First Report of Orange Rust of Sugarcane Caused by Puccinia kuehnii in Ecuador.

Abstract

Orange rust, Puccinia kuehnii (W. Krüger) E.J. Butler, is an important disease of sugarcane (complex hybrid of Saccharum L. species) that causes up to 53% yield loss (3), and can eliminate sugarcane clones in breeding programs. Initially confined to the Asia-Oceania region, P. kuehnii was reported in Florida in June 2007 (2) followed by confirmation in Central and South America. Orange rust pustules were observed on August 5, 2011, in commercial sugarcane fields located in the Ecuadorian Pacific coast of South America. Pustules were observed on cultivar SP79-2233 and sugarcane clones of the CINCAE breeding program (EC06-351, EC06-340, and EC01-744). Low levels of disease incidence and severity were observed in the sugarcane germplasm. Observation under a light microscope showed typical irregularly echinulate urediniospores that were pale in color with thickened apices and paraphyses inconspicuous to absent, such as those reported to be P. kuehnii (4). DNA of urediniospores were extracted and amplified using Pk1F and PK1R qPCR primers (5). Additionally, the 28s large ribosomal subunit DNA was sequenced (1), resulting in a qPCR and 100% sequence identity with a partial sequence of the P. kuehnii 28S ribosomal RNA gene, accession GU058010 (932/932 base pairs, GenBank Accession No. KF202306). Based on urediniospore morphology, DNA amplification, and sequence analysis, the causal agent of the rust observed in Ecuador was confirmed to be P. kuehnii. Commercial varieties have not yet shown symptoms of infections. However, a survey conducted in 2011 and 2012 showed an increase of disease severity from 3% to 28% in the susceptible cv. SP79-2233. Disease symptoms were evident from stalk growth to maturity (7 to 12 months), especially at the beginning of the harvesting season. To our knowledge, this is the first report of the presence, distribution, and disease spread by the sugarcane orange rust pathogen P. kuehnii in Ecuador.