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Common ragweed, Ambrosia artemisiifolia, is a highly allergenic North American plant that has become invasive in some parts of Europe, Asia and Australia following its introduction to many places in the world. Some earlier works suggested that a microcyclic autoecious rust fungus, Puccinia xanthii, known to infect A. artemisiifolia in the USA only, can be considered as a potential classical biocontrol agent (BCA) of this noxious weed in Europe and elsewhere. However, an extensive field survey did not reveal the presence of either P xanthii or any other rusts on common ragweed in 14 US states and two Canadian provinces in 2002 and 2003. Moreover, P xanthii infecting A. artemisiifolia has never been recorded in Canada, although it is known to occur on A. trifida and Xanthium spp. there. Nevertheless, herbarium specimens collected between 1855 and 1963 in five states of the USA confirmed the presence of P xanthii on A. artemisiifolia. It is concluded that currently P xanthii cannot be regarded as a promising BCA of A. artemisiifolia, although it did occur on common ragweed at least a few decades ago in the USA and some forms of this rust species have already been evaluated as effective BCAs of Xanthium in Australia.
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