Abstract

Rhizoctonia solani is a common pathogen that affects the yield and quality of potato (Solanum tuberosum) tubers. R. solani produces sclerotia on the progeny tubers (black scurf) as well as brown sunken lesions (cankers) on the stolons, stems, and roots of the plant (Banville 1989). In addition, Rhizoctonia is associated with atypical symptoms on potato tubers such as deformation, cracking, elephant hide, and pitting (Campion et al. 2003; Muzhinji et al. 2014). R. solani is taxonomically complex, consisting of 13 anastomosis groups (AGs) that differ in morphology, ecology, and host range (Sneh et al. 1998). R. solani AG 3-PT is the predominant AG associated with potato diseases globally (Woodhall et al. 2007). In February 2017, potato tuber samples (cv. Mondial) displaying dark brown scab lesions (elephant hide) (Muzhinji et al. 2014) were collected from the Limpopo, Eastern and Western Free State, potato growing regions in South Africa. The symptoms were similar to those of elephant hide and corky crack symptoms reported by Muzhinji et al. (2014) in South Africa. Small sections of infected tissue were excised, sterilized in 70% ethanol for 30 s and then in 1% NaOCl for 1 min, rinsed in sterile water, plated onto potato dextrose agar (PDA), and incubated at 25°C for 5 days. Hyphal tips of an isolate (R1A) with Rhizoctonia-like colonies were subcultured on PDA and examined for morphological characteristics microscopically. The hyphae were branched at right angles, with constriction at the point of origin, and the mycelium was aerial, brown, and fluffy, characteristics consistent with the Rhizoctonia genus (Sneh et al. 1998). The identity of the fungus was confirmed by sequencing the partial internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) using ITS1-F and ITS4 primers (Gardes and Bruns 1993; White et al.1990). The resulting sequence (MK024251) showed 99% homology with other AG2-2IIIB sequences found in the NCBI GenBank database (GU811673 and FJ492113). Therefore, based on morphological and molecular methods, the isolate was identified as R. solani AG2-2IIIB. The isolate was deposited in the National Collection of Fungi, hosted at the Agricultural Research Council–Plant Health and Protection. Pathogenicity tests were conducted in the greenhouse to fulfill Koch’s postulates. Five PDA plugs of an actively growing culture were mixed with 10 g of sterile barley grains in a 250-ml Erlenmeyer flask and incubated for 14 days until fully colonized. One disease-free mini-tuber (cv. Mondial) was planted in each of 24 5-liter pots and inoculated with 10 g of colonized barley grains. Control plants were inoculated with sterile barley grains only. After inoculation, plants were placed in the greenhouse with a 12-h photoperiod at 25 ± 2°C. Progeny tubers were assessed for elephant hide symptoms at harvest, 92 days after planting. The incidence of elephant hide was 55.7% and severity 1.26. R. solani AG2-2IIIB was consistently reisolated from the symptomatic tubers, and identity of the isolate was confirmed as described previously, thus confirming Koch’s postulates. No symptoms were observed on control plants. The experiment was conducted twice with the same results. Until now, only AG 3-PT was reported to be associated with elephant hide in South Africa (Muzhinji et al. 2014). Therefore, this is the first report of R. solani AG2-2IIIB causing elephant hide on potato tubers in South Africa, and the AG should be considered when developing disease control strategies to effectively manage this disease.

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