First Report in China of Soft Rot of Ginger Caused by Pythium aphanidermatum

Ginger (Zingiber officinale Roscoe) is an important commercial crop planted on more than 13,000 ha annually in Anqiu city, Shandong Province, China. From 2010 to 2011, the incidence of Pythium soft rot disease on cv. Laiwu Big Ginger reached 40 to 75% in Anqiu and yield losses of up to 60% were observed. The disease symptoms included brown spots on ginger rhizomes followed by soft rot, stems and leaves above ground becoming withered and yellow, and water soaking on the collar region. The soft rot did not produce offensive odors, which is different from bacterial rots (2). Forty symptomatic rhizomes were sampled from eight farms. Martin's method (1) was used to isolate the pathogen. Ten pieces from each rhizome were washed with sterile distilled water for 30 s and plated on Martin's selective medium at 26°C in a chamber without light. Colonies grew with cottony aerial mycelium. Main hyphae were 5.7 to 9.6 μm wide. Globose sporangia consisting of terminal complexes of swollen hyphal branches were 11.4 to 18.3 μm wide. The average diameter of zoospores was 9.2 μm. The oogonia were globose and smooth, with a diameter of 21 to 33 μm. The sequences of the rRNA gene internal transcribed spacer (ITS) regions 1 and 2 and the 5.8S gene of five isolates were amplified using primers ITS1 and ITS4 (4), and the nucleotide sequence was the same as isolate No. 2, which was deposited in GenBank (Accession No. KC594034). A BLAST search showed 99% identity with Pythium aphanidermatum strain 11-R-8 (Accession No. JQ898455.1). Pathogenicity tests of five isolates were carried out in a greenhouse. Sixty plants (cv. Laiwu Big Ginger) were grown for 30 days in plastic pots (diameter 20 cm) in sandy soil (pH 5.48) and inoculated. Ten plants were used as untreated controls. Five isolates were grown on Martin's liquid medium for 72 h and the spores were harvested in sterile distilled water. Aqueous spore suspensions of the five isolates were adjusted with deionized water to 1 × 10 8 CFU/ml and injected with a syringe into the soil around the rhizome of the plants. Plants were then placed in the greenhouse at 24 to 26°C and assessed for rhizome rot on the 14th day after inoculation. The inoculated isolates were recovered from the diseased rhizomes, confirming their pathogenicity. To our knowledge, this is the first report of ginger Pythium soft rot caused by P. aphanidermatum in China. Ginger Pythium soft rot caused by P. myriotylum is reported in Taiwan (3). References: (1) F. N. Martin. Page 39 in: The Genus Pythium. American Phytopathological Society, St. Paul, MN, 1992. (2) E. E. Trujillo. Diseases of Ginger (Zingiber officinale) in Hawaii, Circular 62, Hawaii Agricultural Experiment Station, University of Hawaii, December 1964. (3) P. H. Wang. Lett. Appl. Microbiol. 36:116, 2003. (4) T. J. White. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.