First Report of Rhizome Blight of Ginger Caused by Binucleate Rhizoctonia AG-R in China.

Abstract

In 2004, rhizome blight of ginger (Zingiber officinale (Willd.) Roscoe) (cv. Yunnanxiaojiang) occurred in the Kunming District of China. The surface of ginger rhizome after harvest was crimpled and covered with white hyphae. Initial symptoms on ginger were wilting on the stem and the color of the rhizome turned from white to light brown with no lesion formation. After 2 weeks, the surface of ginger rhizome was covered with white hyphae and a dry rot set in under humid conditions. The yield loss in ginger almost reached 50% because of the disease. An AG-R tester isolate paired with the unknown 37 isolates of Rhizoctonia spp. from the diseased ginger rhizomes caused a C2 reaction that confirmed their identity. Isolates of AG-R (GenBank Accession Nos. DQ885780 and DQ885781) had 100% sequence similarity with 5.8S rDNA-ITS with the AG-R tester isolate (GenBank Accession No. AF354082). To produce infected soil inoculum, 10 isolates were cultured on potato dextrose agar in a 9-cm petri dish for 3 to 4 days and then covered with approximately 20 g of autoclaved soil and kept at 25°C for 3 to 4 days. Seedlings of ginger (cv, Yunanxiaojiang) were planted in natural potting soil at a density of one plant per vinyl pot (8 cm in diameter, 9 cm high) and grown in the greenhouse for 7 days. Each seedling was inoculated with 7 g of infested soil by placing it around the rhizome. Control plants were inoculated with autoclaved soil. The experiments were carried out three times, each time with three replicates in a growth chamber kept at 25 and 16°C with a 16-h light and 8-h dark photoperiod. After 14 days, the disease severity was recorded based on a scale in which - = no symptoms; + = small lesions on seedlings, no blight; ++ = seedling blight; and +++ = plant dead. All of the 10 tested AG-R isolates caused ginger seedling blight. Rhizoctonia spp. was reisolated from these plants, confirming its pathogenicity. To our knowledge, this is the first report of rhizome blight of ginger caused by Rhizoctonia spp. and binucleate Rhizoctonia AG-R in China.