RESUMEN
In February 2022, rust-like lesions were observed on commercial sugarcane cultivar N76 in the North Coast area of KwaZulu-Natal, South Africa (29°23'45.95"S; 31°10'3.91"E). Moderate to severe leaf symptoms were present throughout the 0.2 ha field that was planted in August 2021. The symptoms differed from those of the two sugarcane rusts commonly observed in South Africa, namely, brown rust (Puccinia melanocephala H. & P. Sydow) and tawny rust (Macruropyxis fulva L.A. Martin, S.A. McFarl. and L.A. Castl.) (Martin et al., 2017). Similar symptoms were subsequently observed on several additional cultivars in other regions of the industry. Up to five leaves were collected from each contaminated field and examined under a dissecting microscope. Uredinial lesions were orange-brown, cinnamon-brown to red-brown in colour and were 0.5 to 5 × 0.2 to 0.5 mm. Uredinia within the lesions were 300 to 820 × 28 to 40 µm, mainly hypophyllous although epiphyllous uredinia were occasionally observed on severely infected leaves. Uredinia were ellipsoid to fusiform in shape. Urediniospores were transferred to microscope slides and examined at 400x magnification. The urediniospores were morphologically similar to Puccinia kuehnii E. J. Butler, causal agent of sugarcane orange rust (Glynn et al., 2010). They were orange to cinnamon-brown in colour, mostly obovoid to pyriform in shape and moderately echinulate. They were variable in size (25 to 55 × 22 to 27 µm), most with obvious apical thickening of up to 10 µm. Inconspicuous paraphyses were occasionally present but no telia or teliospores were observed. Identity of the pathogen was confirmed using primers Pk2-F and Pk1-R, targeting the internal transcribed spacer (ITS1-5.8S-ITS2) region (Glynn et al., 2010). The 384-bp sequence (GenBank Accession no: ON329808) matched 100% with P. kuehnii submissions on Genbank and was distinct from those of P. melanocephala and M. fulva. Rust symptoms observed on cultivar Co301 in South Africa from 1941 were previously reported to be caused by P. kuehnii (Gorter, 1977; Crous et al., 2000). However, the pathogen was later correctly identified as P. melanocephala (Bailey, 1979). This is therefore the first report of P. kuehnii infecting sugarcane in South Africa, after being detected periodically on a spore trap located in Komatipoort, Mpumalanga (25°33'2.83"S 31°57'22.19"E) since 2016. An Emergency Response Plan, developed in anticipation of an orange rust incursion in South Africa, guided the initial response of the industry. This included the application of a fungicide (a.i. azoxystrobin / cyproconazole) pre-emptively registered for orange rust on sugarcane in South Africa in infected fields, notification of key contacts in southern Africa, and industry-wide surveys to determine prevalence and provide an indication of varietal susceptibility. Long term management will include screening for varietal resistance and the registration of additional fungicides against the disease. Research on the conditions favouring infection will allow the inclusion of orange rust in the local sugarcane rust forecasting model, while trials to assess the effect on yield will provide an indication of the impact of the disease on production.
RESUMEN
Many viral diseases of sugarcane negatively affect yield. A sugarcane accession originating from South Africa exhibiting mosaic symptoms was processed for high-throughput sequencing. Bioinformatic analysis revealed two known sugarcane viruses and a contig of around 2,800 nucleotides resembling umbra-like viruses of the family Tombusviridae. The sequence of the viral contig was confirmed by cloning and Sanger sequencing, and the ends of the virus sequence were determined. Open reading frame analysis revealed the presence of four ORFs. Phylogenetic analysis of the complete virus sequence showed that this virus clusters with other umbra-like viruses of the family Tombusviridae.
