Siraita grosvenorii (Luo Han Guo; Cucurbitaceae) is a new host of Ralstonia solanacearum in China

Siraita grosvenorii (common names: Luo Han Guo, Arhat fruit) is one of the most economically important crops in the north of Guangxi, China. In 2003, a new wilt disease was observed in Luo Hang Guo growing in the lowland fields. Disease incidence ranged from 5 to 8%, with symptoms appearing 8-10 weeks after transplanting. Initially, the upper leaves or side shoots of the affected plants became wilted. Three to five days later, all the leaves and side shoots of the diseased plants wilted (Fig.1). The whole plants wilted and died 7-10 days after the first appearance of the symptoms.

Bacteria were isolated from ten samples, each of two plants, from ten fields in Yongfu, Yongan or Lingui counties, using an agar medium containing (g/l): peptone, 5; yeast extract, 1; beef extract, 3; glucose, 10. Bacteria with similar colony morphology were isolated from all samples. Two purified single colonies from each sample were each inoculated into 20 Luo Han Guo plants using the stem technique (Winstead & Kelman, 1952). The same number of control plants were inoculated in a similar way with autoclaved water. Typical symptoms were seen on the inoculated plants 3 days post inoculation and by 7-10 days all plants had wilted and died. Bacteria with the same colony morphology as those inoculated were re-isolated. No symptoms developed on control plants. All twenty strains induced hypersensitive reactions on tobacco (Klement et al., 1964).

The DNA region coding for 16S ribosomal RNA of two representative strains (LHG1 and LHG8) was amplified using the universal primers f27 and r1492 (Lane, 1991). Both ends of each 1.5 kb PCR product were sequenced (GenBank accession numbers: AY775123, AY775124, AY775125 and AY775126). A GenBank search revealed that the obtained sequences were identical with the corresponding sequences of the 16S rRNA gene of a number of Ralstonia solanacearum strains such as strain GMI1000 (accession number AL646081). Both strains utilised mannitol, sorbitol, dulcitol and maltose but were unable to utilise lactose and cellobiose (Hayward, 1964). Thus, the pathogen causing the wilt disease of S. grosvenorii was identified as R. solanacearum. This is first report of a member of the Cucurbitaceae family plant as a host of R. solanacearum.