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First Report of Orange Rust Caused by Puccinia kuehnii in Sugarcane in Louisiana

In June 2012, lesions typical of rust were observed on sugarcane cultivar Ho 05-961 (a complex hybrid of Saccharum L. spp.) on a farm near Schriever, Louisiana. Incidence and severity of disease symptoms were low. Two types of pustules were observed on leaves of the infected plants. One pustule type was reddish-brown in color turning brown with age, characteristic of brown rust which has been observed in Louisiana since 1979 (2). The other pustule type was orange and did not turn brown with age. Urediniospore samples from the two pustule types were collected. The morphology of the urediniospores from the reddish-brown pustules was consistent with that described for Puccinia melanocephala Syd. & Syd., the fungus that causes brown rust of sugarcane, while the morphology of the urediniospores from the orange pustules was consistent with those described for P. kuehnii E.J. Butler, the causal organism of orange rust of sugarcane (3). Telia and teliospores were not observed. The identity of the two species of Puccinia causing the brown and orange rust lesions was verified using the species-specific quantitative PCR assays (1). Two DNA samples extracted from the pustules identified as P. kuehnii were independently subjected to PCR amplification using primers Pk1F and Pk1R (1) to yield a product from the rDNA that was then bidirectionally sequenced using the same primers. The resulting 480-nt sequences were identical to each other, and a BLAST search of GenBank revealed 100% identity to 19 previously reported isolates of P. kuehnii but not more than 89% similarity to any isolate of P. melanocephala (4). To our knowledge, this is the first report of orange rust in Louisiana. In the 4 months following the detection of orange rust, observations of the disease have been limited to Ho 05-961. Seed cane increase plots of this newly released cultivar were surveyed, and orange rust symptoms and urediniospores were detected in 17 of 38 (45%) fields. The incidence and severity of the disease remained low, and the distribution appeared to be limited to the southern portion of the Louisiana sugarcane production area.

References: (1) N. C. Glynn et al. Plant Pathol. 59:703, 2010. (2) H. Koike. Plant Dis. 64:226, 1980. (3) C. C. Ryan et al. Page 189 in: Diseases of Sugarcane: Major Diseases. C. Ricaud et al., eds. Elsevier, Amsterdam, 1989. (4) E. V. Virtudazo et al. Mycoscience 42:447, 2001.