Puccinia modiolae, a rust fungus pathogen of Carolina bristlemallow, Modiola caroliniana (Malvaceae), is newly reported from North America, appears to be well established along the Gulf coast and is likely to have been introduced from South America. Its taxonomy, distribution and natural host range are discussed and a lectotype designated for this species. Malva sylvestris and Alcea rosea are reported as new hosts for the rust. Additional new records for Malvaceae rusts are made for P. modiolae on Alcea rosea from Brazil, P. heterospora on Herissantia crispa in Florida and P. heterogenea on Malva sp. in Peru. Finally, an identification key for the microcyclic Puccinia species on members of Malvaceae in North America is provided.

Neomycetes, Phytopathogens, Pucciniales , Uredinales

Neomycetes are alien fungi entering a new area (country or continent), typically as a result of non-intentional human activity, that become established in the new region (Kreisel and Scholler 1994, Negrean and Anastasiu 2006). The most common origin for alien species of rust fungi in the USA appears to be South and Central America. In many cases, the pathogens are introduced concurrently with their host species, e.g. on crop plants, ornamentals or weeds.

Puccinia modiolae P. Syd. & Syd. (Pucciniaceae, Pucciniales) is a microcyclic rust fungus that was originally reported on Modiola prostrata A.St.-Hil. (=M. caroliniana (L.) G. Don; Malvaceae) from South America on the basis of specimens from Argentina and Uruguay (Sydow and Sydow 1904). Modiola caroliniana is the only species in the genus Modiola, grows in disturbed vegetation and at forest margins and flowers in all seasons (Kearney 1951, Fryxell 1988). Modiola caroliniana is believed to be native to northern Argentina and the Paraná basin of South America and probably came to the USA from southern South America in wool or cotton (Hanes 2015). Today, it is widely distributed as a weed in warmer parts of the world and is naturalised from the southern United States to northern Argentina including the West Indies. Despite the wide distribution of M. caroliniana, its parasitic rust, P. modiolae, has only been reported from Argentina and Uruguay (Lindquist 1982).

In this study, we examine numerous fresh collections and herbarium materials and conduct phylogenetic analyses of the 28S rDNA locus to provide the first reports of P. modiolae from North America, discuss its host range and distribution and establish a lectotype for this taxon. A key to the microcyclic Puccinia species on Malvaceae in North America is provided.

Materials studied here were obtained from the Arthur Fungarium (PUR), the U.S. National Fungus Collections (BPI) and from fresh collections (listed in specimens examined below). Voucher specimens for new material are deposited in PUR. Rust spores and cross sections were routinely mounted in lactic acid in glycerol. Light microscopic analyses were performed using a Nikon Eclipse 80i microscope. Photomicrographs were obtained with a DS-Fi1 Nikon camera. In all studied specimens, thirty spores were randomly selected and measured.

DNA was extracted and the 5’ end of the nuclear 28S rDNA, amplified with rust-specific primers and sequenced following previous published protocols (Aime 2006, Aime et al. 2018). Sequences were edited using Sequencher 5.2.3 (Gene Codes Corp., Ann Arbor, MI) and aligned using the MUSCLE algorithm in Geneious 9.1.5 (Biomatters Ltd., Newark, NJ). Additional sequences of Puccinia species on Malvaceae were included for context from the studies of Aime (2006), Demers et al. (2015) and McTaggart et al. (2016). Phylogenies were reconstructed using maximum likelihood in RaxML v.2.2.3 via the CIPRES portal (Miller et al. 2010). Trees were visualised in FigTree v1.4.2 (http://tree.bio.ed.ac.uk/software/figtree/) and edited in Inkscape v2 (Free Software Foundation Inc., Boston, MA). Newly generated sequences are deposited in GenBank, accessions MH742974–MH743006.

Study of recently collected materials of malvaceous plants from Texas, Louisiana and Indiana revealed the widespread presence of Puccinia modiolae along the Gulf coast on Modiola caroliniana and occurring as far north as Indiana on new hosts Alcea rosea L. and Malva sylvestris L. Examination of herbarium material also reveals P. modiolae as far south as Brazil on A. rosea (PUR N15322). Additional new records for Malvaceae rusts are made for P. heterospora on Herissantia crispa in Florida and P. heterogenea on Malva sp. in Peru. In total, we generated 28S rDNA sequences for 32 collections of Puccinia species on Malvaceae, including ten collections of P. modiolae for phylogenetic analyses (Fig. 1); all sequences of P. modiolae shared 100% identity across the locus.

Figure 1. 

Maximum likelihood tree, based on 28S sequences, of Puccinia species on Malvaceae. Sequences newly generated for this study indicated in bold type. Numbers at nodes represent bootstrap support values. Pucciniosira pallidula was used as outgroup for rooting purposes.

Phytoparasitic Neomycetes have the potential to cause great losses across the world via infestation of crops, ornamental plants and native flora (Scholler and Aime 2006). Introduction of alien phytoparasitic fungi also has ecological consequences which have been little investigated (Scholler 1999). There is no updated list of Neomycetes in the United States. However, alien rust fungi have had conspicuous economic and ecological consequences in North America. Here we report another introduced rust fungus, P. modiolae, as a new neomycete in the USA.

Pennington (1903) was the first to realise the difference between rust populations on Modiola compared to those on other members of the Malvaceae. He named the Puccinia species on Modiola as P. malvacearum var. modiolae, based on material collected from Río Paraná, Argentina. Sydow and Sydow (1904) described the rust population on Modiola as a separate species based on different material (syntype) collected from Argentina and Uruguay, but designated no holotype for the species. They later considered P. malvacearum var. modiolae as a synonym of P. modiolae in the appendix of their book (appendix to the first volume of Monographia Uredinearum, p. 892). Our phylogenetic analyses show P. modiolae and P. malvacearum are distinct species (Fig. 1); designation of a lectotype and isolectotype are made herein to stabilise the taxonomy for this species.

Puccinia modiolae is a native rust fungus of South America and was most likely introduced in the USA by accompanying its host plant Modiola. The rust species is quite common on Modiola caroliniana in Louisiana and was also found in Texas, making the Gulf coast a likely site for the original introduction of the rust species in North America. We are unable to pinpoint when P. modiolae was introduced into the USA. However, we were unable to locate any historical North American herbarium material of P. modiolae in BPI or PUR, nor were we able to find records of any rust species on Modiola in the USA, Canada or Mexico in all available literature, making it likely that P. modiolae became established in the southern USA probably no earlier than the second half of the 20^th century. Before the present study, P. modiolae was only known from Argentina and Uruguay. In Argentina, Althaea officinalis L., Lavatera arborea L. and Malva parviflora L., in addition to M. caroliniana, have been reported as the natural host range of the rust species; only M. caroliniana is a reported host in Uruguay (Lindquist 1982). We have identified Alcea rosea and Malva sylvestris as new hosts for this rust species, ranging from southern Brazil to the upper Midwest USA.

The presence or absence of spermogonia is one of the morphological features for distinguishing microcyclic rust fungi on Malvaceae members (Lindquist 1982). Our study revealed that this feature is stable and meaningful for separating Puccinia spp. on Malvaceae. All studied specimens of P. modiolae in this research produced spermogonia in close connection to telia (Fig. 4). Eight microcyclic Puccinia species have been reported on Malvaceae in North America thus far.

MCA gratefully acknowledges funding from the National Science Foundation (CSBR program: DEB-1458290; TCN program: DEB-1502887) for the Purdue University herbaria, without which this work would not have been possible. This work was also supported by the USDA National Institute of Food and Agriculture Hatch project 1010662.