with fungi (Watson 1991; Hajek 2004). Rush skeletonweed is native to Mediterranean Europe and was a problematic weed in Australia until the deliberate introduction of the rust fungus as a control agent. Introduction of the fungus was followed by a rust disease epidemic, which caused widespread reductions in rush skeletonweed plant height, flower numbers, and viable seed production. Rush skeletonweed mortality was over 90% in areas of Australia where the climate was most favorable for infection (Cullen et al. 1973; Cullen 1985). Another successful example of the classical approach is the introduction of Uromycladium tepperianum, a rust fungus from Australia, into South Africa to control the invasive tree Acacia saligna (Morris et al. 1999). This fungal pathogen, which causes severe gall formation and tree death, has reduced A. saligna populations by 90-95% throughout the plant’s range in South Africa; successful results were evident within 8 years after the release of the fungus (Morris 1997; Wood and Morris 2007).

In the United States, three rust fungi have been released during the last 4 decades to control invasive weeds: Puccinia chondrillina to control rush skeletonweed, P. carduorum for musk thistle (Carduus thoermeri), and P. jaceae var. solstitialis for yellow starthistle (Centaurea solstitialis) (Baudoin et al. 1993; O’Brien et al. 2010). P. jaceae var. solstitialis was found to be ineffective in controlling its target weed. P. chondrillina and P. carduorum were found to have caused damage to their target species (reduced plant density and reduced seed production), but these observations were made in field sites where insect species had also been introduced (Supkoff et al. 1988; Baudoin et al. 1993; O’Brien et al. 2010).

TABLE 2-1 Classical vs Bioherbicide Approach

Feature Classical (Inoculative) Bioherbicide (Inundative)
Target plants Exotic, aggressive, widespread, generally perennials Native or naturalized, generally annuals
Biological control agents Exotic arthropods and pathogens Locally occurring pathogens (a few insects)
Habitats Marginal lands (such as rangelands) and natural systems Intensive croplands
Host specificity Very narrow (monospecific) May be broader
Delivery Small (inoculative) population released Mass-reared and large inundative applications (releases)
Establishment Several years to become established Immediate effect
Control objective Sustained long-term control Transient nonsustainable control
Approach Ecological Technological

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