Orange rust of sugarcane caused by Puccinia kuehnii was detected in Florida in 2007 (1). It was hypothesized that the pathogen originated from Africa because brown rust of sugarcane (synonym common rust) was introduced to the Western Hemisphere from Africa (3). Requests for rust-infected sugarcane samples were made to several western and central African countries to investigate if orange rust of sugarcane was present but as yet undetected. Orange rust had not previously been reported from western or central Africa. At Zuénoula, Ivory Coast in July 2009, symptoms of sugarcane rust were observed on cvs. SP 71-6180 and Co 997 and appeared distinct to those of brown rust of sugarcane. A year later (May 2010), rust-infected specimens of SP 71-6180 and Co 997 from the same location and also from Borotou in Ivory Coast were sent to the USDA-ARS Systematic Mycology and Microbiology Laboratory in Beltsville, MD for identification. Also in May 2010, sugarcane rust was observed at Mbandjock and Nkoteng in Cameroon on cvs. D 88172, FR 87482, and RB 72-454 and on breeding clones RCmr 08/319 and RCmr 08/1121. All specimens had orange uredinial lesions that ranged from 0.6 to 6.5 mm × 200 to 300 μm and were ellipsoidal to elongate. Urediniospores were consistent with P. kuehnii E.J. Butler observed on specimens from Florida (1). DNA isolated from all samples was successfully amplified with P. kuehnii specific primers targeting ITS1 of rDNA (2). The nuclear large subunit region of rDNA of the rust specimens from Ivory Coast (BPI 881015–881017, GenBank Accession No. HQ666888) and Cameroon (BPI 881010–881014, GenBank Accession Nos. HQ666889–HQ666891) were sequenced. DNA sequences for all were identical to sequences of P. kuehnii and distinct from known sequences of P. melanocephala available in GenBank. To our knowledge, this is the first confirmed report of orange rust of sugarcane in western and central Africa. There is evidence that brown rust of sugarcane was introduced to the Western Hemisphere from this region of Africa (3) making it also the likely source of introduction of orange rust. Further experimentation is required to confirm this hypothesis.
References: (1) J. C. Comstock et al. Plant Dis. 92:175, 2008. (2) N. C. Glynn et al. Plant Pathol. 59:703. 2010. (3) H. L. Purdy et al. Plant Dis. 69:689, 1985.