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First Report of Rhizoctonia solani AG 3‐PT causing black scurf on potato tubers in Namibia

2022; Wiley; Volume: 45; Issue: 1 Linguagem: Inglês

10.1002/ndr2.12066

2044-0588

R. Tjimune, E. Mangwende, Moleboheng Lekota, Norman Muzhinji,

Plant Pathogens and Fungal Diseases

Potato (Solanum tuberosum) is one of the most important vegetable crops in Namibia, contributing to food and nutritional security. The annual production of potatoes in Namibia is approximately 10,000 tonnes (Namibia Agronomic Board, 2020). Rhizoctonia solani is a common pathogen of potato and affectd the yield and quality of tubers. Rhizoctonia solani develops sclerotia on the tubers (black scurf), brown sunken lesions (cankers) on the stolons, stems and roots of the plant (Banville, 1989) as well as tuber deformation, cracking, "elephant hide" and pitting (Woodhall et al., 2007; Muzhinji et al., 2015). The pathogen is taxonomically complex, consisting of 13 anastomosis groups (AGs) but AG 3-PT is the predominant group associated with potato disease (Woodhall et al., 2007). In April 2021, potato tubers (cv. Sifra) with symptoms typically associated with black surf were collected from a farm in Tsumeb (15°15'S; 17°15′E), in the Oshikoto region of Namibia. Small tuber pieces, 5 mm in diameter; were excised, sterilised in 70% ethanol for 30 seconds and then in 1% NaOCl for 1 minute, then rinsed in sterile water, plated on potato dextrose agar (PDA) and incubated at 25°C for 48 hrs. The cultures were observed microscopically and two isolates (RT01 and RT02) with Rhizoctonia-like structures (Figures 1 and 2), i.e., aerial mycelium, right angled branches having a constriction at the point of origin (Ogoshi et al., 1983), were sub-cultured on PDA and incubated at 25°C for 48 hrs. 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 & Bruns, 1993; White et al., 1990). The resulting sequence (GenBank Accession No. OL353907-OL353908) showed 99.7% identity with other AG 3-PT sequences in GenBank (KJ777666 and AB905388). Therefore, based on morphological and molecular sequencing, the isolates were identified as R. solani AG 3-PT. To test pathogenicity, a pure culture of the RT01 and RT02 isolates was grown on sterilised barley grains in 250 ml conical flask for seven days. Pathogen-free certified seed tubers (cv. Sifra) were planted in 2-litre greenhouse pots and inoculated with 10 g of R. solani-colonised barley grains. Control tubers were inoculated with sterile barley grains only. After inoculation, plants were maintained in the greenhouse with a 12 h photoperiod at 25 ±2°C. Approximately 90 days after inoculation, black scurf symptoms resembling those observed from potato tubers harvested in the field were observed on progeny tubers (Figure 3A) from inoculated plants, whereas no symptoms were observed on tubers from the control plants (Figure 3B). Rhizoctonia solani AG-PT was consistently re-isolated from tubers with black scurf and identity of the isolate was confirmed using morphological and molecular methods as described previously, fulfilling Koch's postulates. The experiment was repeated twice with five plants per treatment. Rhizoctonia solani AG 3-PT has been reported previously as a pathogen causing black surf symptoms on potato tubers (Muzhinji et al., 2015; Woodhall et al., 2007). To our knowledge, this is the first report of R. solani AG 3-PT causing black scurf in Namibia. The occurrence of black surf reduces the qualitative and quantitative yield of potato tubers and management strategies should be developed to control this pathogen.