First Report of Puccinia kuehnii causing Orange Rust of Sugarcane in Texas, USA

In April 2017, uredinial lesions typical of orange rust caused by Puccinia kuehnii E.J. Butler were observed on the experimental sugarcane clone HoCP 14-801 (a complex hybrid of Saccharum L. spp.) in a cultivar trial in Hidalgo County, Texas. Additional observations were made in cultivar trials at three other sites in Hidalgo, Cameron, and Willacy Counties during the following 5 months of 2017. Orange rust symptoms were observed on 12 experimental clones and one commercial check cultivar, CP 89-2143, at one or more of the three trial locations. Cultivar trials were embedded in commercial fields. Uredinial lesions were orange and linear, measuring 650 to 850 × 26 to 32 µm. The orange to light cinnamon brown urediniospores were mostly obovoid or pyriform and variable in size (30 to 60 × 25 to 30 µm). Walls were 1 to 2 µm thick on the sides with a pronounced apical thickening of 5 µm or more and were echinulate with evenly distributed spines 2 to 4.5 µm apart. These morphology features were consistent with those described by Ryan and Egan (1989) and others for Puccinia kuehnii. Telia and teliospores were not observed. Leaf samples containing rust lesions were collected from cultivar CP 89-2143 and experimental clones HoCP 14-801, HoCP 14-874, and CP 11-2013. Identification of the species causing the orange rust lesions was verified by PCR assays conducted independently in two laboratories (Sugarcane Research Unit, ARS, USDA, Houma, LA, and Soybean/Maize Germplasm, Pathology, and Genetics Research Unit, USDA-ARS, Urbana, IL). Primers PkPmF and PkPmR were used that amplify fragments of 606 bp from P. kuehnii and 585 bp from P. melanocephala, the causal agent of brown rust of sugarcane (Glynn et al. 2010). Only the 606-bp fragment corresponding to P. kuehnii was amplified from each sample. No fragments were amplified when a PCR analysis with primers AR-F2 and AR-R1, which amplify Macruropyxis fulva, the pathogen causing tawny rust on sugarcane (Martin et al. 2017), was conducted on the samples. Sequences of the fragments were identical to each other, and a BLAST search of GenBank revealed 100% identity to previously reported isolates of P. kuehnii including an isolate from Florida, U.S.A. (GenBank accession no. EU176008). Sequences from P. kuehnii infecting CP 11-2013 (GenBank accession no. MN547621) and HoCP 14-874 (GenBank accession no. MN547622) were submitted to GenBank with voucher collection maintained at the USDA, ARS, Sugarcane Research Unit in Houma, LA. This is the first report of orange rust in the sugarcane production area of Texas. In 2018, approximately 50% of the sugarcane production area in Texas was planted with cultivar CP 89-2143. Although this cultivar was known to be susceptible to orange rust, it was introduced and expanded prior to the observation of orange rust in the industry. Orange rust was observed in commercial fields of cultivar CP 89-2143; however, disease severity was less than that observed in some experimental clones. In Florida, overall losses from orange rust have been estimated to be 10% with reductions as high as 43% in a highly susceptible cultivar (Rott et al. 2014). As researchers attempt to identify new sugarcane cultivars that are adapted to the Texas production area and provide diversity among the cultivars, susceptibility to orange rust must be considered.