AgResearch to explore the costs and benefits of a biocontrol programme over a 25-year period for each of the three impact scenarios above. A cost−benefit analysis model showed that benefits should exceed costs for all three scenarios as long as the biocontrol agent(s) achieved 90% of their maximum impact on populations within 30−35 years. “We would usually reject biocontrol agents that reduced seed production and/or plant growth by as little as the 5, 10 or 15% scenarios, and it is certainly realistic to expect they could do much more than this,” said Jane. So the results of this SFF project suggest a biocontrol programme for nassella tussock in New Zealand is definitely worthwhile from an economic viewpoint. This finding is being shared with land managers, and their interest in pursuing biocontrol for nassella tussock is being determined.
The study into the feasibility of biocontrol for nassella tussock was funded by the Sustainable Farming Fund (SFF 404930), with co-funding from Environment Canterbury, Marlborough District Council, Hawke’s Bay Regional Council and Beef+Lamb. Jane Barton and Freda Anderson are contractors to Landcare Research.
Contact: Jane Barton (Jane.Barton@ihug.co.nz) or Shona Lamoureaux (shona.lamoureaux@agresearch.co.nz)