Black scurf and stem canker on potato (Solanum tuberosum L.), caused by Rhizoctonia solani, is one of the most important soil-borne diseases throughout the world. Isolates of R. solani anastomosis group (AG) 3-PT have been reported as the predominant cause of the disease on potato (Carling 1996) and the same results were also obtained in Heilongjiang Province, China (Yang et al. 2017). In October 2020, 14 diseased potato tubers (cv. Youjin-885) with symptoms typically associated with black scurf were found in Hegang City of Heilongjiang in Northeast China, where potatoes are grown for propagation in the breeding nursery. Pieces of sclerotia were removed from the surface of the potato and were surface sterilized with 70% ethanol for 30 s and 0.5% NaClO for 1 min, then rinsed three times with sterile distilled water and placed on potato dextrose agar (PDA) at 25°C in the dark. After incubation for 48 to 72 h, mycelia resembling Rhizoctonia were microscopically examined for morphological characteristics, and hyphal tips transferred to fresh plates of PDA. The characteristics of the observed isolate were typical of R. solani Kühn, which include hyphal branching at right angles, a septum near the branching point and a slight constriction at the branch base (Yang et al. 2015). Hyphal cells were also determined to be multinucleate by staining with 1% safranin O and 3% KOH solution (Bandoni 1979). PCR amplification and DNA sequencing of the internal transcribed spacer (ITS) region of the ribosomal DNA (rDNA) was performed by using the universal primers ITS4/ITS5 (White et al. 1990). The resulting sequence of 700 bp (GenBank accession no. OL770460) showed more than 99% identity to AG 2-2IV isolates present in GenBank (e.g. AB911322; KR259910). On the basis of morphological characteristics and molecular analysis, the isolate was identified as R. solani AG 2-2IV. Pathogenicity of the isolate was tested in greenhouse conditions. Pathogen-free minitubers (cv. Favorita) of approximately the same size (10 to 20 g) were allowed to sprout at room temperature for 10 days. The minitubers were then planted in autoclaved soil in a plastic pot (4 L capacity), placed in a greenhouse at 18 - 27°C (night-day) with 50% relative humidity and watered as required. The pots were inoculated with 7-mm-diameter mycelial plugs (from one PDA petri plate) near the minituber, which was then covered with potting mix. The control pots were inoculated with sterile plugs of PDA. Each treatment consisted of 10 plants, and the experiment was repeated three times. Two months after stems emerged, plants and progeny tubers were harvested and assessed for disease. Stem cankers typical of R. solani infection and black scurf were observed on plants grown in pots inoculated the mycelial plugs, but the control plants remained disease free. Fungi reisolated from symptomatic stems and tubers were identified as R. solani AG 2-2IV using morphological characters and ITS sequences.Sclerotia were observed on PDA by incubating at 25oC in the dark. Although eight AGs have been previously shown to cause black scurf and stem canker in Heilongjiang (Li et al. 2014; Yang et al. 2015; Yang et al. 2017; Yang et al. 2019; Yang et al. 2020), to our knowledge, this is the first report of AG 2-2IV causing disease on potatoes in Heilongjiang Province, the main potato seed production area of China. Early detection of R. solani AG 2-2IV during potato seed production is necessary to prevent its dispersal via infected tubers to other fields across China. The information of which AG is present will assist in developing management strategies for this disease.
Keywords: Rhizoctonia solani; black scurf; first report; potato.