whole leaf clearing and staining technique used in specificity studies of rust fungi is described. The method is effective on a large number of plant/rust fungus associations.

The native flora of Re´union Island in the Indian Ocean is threatened by invasive woody plants introduced for agriculture, forestry or as ornamentals. We reviewed archives to identify woody plants introduced since colonisation and ranked the biological impact of the main non-indigenous species on native vegetation. The relationship between cumulative number of non-indigenous plants and population follows an s-shaped curve, but the rate of introduction was mainly determined by historical periods with five main introduction phases identified. A total of 318 introduced woody species were recorded with 132 identified as naturalized in natural ecosystems. Of these, 26 of the more serious invasive species (i.e. having a large biological impact on the new environment) were selected and ranked by biological impact to native ecosystems. Hiptage benghalensis, Ligustrum robustum var. walkeri, Acacia mearnsii, Ulex europaeus and Psidium cattleianum were identified as having the highest invasiveness capacity and the greatest impacts on succession and utilization of resources in natural areas. A strategy is required to manage highpriority invasive species and reduce the rate of nonindigenous plant introductions to the island.

S U M M A R YThe host specificity of the rust fungus Phragmidium violaceum, a potential biological control agent of European blackberry (Rubus fruticosus) was studied by inoculating a mixture of 15 isolates of the rust on 108 plants of importance to the Australasian region. A scale of infection types was developed based on the results of microscopic and macroscopic observations of the reaction of host and non-host plants to the rust. The results showed that P. violaceum has a limited host range in the genus Rubus. The rust was able to reproduce on 17 taxa of Rubus previously unrecorded as hosts, including Australasian species of Rubus subgenera Dalibarda and Lampobatus. All other taxa attacked were species of Rubus subgenus Eubatus and the majority were hybrid cultivars containing European blackberry species.

SUMMARY Phragmidium violaceum is a rust fungus with potential for the biological control of European blackberry (Rubus fruticosus) in Australia. The collection, selection, purification and multiplication in Europe of isolates of the rust is described. Species of European blackberry naturalised in Australia showed different levels of susceptibility when inoculated with a pool of 15 isolates highly pathogenic to Rubus procerus. Selection of individual isolates on the four most widespread blackberry species showed that only two of the isolates would be required to obtain the best attack on these four species.

Willows (Salix spp.) are Weeds of National Significance in Australia where a large number of taxa are naturalised in temperate regions and can cause serious environmental degradation of riparian and wetland habitats. Several species are of economic or ornamental value and conflicts of interest could arise with planning their suppression. Biological control of six willow species (S. alba L., S. cinerea L., S. fragilis L., S. nigra Marshall, S. viminalis L., S. x rubens Schrank) is under consideration in Australia using three approaches based on targeting different morphological structures. A rich phytophagous biota associated with Salix in their regions of origin indicate a range of organisms are available for development as biological control agents. Implementation strategies will depend on the host species targeted for biological control, their reproductive strategies, the level of control required to reduce environmental impacts and the level of conflict of interest associated with a biological control program.