Abstracts: Theme 3 – Risk analysis Argentinian fungi for Bathurst burr fail preliminary host-specificity tests Louise Morin,1 Bruce Auld,2 Harry Evans,3 Carol Ellison,3 Robert Reeder,3 Freda Anderson4 and Bill Pettit4 1Cooperative Research Centre for Australian Weed Management, CSIRO Entomology, GPO Box 1700, Canberra, ACT 2601, Australia 2Orange Agricultural Institute, NSW Agriculture, Forest Road, Orange, NSW 2800, Australia 3CABI-Bioscience, Silwood Park, Buckhurst Road, Ascot, SL5 7TA, UK 4CSIRO Entomology, c/o CERZOS, Universidad Nacional del Sur, CC 738 (8000), Bahia Blanca, Argentina Xanthium spinosum (Bathurst burr) is a widespread summer annual weed in rangeland, pasture and crops in eastern Australia. Prospects for classical biological control of this weed were investigated in the 1990s by carrying out a series of surveys for fungal pathogens attacking X. spinosum in Argentina, the putative country of origin of this plant. The powdery mildew Erysiphe cichoracearum and the facultative parasite Cercospora xanthicola were the most frequently recovered pathogens and were widely distributed within the regions of Argentina surveyed. Significant damage was associated with the presence of E. cichoracearum, which sporulated profusely on both leaf surfaces, stems and shoots. Infection by C. xanthicola was spectacular at several sites in northern Argentina in March 1995, but it appeared that the pathogen was favoured by humid environmental conditions. In following surveys, infection was scattered, restricted to lower leaves and rarely damaging. The pathogenicity of isolates of E. cichoracearum was tested on Bathurst burr, other Xanthium spp. and a selection of species from related genera of Asteraceae. Severe infection and heavy sporulation developed on Bathurst burr plants while the other Xanthium spp. developed only mild symptoms. All other Asteraceae tested proved to be immune or resistant, but three of the eight sunflower cultivars tested became heavily infected. The most aggressive isolate of C. xanthicola tested in the laboratory produced necrotic lesions on Bathurst burr that expanded to cover most of inoculated leaves within 3 weeks. However, no stem lesion ever developed and plants recovered rapidly. The fungus required a minimum of 2 days under humid conditions to infect plants. All sunflower cultivars tested were susceptible to the pathogen and developed necrotic lesions. The lack of specificity of E. cichoracearum and C. xanthicola militates against their possible use as biological control agents for Bathurst burr in Australia. Biological control of saffron thistle with fungi: limited prospects Louise Morin1 and Mireille Jourdan2 1Cooperative Research Centre for Australian Weed Management, CSIRO Entomology, GPO Box 1700, Canberra, ACT 2601, Australia 2 CSIRO European Laboratory, Campus International de Baillarguet, 34980 Montferrier-sur-Lez, France Carthamus lanatus (saffron thistle), a native of the Mediterranean region, affects livestock, pasture and grain production throughout temperate and subtropical Australia. Herbicides are effective in controlling this weed, but the cost of this control method prohibits its use over the vast areas infested. Biological control, if successful, is likely to be the only solution to effectively manage saffron thistle populations in Australia. Following surveys carried out in Greece, two pathogens, Puccinia sommieriana (microcyclic rust) and Septoria centrophylli (facultative parasite), were identified as potential candidates for classical biological control of saffron thistle. A preliminary study was conducted to determine the susceptibility to these pathogens of Australian accessions of saffron thistle and cultivars of the closely related crop, safflower (Carthamus tinctorius). All isolates of Puccinia sommieriana tested produced, within 5–6 days after inoculation, small chlorotic flecks on leaves of all Australian accessions of saffron thistle tested. Flecks had developed into mature telia by 14 days after inoculation. However, the rust also infected leaves, bracts and stems of safflower cultivars and developed mature telia within the same time frame. 351