RESUMO
In September 2007 at Masagua, Escuintla Department, Guatemala, uredial lesions that appeared different from those of brown rust were observed on a sugarcane (a complex hybrid of Saccharum L. species) cultivar (CP 72-2086) considered resistant to brown rust caused by Puccinia melanocephala Syd. & P. Syd. Samples were sent to the USDA-ARS Systematic Mycology and Microbiology Laboratory in Beltsville, MD for identification. Observed morphological features were consistent with P. kuehnii E.J. Butler and appeared similar to orange rust samples obtained from Florida in July (2). Uredinial lesions were hypophyllous, orange, and variable in size measuring 650 to 850 × 26 to 32 µm. Urediniospores were mostly obovoid to pyriform or broadly ellipsoidal, variable in size, 32 to 45 × 25 to 30 µm, and moderately echinulate with spines evenly distributed, 3 to 5 µm apart. Urediniospore walls were orange-to-light cinnamon brown, 1 to 2.5 µm thick with a pronounced apical wall and four to five equatorial pores. Telia and teliospores were not observed. The nuclear large subunit rDNA region of the rust infecting cv. CP 72-2086 (BPI 898289, GenBank Accession No. EU344904) and the ITS1, 5.8S, and ITS2 rDNA regions (GenBank Accession No. EU543434) were sequenced (1,3). DNA sequences matched sequences of P. kuehnii in GenBank and were distinct from known sequences of P. melanocephala available in GenBank (3). Thirteen cultivars were rated as to their relative resistance using severity of orange rust symptoms; CG 96-59, CG 96-135, CP 72-1312, CP 73-1547, and CP 88-1165 were resistant; CG 96-40, CG 98-121, CP 72-2086, CP 88-1508, and CP 89-2143 were intermediate; and CG 96-52, CG 98-0115, and SP 79-2233 were susceptible. Orange rust was previously reported in Florida (2), but to our knowledge, this is the second report of its occurrence in the Western Hemisphere. References: (1) M. C. Aime. Mycoscience 47:112, 2006. (2) J. C. Comstock et al. Plant Dis. 92:175, 2008. (3) E. V. Virtudazo et al. Mycoscience 42:447, 2001.