The rust fungus Phragmidium tuberculatum Jul. Müll. is a common pathogen on Rosa spp., on which all life cycle stages are formed. Symptoms occur in spring and may include distorted stems, yellow spots on the upper leaf surface, and a bright orange spore mass formed on the abaxial leaf surface. In late summer, sori become speckled with black as fascicles of teliospores develop. The current known distribution of P. tuberculatum is mostly limited to Europe with some occurrence in Asia and into Australasia (2). There is some documented occurrence in North America (Alaska, Connecticut, and Canada ), where most rose rust disease is attributed to P. mucronatum (Pers.) Schltdl. This study used a combination of molecular and morphological analyses on newly collected material from across North America (California: BPI877978, PURN7783; Oregon: BPI877980; Massachusetts: BPI877977; and Quebec: BPI877979) and herbarium material from South and Central America (Honduras: BPI864186; and Argentina: BPI843677; both previously identified as P. mucronatum) to document a much broader distribution of P. tuberculatum. Collectively, teliospores from these collections are 4 to 6 celled, dark to black-brown, warted, elongated to cylindrical, 64.7 to 92.4 μm in length by 23.1 to 39.3 μm in width (average 77.6 × 30.0 μm) (30 teliospores from 2 leaves), with 2 to 3 pores/cell and a pronounced hyaline apiculus 4.6 to 18.5 μm long (average 8.3 μm). P. tuberculatum is similar morphologically to P. mucronatum, but sensu Gäumann (3) differs in having wider (30 to 36 μm) and longer (65 to 110 μm) teliospores with an average of 6 to 8 cells/spore. However, the two are easily distinguished by DNA analyses (4). The 28S sequences were amplified using the protocols described in Aime (1) and compared phylogenetically to 28S sequences available in the GenBank database for P. tuberculatum, P. mucronatum, and other Phragmidium spp. (4). In a maximum likelihood analysis, all isolates formed a 99% bootstrap supported clade with P. tuberculatum sequences from Germany, and shared 100% sequence identity with JF907675 P. tuberculatum. In contrast, comparison with HQ421646 P. mucronatum produced only 92% identity (e.g., 836/911 bp for PURN7783). This information indicates that P. tuberculatum is likely to be widespread in the Americas but simply misidentified as P. mucronatum, as was found to be the case for the two herbarium specimens sampled. Detailed examination of historical herbarium material may help to pinpoint how long the fungus has been present and the current extent of its distribution. The rose rust fungus is not considered to be a problem economically, but its spread within North America may be an indicator of commercial practices that serve as a vector for other diseases on ornamental plants. Voucher specimens have been deposited in the U.S. National Fungus Collections (BPI) and Arthur Fungarium (PUR); voucher sequences are deposited in GenBank (Accession Nos. KJ841917 to 23).
References: (1) M. C. Aime. Mycoscience 47:112, 2006. (2) J. F. Arthur. Manual of the rusts in United States and Canada. Purdue Research Foundation, 1934. (3) E. Gäumann. Die Rostpilze Mitteleuropas mit besonderer Berücksichtigung der Schweiz. Büchler, Bern, 1959. (4) C. M. Ritz et al. Mycol. Res. 109:603, 2005.