Trying Again with a Tough Tussock
Nassella tussock (Nassella trichotoma) is unpalatable to stock and reduces the livestock carrying capacity of infested pasture. This weed is not the easiest target for biological control, but a new Sustainable Farming Fund grant awarded to the Marlborough Farmers Nassella Tussock Group (but administered by Manaaki Whenua – Landcare Research) is allowing us to have another crack.
Suspected crown rot fungus in Australia we hope to find again
The weed is present in Auckland, Hawke’s Bay and Northland, but is most problematic in the South Island, where it now infests at least 524,000 ha. Worryingly, mapping based on current climate patterns undertaken by AgResearch suggests this represents only 6% of its potential range in New Zealand. Climate change is likely to make the situation worse, as more frequent droughts predicted to occur on the east coast of New Zealand are likely to lead to increased bare ground and reduced competition from pasture grasses.
Managing nassella tussock is expensive, and usually involves manual removal (grubbing) of plants to keep weed populations below levels that are economically damaging. Recently the herbicide fluproponate has been introduced to New Zealand, but it has been found to cause damage to desirable pasture plants. Also, in Australia nassella tussock has developed resistance to this herbicide, and the same could happen here.
Biocontrol could potentially provide a more cost-effective and sustainable solution. “Modelling studies by AgResearch have shown that if we could reduce seeding by 10% and plant growth by 15% (reductions that should easily be achievable) that would result in a 76% reduction in the size of the Canterbury nassella tussock population,” explained Seona Casonato, who is leading the new project.
Researchers and resources from three countries − Argentina, Australia and New Zealand − will be involved in this 3-year project. Argentina is the source of the weed, while Australia shares our desire to control it, having an even worse problem than New Zealand. The three countries have previously collaborated on biocontrol of the closely related Chilean needle grass (Nassella neesiana), which also originated in South America. This successful partnership led to the identification of a host-specific rust fungus (Uromyces pencanus), which we believe will make an excellent biocontrol agent for Chilean needle grass once it can be exported from Argentina.
When seeking to use disease-causing micro-organisms (pathogens) to control weeds, there are two strategies that can be used: classical control and inundative control. Classical biocontrol involves finding a pathogen that damages the weed somewhere else (usually in its centre of origin) and then releasing it where it is needed. Classical biocontrol agents are expected to maintain and spread themselves once released. Inundative biocontrol, by contrast, uses pathogens that already occur on the weed where it is causing problems. Humans can help these pre-existing microbes to cause severe disease epidemics by formulating them to overcome unfavourable environmental conditions (e.g. a lack of moisture) and by applying them to the weed in large quantities (e.g. a million spores per millilitre of liquid). One draw-back of this method is that the help we provide needs to be ongoing. Still, a herbicide based on a pathogen (a bioherbicide) can be a useful tool, especially if the pathogen is relatively host specific, and so can be applied over non-target plants without damaging them (in contrast, for example, with the chemical herbicide fluproponate mentioned above). Because a bioherbicide is based on a complex living organism, it is also less likely that a weed would develop resistance to it.
For nassella tussock we intend trying both strategies. There have already been surveys undertaken in Argentina back in the 1990s looking for pathogens with potential as classical biocontrol agents. But when the three pathogens that seemed to have the most promise all looked decidedly less promising after further study, the limited funds were channelled towards Chilean needle grass instead. While our reasons for rejecting the rust fungus (Puccinia nassellae) and the smut fungus (Tranzscheliella sp.) are still valid, technology has advanced considerably since the crown rot fungus was rejected for its intransigence in the lab and our inability to identify it. “We believe that if we could discover this pathogen again in Argentina, this time we could use molecular techniques to identify it. Once we know what sort of organism it is, this should help us to work out what conditions are needed to establish a colony in the glasshouse for further study,” explained Freda Anderson (CERZOS-CONICET, Argentina), who has been involved with this project since the first surveys.
Consequently, new surveys in Argentina will be undertaken as part of the new project. Researchers will be looking particularly for this crown rot fungus, and other pathogens that appear to help it along in the field (e.g. Fusarium species), because it seems that when severe damage occurs it is due to a combination of pathogens and/or insects. “We will also look for the crown rot fungus in Australia as, after looking back through some old photos, I think I might have encountered it there more than 20 years ago,” said Seona.
There is also another pathogen found on nassella tussock in Australia that might be useful for New Zealand. Zinzipegasa argentinensis was first described on a Nassella species in Argentina in 1911, so it was probably imported to Australia along with the weed. Given the problems we have been having getting permission to export other biocontrol agents from Argentina, it would be good to have an option of importing potential agents from just across the ditch. Zinzipegasa argentinensis causes black lesions on flowering stems of nassella tussock but doesn’t cause enough damage on its own in Australia to be really useful. We will try to re-collect this fungus and test its pathogenicity on our nassella tussock plants in the hope that they are more susceptible to it, and see if damage can be increased when combined with other pathogens.
The third prong in our planned attack is to try to develop one or more fungi we already have here as a bioherbicide. We surveyed nassella tussock in New Zealand in 2000 and found 42 fungi associated with it, but only six were judged to have caused disease symptoms and to be potentially host specific. None of these were identified to the species level, but were placed in the genera Ascochyta (one species), Fusarium (three species), Phoma (one species) and Pyrenophora (one species), and no further research was done. If these pathogens could be reisolated from the sites where they were collected previously, they could now be identified further (using molecular techniques) and tested as potential bioherbicides. The Fusarium species would be of particular interest, as such fungi may enhance the ability of the crown rot to kill host plants in Argentina, and perhaps we could encourage them to do the same here.
“What makes this project interesting is that it will combine all the ‘old’ knowledge of experienced researchers from three countries with ‘new’ technology,” enthused Seona. Combining the classical and inundative techniques will also be a novel experience for New Zealand, but there is no reason why, with sufficient resources, it could not be done. Biocontrol will never eradicate a weed completely, but it would certainly be a welcome additional tool in our ongoing battle against this tough tussock.
This project is funded by the Sustainable Farming Fund administered by the Ministry for Primary Industries, with co-funding from Environment Canterbury, Marlborough District Council, the National Biocontrol Collective, Manaaki Whenua – Landcare Research, and the Victoria Serrated Tussock Working Party and AgriBioscience in Australia.
CONTACT: Seona Casonato (casonatoc@landcareresearch.co.nz)