Puccinia tanaceti : specialist or generalist ? George NEWCOMBE Department of Forest Resources, University of Idaho, Moscow, ID 83844-1133, USA. E-mail : georgen@uidaho.edu Received 17 February 2003; accepted 30 April 2003. Rust is reported for the first time in North America on tansy (Tanacetum vulgare), a widespread and invasive Eurasian plant that was introduced into North America in the 17th century. Morphologically, the rust fungus corresponds exactly to Ga¨umann’s description of Puccinia tanaceti, and to European specimens of P. tanaceti on T. vulgare. An inoculation study confirmed the narrow host specialization of P. tanaceti that Ga¨umann described, in that T. vulgare was successfully inoculated whereas four other species of the tribe Anthemidae of the Asteraceae were entirely resistant, i.e. Leucanthemum maximum (Shasta daisy), L. vulgare (ox-eye daisy), Artemisia absinthium (wormwood), and A. tridentata (sagebrush). This confirmation of a specialized P. tanaceti invalidates P. tanaceti s. lat. The latter complex has resulted from synonymies, and compilation of host records of rust fungi resembling P. tanaceti on at least 105 species representing 19 genera from six tribes of the Asteraceae from both New and Old Worlds. INTRODUCTION Do some rust fungi have host ranges broad enough that they could be considered ‘generalists’? This ques- tion may seem unusual since rust fungi are generally thought to be narrowly specialized parasites of plants. Nevertheless, apparent ‘ generalists ’ have resulted from the practice of compilation of host range from host records in herbaria. One apparent generalist is Puccinia tanaceti. Records compiled by the US National Fungus Collection show that P. tanaceti s. lat. has become a rust fungus that one could expect to find on any species in the tribe Anthemidae of the Asteraceae (Farr et al. 2002). Among the 108 genera of the Anthemidae are Tanacetum, Arte- misia, Achillea, Chrysanthemum, and Anthemis itself ; the species total for the tribe is at least 1741 (Bremer & Humphries 1993). Morphology-based determinations of P. tanaceti on many species in the Anthemidae and other tribes of the Asteraceae were largely sanctioned by Cummins (1978) who synonymized P. chrysanthemi and P. absinthii with P. tanaceti, even though others (e.g. Punithalingam 1968) had not. Arthur (1962) had earlier synonymized P. similis with P. absinthii, and Wilson & Henderson (1966) added P. artemisiella to the synonymy and simply united the rusts on Artemisia and Chrysanthemum since ‘there are not clear mor- phological differences between them. ’ The process of indirect compilation of host range has even spilled beyond the boundaries of Anthemidae because records of P. tanaceti on five other tribes of the Asteraceae (i.e. Astereae, Helenieae, Heliantheae, Lactuceae and Vernonieae) are common in the US National Fungus Collection (BPI). The implications of this process of indirect compilation of host range are that P. tanaceti s. lat. possesses a host range that could potentially total in the thousands of host species. However, there is an opposing view of P. tanaceti. Based on direct, host-range determinations or inocu- lations, Mayor (1946) and Ga¨umann (1959) believed P. tanaceti to be narrowly specialized. Mayor and then Ga¨umann performed experiments in which P. tanaceti s. str. was successfully inoculated onto its type host, the Eurasian Tanacetum vulgare. At the same time, inocu- lations of T. balsamita, Chrysanthemum cinerar- iaefolium, C. corymbosum, C. inodorum, C. parthenium and C. segetum resulted in no rust (i.e. these species appeared resistant). T. vulgare, the type host of P. tanaceti s. str., pos- sesses a native distribution in Europe and western Asia (Bremer & Humphries 1993). But through deliberate and accidental introduction, T. vulgare has become one of the most widespread plants in the world. In North America it is found across Canada and in all of the 48 contiguous states except Texas and a few south-eastern states. It was deliberately introduced in gardens of the Mycol. Res. 107 (7): 797–802 (July 2003). f The British Mycological Society 797 DOI: 10.1017/S0953756203008025 Printed in the United Kingdom.