Definitive identification of orange rust of sugarcane caused by Puccinia kuehnii in Cuba

In November 2008, leaf lesions, pale yellow to yellow-orange, sometimes coalescing in large necrotic areas (Fig. 1), were observed on hybrid sugarcane clones (Saccharum spp.) CSG 86-504, CSG 24-92, CSG 204-92 and CC 87-409 at the Sugarcane Research Institute (INICA) Station in Villa Clara province, in central Cuba. Specimens were submitted to the Central Plant Quarantine Laboratory for identification. The observed morphological features were consistent with those described for Puccinia kuehnii (Virtudazo et al., 2001; Comstock et al., 2008). Uredinial lesions, variable in size (3-4 mm in length), were generally light brown in colour, but occasionally yellow-orange to cinnamon brown (Fig. 2). Hypophyllous uredinia were ellipsoidal to fusiform in shape, distinctly lighter and less uniformly distributed than for P. melanocephala. Urediniospores (Fig. 3) were mostly obovoid to pyriform, almost ellipsoidal in shape, variable in size, 27.5-55.0 (41.9) x 17.5-37.5 (26.0) μm, echinulate with evenly distributed spines (Fig. 4A and B). Walls (orange to light cinnamon brown in colour) were 1-2.5 μm thick, often with a pronounced apical thickening, 3-7 μm (mode 5 μm). Four to five equatorial pores were present. In contrast to P. melanocephala, paraphyses were not visible in stereoscopic views of the uredinia and were absent in most specimens. Telia and teliospores were not observed.

Total DNA isolation from symptom-bearing leaves with uredinia was performed according to Lin et al. (2001). A real time PCR was conducted using the SYBR green technique with primers synthesized from exact sequences of ITS1, 5.8S, and ITS2 of the large subunit rDNA region reported in GenBank Accessions EU164549 and EU176009 (Comstock et al., 2008). Results confirmed the identification as P. kuehnii. Previous reports of P. kuehnii in Cuba (CMI, 1969) were based on misidentifications of P. melanocephala. To the authors’ knowledge, P. kuehnii has not been reported by any diagnostic laboratory in Cuba in the last 30 years.In addition, Cuba was not included in the latest P. kuehnii distribution map (CAB International, 2007). Therefore, this is the first confirmed report of the presence of P. kuehnii in Cuba .The disease has since been found in different localities in the central and eastern regions of Cuba , without any reported economic losses to date.