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14 Plant Protection Quarterly Vol.15(1) 2000 Final host specificity testing of the two isolates (FR3, IT2) was completed by May Biological control of Carduus pycnocephalus and C. 1993 and, following permission from the tenuiflorus using the rust fungus Puccinia cardui- Australian Quarantine Inspection Service and the then Australian Nature Conserva- pycnocephali tion Agency (now Environment Aus- tralia), the isolates were released in the J.J. BurdonAB, P.H. ThrallA, R.H. GrovesAB and P. ChaboudezC field across southern Australia from Octo- A Centre for Plant Biodiversity Research, B CRC for Weed Management ber 1993 onwards. Systems, CSIRO Plant Industry, GPO Box 1600, Canberra, ACT 2601, The release program Australia. C At all release sites a mixed inoculum of le Fontenelle bât. G2, 663 rue Pré aux Clercs, Montpellier 34000, France. both isolates of P. cardui-pycnocephali was released by spraying a suspension of 300 mg of urediospores in 15 L of local tap Summary identified more than 20 insect species water onto 10 m2 patches of either or both Following extensive host-specificity test- feeding on the Carduus species but indi- C. tenuiflorus and C. pycnocephalus. A ing in southern France and in quarantine vidually none appeared to limit the size of mixed inoculum was used to prevent facilities in Canberra, two isolates of the host populations in the field (Sheppard et problems associated with distinguishing rust fungus Puccinia cardui-pycno- al. 1991). In contrast, extensive field obser- between these two closely related thistle cephali were released in southern Aus- vations of natural populations of Carduus taxa by untrained personnel. Plots were tralia in October 1993 as biological con- in southern France during the period inoculated in the late afternoon, covered trol agents for the slender thistles 1988-1991 and subsequent experiments with plastic sheeting overnight to main- Carduus pycnocephalus and C. tenui- indicated that the rust fungus Puccinia tain high humidity, and then uncovered florus. Initially, establishment was poor cardui-pycnocephali had a significant effect the following morning. Disease symp- due to prolonged dry conditions. Subse- on seed production (Chaboudez et al. toms became visible 10–15 days later de- quent releases in 1994–1995 became well 1993, P. Chaboudez unpublished data). pending on prevailing temperatures. established and consistently cause no- Once P. cardui-pycnocephali was identi- Initially, limited field releases were ticeable disease particularly in western fied as a potential biological control agent, made in New South Wales, Victoria and Victoria and Tasmania. In 1998, compara- two further complications came into play. Tasmania in late spring 1993 (October– tive assessments of fungicide-treated First, P. cardui-pycnocephali was known to November) with broad scale releases be- (disease absent) and untreated (disease have occurred in Australia for at least 50 ing undertaken throughout southern Aus- present) plots in both these areas showed years prior to this control program (Costin tralia during 1994. All these early releases reductions in the size of individual C. 1954). This presented questions regarding suffered from the prolonged drought that tenuiflorus plants and the number of in- the value of importing further strains of an prevailed across much of southern Aus- florescences as a consequence of infec- agent that, to date, had had no noticeable tralia during 1993–1994. While the patho- tion by P. cardui-pycnocephalus. While effect on thistle growth. Second, there was gen became established at several sites the impact of the pathogen alone was in- some suggestion that different isolates of during the winter/spring period, it failed sufficient to reduce thistle densities per- P. cardui-pycnocephali would show differ- to persist over summer probably because manently, it will have a role as an addi- ent degrees of aggressiveness on the two of the extremely dry conditions and con- tional tool in the integrated control of target species. These concerns were ad- sequent very high grazing pressures. these thistles. dressed during initial trials in France (con- Once the drought broke, further releases ducted at the CSIRO Biological Control were made through a network of collabo- Introduction Facility in Montpellier) by comparing the rators in many of the same areas in au- Carduus pycnocephalus and C. tenuiflorus virulence and aggressiveness of several tumn 1995. A strategy of release of P. are two closely related thistle species oc- isolates of P. cardui-pycnocephali collected cardui-pycnocephali at multiple sites rather curring naturally in Mediterranean to in Mediterranean Europe with a ‘control’ than detailed monitoring of just a few sites north-western Europe. Introduced to Aus- isolate collected in Australia. Two Medi- was adopted to maximize spread. A total tralia, probably as a contaminant of feed, terranean isolates (IT2 and FR3) were sub- of 478 ‘starter’ samples of both isolates of these thistles have become widely distrib- stantially more