Micropropagation of wasabi (Wasabia japonica) and identification of pathogens affecting plant growth and quality

Wasabi (Wasabia japonica Matsum.) is grown for its rhizome which is a culinary ingredient. Wasabi plantlets were regenerated from meristem-tips taken from axillary buds on greenhouse-grown plants. Axillary shoots developed at 3.8±0.13 per meristem-tip every 8 weeks on 0.8 mg/L N6-benzyladenine (BA) and rooted on hormone-free Gamborg's B5. These plants were used in subsequent pathogenicity studies. Pythium dissotocum and Pythium intermedium were recovered from plants with root loss. Hydroponically-grown plants were inoculated with mycelial plugs of each Pythium spp. and after 2 months, root dry weight was 16% of the controls. Isolation studies from blackened rhizome vascular tissue were also conducted. Microscopic observations and isolation data showed that bacteria were associated with symptomatic plants. Pectobacterium carotovorum subspecies carotovorum (Pcc) was identified. Symptoms developed when roots were wounded and inoculated with Pcc. Pcc was recovered from the rooting medium used by growers and found to be identical to the wasabi isolate.