First Report of Damping-Off of Swiss Chard Caused by Rhizoctonia solani AG-4 HG I and Binucleate Rhizoctonia AG-A in China.

During July, 2003, damping-off of Swiss chard (Beta vulgaris subsp. cicla L.) was observed in a seedling (approximately 1 month after germination) field (approximately 2 ha) in Yuanmou County in the Cuxiong District of Yunnan, China. More than 80% of the seedlings showed symptoms of the disease. Symptoms on newly emerged plants consisted of wilting, a brown necrosis of the lower taproot, and eventual death of seedlings. Among the 15 isolates of Rhizoctonia spp. isolated from Swiss chard with damping-off symptoms, 12 isolates of Rhizoctonia solani with dark brown sclerotia on potato dextrose agar (PDA) anastomosed with tester isolates of each subgroup AG-4 HG I, AG-4 HG II, and AG-4 HG III, giving a C2 hyphal fusion (1) reaction at a high frequency. The other three binucleate Rhizoctonia spp. (BNR) isolates whose mycelia were white with floccose aerial hyphae on PDA anastomosed freely with two BNR AG-A tester isolates producing a C2 hyphal reaction. The 5.8S rDNA-ITS of a single isolate of R. solani and a single isolate of BNR was sequenced. The sequence of the AG-4 isolate (GenBank Accession No. EF679777) exhibited 99 to 100% homology with isolates of R. solani AG-4, subgroup 4HG I (GenBank Accession No. AY154307). The sequence from the AG-A isolate (GenBank Accession No. EF679778) exhibited 98% homology with BNR AG-A (GenBank Accession Nos. AB000040 and AF354092). Swiss chard (cv. Baijin) seedlings (approximately 5 cm high) were planted in potting soil at a density of one seedling per vinyl pot (8 cm diameter, 9 cm high). Two isolates each of R. solani and BNR were used in pathogenicity testing. Each seedling was inoculated in the root zone with approximately 7 g of artificially infested soil. Control plants were inoculated with autoclaved soil. The experiments were conducted three times, each time with three replicates, in a greenhouse with a photoperiod of 16 h of light and 8 of h dark at 30 and 16°C, respectively. After 7 days, disease severity was measured based on a scale in which 0 = no symptom; 1 = small lesions on seedlings, no blight; 2 = leaves blight, no stem blight; 3 = stem blight; and 4 = plant dead. The two AG-4 and two of AG-A isolates were pathogenic on the Swiss chard seedlings and caused damping-off symptoms with a disease index 1.7 to 4.0, and there were no significant differences (P = 0.05) among them. We reisolated and confirmed the presence of R. solani and BNR AG-A from diseased plants. AG-3 isolates were reported to cause the damping-off of Swiss chard in the United States (2). To our knowledge, this is the first report of damping-off of Swiss chard caused by Rhizoctonia solani AG-4 HG I and BNR AG-A. References: (1) D. E. Carling. Page 37 in: Grouping in Rhizoctonia solani by Hyphal Anastomosis Reaction. Kluwer Academic Publishers, Dordecht, the Netherlands, 1996. (2) S. T. Koike and K. V. Subbarao. Plant Dis. 83:695, 1999.