Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2007
Publication Date: 1/25/2008
Citation: Comstock, J.C., Sood, S.G., Glynn, N.C., Shine, Jr., J.M., Mckemy, J.M., Castlebury, L.A. 2008. First report of Puccinia kuehnii, causal agent of orange rust of sugarcane, in the United States and Western Hemisphere. Plant Disease. 92:175. Interpretive Summary: Puccinia kuehnii is a rust fungus that causes the orange rust disease of sugarcane in Asia, Africa and Australia where it has caused extensive losses. This paper shows that P. kuehnii is now present in Florida infecting cultivars of sugarcane. The detection of Puccinia kuehnii in Florida is significant in that it is believed to be the first verified report in the Western Hemisphere. Orange rust symptoms were moderately severe on cultivars that occupy approximately 25% of the commercial acreage in Florida as well as numerous clones in the cultivar development program. DNA sequences and morphological features were used to identify P. kuehnii and distinguish it from P. melanocephala, another rust fungus on sugarcane. This information will be useful to sugarcane breeders and producers in determining which cultivars should be planted. The impact on commercial production and the cultivar development program is not known at this time.
Technical Abstract: In June 2007, approximately 5 miles east of Belle Glade, Florida, a rust disease was observed on a sugarcane (a complex hybrid of Sacharum L. species) cultivar (CP 80-1743) considered resistant to brown rust caused by Puccinia melanocephala Syd. & P.Syd. Six miles south of Canal Point, Florida, another cultivar (CP 72-2086), also considered resistant to P. melanocephala, was found to be infected with a rust. Observed morphological features were consistent with Puccinia kuehnii E.J. Butler. Uredinial lesions were orange and variable in size, measuring 650–850 x 26–32 µm, hypophyllous, ellipsoidal to fusiform in shape and were distinctly lighter than pustules of P. melanocephala. Urediniospores were mostly obovoid to pyriform to broadly ellipsoidal, variable in size, 32–45 x 25–30 µm, and moderately echinulate with mostly evenly distributed spines 2–4.5 µm apart. Walls were orange to light cinnamon brown, 1–2.5 µm thick with a pronounced apical wall thickening of up to 7 µm and 4–5 equatorial pores. Telia and teliospores were not observed. The nuclear large subunit rDNA region of the rust infecting cultivar CP 80-1743 and the ITS1, 5.8S and ITS2 rDNA regions were sequenced from the rust infecting CP 80-1743 and CP 72-2086. All sequences were found to be distinct from known sequences of P. melanocephala and identical to sequences of P. kuehnii. Similar orange-colored uredinial lesions were subsequently observed on the same two varieties and several other varieties at different locations in South Florida. The ITS1, 5.8S and ITS2 sequences from rust lesions on two additional varieties (CPCL99-1777 and CPCL01-1055) were identical to those from CP80-1743 and CP72-2086. To our knowledge, this is the first record of P. kuehnii infecting sugarcane in the Western Hemisphere and the disease appears to be distributed widely through the South Florida sugarcane growing area.