Biological control of slender thistles using an introduced rust fungus
Slender thistles, Carduus pycnocephali and C. tenuiflorus are winter annual pasture weeds of European origin that are widely distributed in the southern states. Currently infestations are worst in parts of Victoria and Tasmania. In Western Australia, South Australia and New South Wales the thistles are usually less extensive but are often locally very important.
The rust fungus, Puccinia cardui-pycnocephali, is one of the organisms that cause most damage to stands of these thistles in Europe. Indeed, surveys conducted in southern France found this rust to be more important in limiting the size of thistle populations than any of more than 20 insect species found feeding on the plant. The rust was therefore chosen as a potential biological control agent for use in thistle control.
Interestingly, strains of this rust fungus have been in Australia since at least 1947. However, those strains were not aggressive and had no impact on thistles. Following research conducted at the CSIRO Biological Control Facility in Montpellier, southern France, two new isolates of the rust were identified that were substantially more aggressive towards the thistles than those already in Australia.
These two isolates were subjected to exhaustive host specificity testing on a range of plant species with a particular emphasis on members of the daisy family (Asteraceae). With the exception of seedlings of globe artichoke where pustules developed on cotyledons and the first true leaves only, P. cardui-pycnocephali only attacked plants of C. pycnocephalus and C. tenuiflorus. Final host specificity testing of the two isolates was completed by May 1993 and, following permission from the Australian Quarantine Inspection Service and Environment Australia, the isolates were released in the field across southern Australia from October 1993 onwards.
The effect of the rust on slender thistle stands
The most obvious symptoms of the presence of the rust are very dark brown-black pustules or spots on leaves and stems of infected plants. These are often more prominent on the underside of leaves. Heavily infected leaves dry up and wither prematurely. Depending on environmental conditions, the rust fungus may have many generations each year. After infection, new pustules erupt 14-21 days later to produce tens of thousands of spores that are carried from plant to plant on the wind.
Since 1995, anecdotal reports from property owners and State government weed inspectors have suggested that the rust was generating sufficient disease pressure to cause reductions in the size and seed production of affected slender thistle stands. During 1998 we carried out an assessment of the effect of these new isolates by comparing the effect of a natural epidemic of the rust on one species of slender thistle (C. tenuiflorus) with the growth of plants protected from the rust with fungicide sprays. This comparison was carried out in improved pasture in northern Tasmania (147o15’E, 41o04’E) and south-western Victoria (141o 26’E, 37o27’S).
Both these sites were near locations at which the new isolates of rust had been released within the previous two years. At both sites, all thistle plants in a number of fungicide-sprayed (rust absent) and unsprayed (rust present) were separately harvested and bagged (a very prickly job!) and a visual estimate of disease severity (% disease cover of green leaves and stems) was made.
This comparative assessment of the effect of the rust on the growth and flowering of the slender thistle clearly showed that when the rust was present throughout the growing season the number of inflorescences produced (Fig. 1a) and the size of individual C. tenuiflorus plants (Fig. 1b) was reduced as a consequence of infection by P. pycnocephalus. This effect was detected at both sites with the average size of infected plants being 28 and 22% lower than disease-free plants at the Tasmanian and Victorian sites respectively.
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Fig. 1. The effect of the absence (fungicide-treated plots; filled bars) or presence (control plots; cross-hatched bars) of the rust fungus P. cardui-pycnocephali on: (a) the mean number of inflorescences per plant; and (b) the size of C. tenuiflorus plants growing at locations in northern Tasmania and western Victoria in 1998.
The release of the two aggressive strains of the rust P. cardui-pycnocephali for biological control of slender thistles has resulted in successful establishment of the pathogen at a range of sites across southern Australia. Clearly under favourable conditions in the field, the rust can reduce the size and flower production of C. tenuiflorus. However, the data from this study in Tasmania and Victoria and casual observations indicate that for most years these reductions alone will be insufficient to permanently reduce thistle densities. Rather, the role of P. cardui-pycnocephali will be as an additional tool in the integrated control of these thistles. Indeed, there is already evidence that in some circumstances competition from pasture grasses on its own may limit the weediness of slender thistles. In such situations, the impact of P. cardui-pycnocephali will provide an additional negative impact, further depressing the plant’s competitive ability.
The authors are grateful to (i) AWRPO for funding the testing and initial release program; (ii) The CRC for Weed Management Systems for providing the financial support needed to establish the field plots; (iii) Sandy Leighton (Tasmania) and Brad Roberts and Nick Harvey (Victoria) who established and sprayed the plots; (iv) the Tamar Valley Weeds Group (Tasmania) for their support; and (v) especially to Ian Sauer for his enthusiastic interest and generosity in making his property ‘Farley’ available for this work.
A full account of this work is being published in the journal Plant Protection Quarterly.
Report by: Jeremy J. Burdon and Peter H. Thrall, Centre for Plant Biodiversity Research & CRC for Weed Management Systems, CSIRO-Plant Industry, GPO Box 1600, Canberra, A.C.T. 2601. Email: J.Burdon@pi.csiro.au