aggressive towards C. P. cardui-pycnocephali were distributed be- uted across southern Australia from pycnocephalus or C. tenuiflorus than the tween October 1993 and June 1995 (Figure Queensland to Western Australia and Tas- Australian isolate. Although both isolates 1). These post-drought releases were mania (Parsons and Cuthbertson 1992). In attacked both Carduus species, isolate IT2 much more successful, with reports of es- favourable sites, they may form dense induced a slightly more susceptible reac- tablishment and persistence coming from stands, leading to reduced growth of more tion on C. pycnocephalus than isolate FR3, collaborators in South Australia, Victoria, desirable pasture plants. Losses to Aus- which was similarly more successful on C. Tasmania and New South Wales. tralian industry resulting from reduced tenuiflorus (Chaboudez et al. 1993). Since 1995, anecdotal reports from stock carrying capacities and vegetable These two isolates of P. cardui- property owners and State government contamination of wool were estimated to pycnocephali were then subjected to ex- weed inspectors have suggested that P. exceed $5 million per annum in 1992. haustive host specificity testing on a range cardui-pycnocephali was generating suffi- As part of a broader strategy to control of plant species with a particular empha- cient disease pressure to cause reductions all weedy thistle species introduced to sis on members of the Asteraceae. With in the size and seed production of affected Australia, C. pycnocephalus and C. tenui- the exception of seedlings of Cynara slender thistle stands. This paper reports florus were identified as candidates for a scolymus (globe artichoke) where pustules an objective assessment of the perform- biological control program. The first step developed on cotyledons and the first true ance of P. cardui-pycnocephali as a biologi- in mounting the control program was to leaves, P. cardui-pycnocephali only attacked cal control agent against one (C. tenui- conduct a series of extensive surveys for plants of C. pycnocephalus and C. florus) of its two thistle hosts. natural enemies associated with the spe- tenuiflorus (Chaboudez et al. 1993, P. cies in their home ranges. Those surveys Chaboudez unpublished data). Plant Protection Quarterly Vol.15(1) 2000 15 significance of treatment effects (fungicide treated vs. control) on per cent disease, fe- cundity (as measured by the number of in- florescences) and final dry weight were based on Type III sums of squares. Plot density was used as a covariate. Regres- sion analysis was used to examine the re- lationship between disease severity and dry weight of infected plants. Results and discussion There were substantial differences be- tween the Tasmanian and Victorian sites in terms of plot densities: mean densities were 27.7 and 52.0 m-2 for the two sites re- spectively. As a consequence of their higher density, individual plant dry weights were much lower in both fungi- cide treated and control plots at the Victo- rian site (Figure 2c). With respect to dis- ease levels, there was little difference in 60 the control plots, although fungicide- treated plots at the Victorian site were al- 124 most entirely disease-free. A low level of disease was present in many of the treated 210 plots at the Tasmanian site. There ap- peared to be little difference in number of 74 inflorescences per plant between the two sites. 10 As expected, there was a significant ef- fect of treatment (control vs. fungicide- Figure 1. General areas and number of releases of ‘starter’ samples of treated) on mean levels of disease for both inoculum of two isolates of P. cardui-pycnocephali from October 1993 to the Tasmanian (F=138.2, P<0.001) and Vic- June 1995. torian sites (F=265.1, P<0.001) (Figure 2a). Disease levels in control plots were rela- Materials and methods plants) were harvested approximately tively low being about 3–4%. However, Twenty 1 m2 plots were established in two weeks later in early December. Al- inspection of dried and withered leaves August 1998 in a large stand of C. though not all plots could be harvested, at still attached to plants indicated that dis- tenuiflorus in an improved pasture in both sites equal numbers of treated and ease levels had been higher in the weeks northern Tasmania (147°15'E, 41°04'E). A control plots were assessed. Within each preceding harvest. These differences were month later a second site was set up in a plot, the above-ground biomass of each reflected in significant effects of the fungi- similar manner in south-western Victoria plant was separately harvested and cide treatment on both the number of in- (141°26'E, 37°27'S). At both sites C. bagged. At that time, a visual estimate of florescences (Figure 2b) and plant dry tenuiflorus was the dominant thistle disease severity (per cent disease cover of weights (Figure 2c) at both sites. Mean present; although C. pycnocephalus was green leaves and stems) was also made plant dry weights for infected plants were present in the stands no individuals oc- and recorded.

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