Throughhill yam (Dioscorea nipponica Makino), a perennial winding herb and a member of the Discoreaceae, is distributed principally in northeast Asia. It is used to produce medicine for treating coronary heart disease, diabetes mellitus, and inflammation. In China, this species is cultivated in many provinces such as Liaoning, Jilin, Heilongjiang, Hebei, Inner Mongolia, and Shanxi. In July 2006, seedling blight was observed on D. nipponica with disease incidence ranging from 37 to 75% in commercial fields in Antu County, China. In the early stages of disease development, water-soaked lesions appeared at the stem base and on leaves near the ground. Lesions later turned dark brown and necrotic. Leaves eventually became chlorotic, stem and petioles collapsed gradually, and plants died. Mycelium was observed to be growing on the surface of infected tissues and adjacent plants, and brown, hard sclerotia were produced on stem or petiole surfaces. A fungus with morphological characteristics of Rhizoctonia solani Kühn was consistently isolated from diseased tissues that were plated on potato dextrose agar (PDA). Mycelium was branched at right angles with a septum near the branch and a slight constriction at the branch base. Cells from hyphae grown on 2% water agar at 25°C were determined to be multinucleate when stained with 1% safranin O and 3% KOH solution (1) and examined at ×400. Anastomosis groups were determined by pairing isolates with 12 tester strains representing all subgroups of AG1 to AG5 on 2% water agar in petri plates (2). The anastomosis grouping of isolates Rs1, Rs2, and Rs5 was determined to be AG1-IB and that of isolates Rs3 and Rs6 was determined to be AG2-1. The rDNA internal transcribed spacer (ITS) gene sequence of isolates Rs1, Rs2, and Rs5 (GenBank Accession Nos. GU585667, GU596490, and GU594691) had 100, 99, and 100% nucleotide identity, respectively, with AG1-IB (GenBank Accession No. FG440191). The rDNA-ITS of isolates Rs3 and Rs6 (GenBank Accession Nos. GU596493 and GU594692) exhibited 99% homology with AG2-1 (GenBank Accession No. EU513135). Pathogenicity tests were performed on healthy, potted 2-year-old plants of D. nipponica. Twenty plants were wound inoculated by placing 0.6-cm mycelial plugs from 3-day-old PDA cultures on leaves and stems. Twenty plants were treated with PDA plugs as controls. Plants were maintained at 25°C and 95% relative humidity on a 12-h light/dark regimen. Typical symptoms of leaf and stem rotting identical to those observed in the commercial field appeared 4 days after inoculation and all inoculated plants died within 10 days. No disease symptoms were observed on control plants. Rhizoctonia solani was consistently reisolated from symptomatic tissues. To our knowledge, this is the first report of R. solani causing seedling blight on D. nipponica in the world.
References: (1) R. J. Bandoni. Mycologia 71:873, 1979. (2) C. C. Tu and J. W. Kimbrough. Mycologia 65:941, 1973.
