Rust fungi (Basidiomycota, Uredinales) consist of more than 7000 species of obligate plant pathogens that possess some of the most complex life cycles in the Eumycota. Traditionally, a limited number of synapomorphic characters and incomplete life-cycle and hostspecificity data have hampered phylogenetic inference within the Uredinales. The application of modern molecular characters to rust systematics has been limited, and current contradictions, especially in the deeper nodes, have not yet been resolved. In this study, two nuclear rDNA genes (18S and 28S) were examined across the breadth of the Uredinales to resolve some systematic conflicts and provide a framework for further studies of the group. Three suborders of rusts are recovered. Of the 13 rust families most widely accepted, 8 are supported in full or in part (Coleosporiaceae, Melampsoraceae, Mikronegeriaceae, Phakopsoraceae p.p., Phragmidiaceae, Pileolariaceae, Pucciniaceae, Raveneliaceae), 3 are redundant (Cronartiaceae, Pucciniastraceae, Pucciniosiraceae), and the status of 2 (Chaconiaceae, Uropyxidaceae) could not be resolved. The Mikronegeriaceae and Caeoma torreyae are the most basal rusts sampled. It is concluded that morphology alone is a poor predictor of rust relationships at most levels. Host selection, on the other hand, has played a significant role in rust evolution.

Puccinia hemerocallidis and P. funkiae resemble each other morphologically; however, they are biologically and taxonomically distinct, with telia of the former being restricted to species of Hemerocallis and the latter to Hosta species. However, both fungi share a macrocyclic and heteroecious life cycle with Patrinia villosa as the spermogonial and aecial host. An additional microcyclic rust fungus, P. patriniae, is also known on P. villosa. This microcyclic fungus is similar to the two macrocyclic fungi in its telial structure and teliospore morphology. These similarities in morphology and host relationships suggest the three fungi may also share a close evolutionary relationship. To determine the phylogenetic relationships of the three species, a portion of the nuclear ribosomal DNA repeat encoding the ITS and 5.8S subunit regions was amplified by PCR, sequenced, and analyzed. The resulting phylogenetic trees showed that P. hemerocallidis and P. funkiae share a recent common ancestor and that P. patriniae is closely allied with P. hemerocallidis. The results suggest a possible evolutionary derivation of microcyclic P. patriniae from macrocyclic heteroecious P. hemerocallidis, which fits the evolutionary interpretation of correlated species known as Tranzschel's law.

Species of macrocyclic, heteroecious grass rusts often have been defined with wide host ranges and variation in spore morphology. Consequently, some are species complexes and contain genetically distinct forms. Molecular analyses, together with morphological and biological methods, provide powerful means to dissect these complexes. Puccinia coronata is a complex species that has a broad telial host range including more than 45 genera of grasses and a narrow aecial host range. Phylogenetic analysis of nuclear ribosomal internal transcribed spacer (ITS) DNA sequences from 15 aecial and telial collections grouped P. coronata into six distinct clades supporting separation of this complex into four distinct species. Puccinia andropogonis, a common rust of tall prairie grasses in North America, is also a complex species. However, in contrast to P. coronata, P. andropogonis has a narrow telial host range and a broad aecial host range. DNA sequence analysis grouped 15 collections of P. andropogonis into six distinct clades representing at least four distinct species. Speciation of P. coronata appears to have occurred primarily by radiation onto new telial hosts, whereas in P. andropogonis speciation appears to have occurred primarily by radiation onto new aecial hosts.

We analyzed the phylogenetic relationships of 49 specimens comprising 14 morphologically similar species of Pucciniastrum distributed in Japan based on the sequence data of the large subunit rDNA (D1/D2), 5.8S rDNA, and internal transcribed spacer (ITS) regions. Neighbor-joining and parsimony analyses generated six major groups for both the D1/D2 and ITS regions. Pucciniastrum circaeae and P. epilobii formed a single group. P. hydrangeae-petiolaris, P. coryli, P. fagi, P. hikosanense, P. tiliae, and P. boehmeriae were each a distinct clade, and P. fagi formed a close relationship with P. hikosanense. However, these analyses did not support the monophyly of the following species: P. kusanoi, P. actinidiae, P. corni, P. styracinum, P. yoshinagai, and P. miyabeanum.

Ochropsora ariae was found to host-alternate between Anemone pseudo-altaica and Aruncus dioicus var. tenuifolius and Ochropsora nambuana between Anemone flaccida and Elaeagnus multiflora var. hortensis in Japan. Both species produced a sessile, thin-walled, cylindrical probasidium (teliospore), which turned into a four-celled metabasidium by continuous apical elongation of the probasidium. Several probasidia of O. nambuana were produced from a basal basidiogenous cell in a sorus hymenium. Life cycle and probasidium/metabasidium morphology showed the taxonomic identity of Ceraceopsora elaeagni with O. nambuana. Developmental morphology of the basidium found in the two Ochropsora species raised a question against the taxonomic separation of Ochropsora and Aplopsora

The objective of this study was to find features in microcyclic rust fungi (Uredinales) on wild host plants favorable for extension of the natural range of distribution. Puccinia glechomatis, a leptosporic rust fungus and its herbal host Glechoma hederacea (Lamiaceae), both natives to Eurasia and introduced in North America, were used for this study. Although the host has been known from North America since the beginning of the nineteenth century, the rust fungus was first observed there only in recent years. Favorable features were identified by studying the life cycle of the rust, including nuclear conditions and seasonal characteristics as well as its spread in North America. The life cycle was studied macroscopically by inoculation experiments, by various light microscope techniques, and by scanning electron microscopy. The spread of the pathogen and its host were reconstructed by evaluating host plant herbarium specimens and databases, literature, and field study data. The studies on P. glechomatis show that, generally for microcyclic rust fungi, establishment and potential for spread are based on several favorable features of both the host (e.g., synanthropic occurence and dispersal, genetic stability, regeneration of vegetative plant parts) and the rust fungus (asexual reproduction/genetic stability, homothallism, propagation with host plant, formation of both leptospores and thick-walled teliospores).

Nine species of rust fungi (Uredinales) were found during a 1-day field study in an Acacia koa Metrosideros polymorpha woodland in Volcanoes National Park on Big Island, Hawaii. Two species, both found on Acacia koa, are native (endemic) rusts whereas 7 species are nonnative on nonnative hosts, highlighting the high proportion of introduced species in the Hawaiian flora even in more or less natural habitats. One species, Uromyces linearis on Panicum repens, constitutes the first record of this rust for the Hawaiian archipelago, bringing the total to 93 species on the islands, 70 (75.3%) of which are introduced. The species records are annotated with emphasis on the geographic origin of each taxon. In addition, the study has led to the molecular reevaluation of the genus Racospermyces, indicating that it is synonymous with Endoraecium, and six new combinations are proposed for the species previously placed in Racospermyces. The high number of introduced species in Hawaii coupled with the paucity of native species when compared to other global regions is discussed.