So what's the problem?
Prickly acacia (Acacia nilotica, Mimosacae) was introduced as a shade and fodder tree to Australia during the early 20th century. However, it has since become an invader and has currently spread over approximately six million hectares of arid and semi-arid land in Queensland and has the potential to spread throughout the arid regions of the whole of northern Australia. It greatly affects the environment as well as the economy, particularly the livestock industry and is widely considered as one of Australia’s worst weeds.
There has been a biological control programme for Acacia nilotica since the early 1980s, with surveys for natural enemies conducted in parts of Africa and Pakistan. More recently, the Australian populations of this Afro-Asian tree have been identified as A. nilotica ssp. indica and the focus for survey work has shifted to India, from where this subspecies originates. Taxonomic studies have revealed that two distinct rust fungi are associated with the species in India - Ravenelia acaciae-arabicae and Ravenelia evansii. Both rusts were considered to have potential as biological control agents due to the impact inflicted on their host and their apparent field host specificity.
What is this project doing?
CABI has been tasked with assessing the two rust fungi as potential candidates to control prickly acacia in Australia. Our aim is to clarify aspects of their infection biology, as well as starting assessments of their specificity towards the target weed. In order to achieve this, the project team will inoculate the prickly pear species with the rusts as well as a selection of other Acacia species of importance to Australia.
Results so far
Evaluation of the rust R. acaciae commenced in June 2010. At first the team studied its infection biology and the team undertook the preliminary host-range testing against 17 different Acacia species to test its specificity. We did this in our quarantine facilities at our scientific centre in the UK. The team found that the evaluated non-target Acacia species were wide-ranging in their susceptibility to the rust from expressing no macroscopic symptoms at all to necrotic leaf-spotting. Critical however, was the fact that this rust produced viable spores on Acacia sutherlandii, a species native to Queensland growing alongside A. nilotica ssp. indica in the field in Australia. Although this sporulation was always restricted by severe accompanying defence reactions of the plant, the overall risk to this Australian native and the Australian flora in general was deemed to be too high to consider this pathogen any further.
The focus therefore, shifted to the second rust R. evansii. This pathogen was assessed solely for its virulence and ability to infect A. sutherlandii as a key non-target species. This fungus was equally able to produce spores on this non-target Acacia species. Consequently any further evaluation of R. evansii will also not be pursued. Thus neither of the two pathogens can be considered as suitable biocontrol agents for Acacia nilotica in Australia, despite their apparent field host specificity in India.
In order to solve a remaining taxonomic question linking rust aecia observed to cause the formation of galls on A. nilotica ssp. indica in India with either R. acaciae-arabicae or R. evansii inoculation studies will be undertaken under quarantine conditions in the UK in 2012 using fresh material shipped from India.
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