First Report of Orange Rust Caused by Puccinia kuehnii on Sugarcane in South Africa

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 October 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 and were 0.5 to 5 × 0.2 to 0.5 mm. Uredinia within the lesions were 300 to 820 × 28 to 40 μm and 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 400× magnification. The urediniospores were morphologically similar to Puccinia kuehnii E. J. Butler, the causal agent of sugarcane orange rust (Glynn et al. 2010). They were orange to cinnamon-brown, mostly obovoid to pyriform, 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 (Crous et al. 2000; Gorter 1977). 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 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 favoring 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.