Ginger rhizome rot is a major problem in Bhutan. Though actual crop loss has not been assessed, problems in the fields have been quite noticeable.

Initial work on rhizome rot indicated the presence of soil-borne organisms like Rhizoctonia sp., Fusarium sp., Pythium sp. and Pseudomonas sp. in soil samples collected from ginger fields (Om, pers. comm.). However, works on pathogenicity of these organisms were not done. In 2018, the Plant Protection Sector under Agriculture Research & Development Centre (ARDC) in Samtenling raised the need for identification of the causal agents of ginger rhizome rot.  To address this issue, the National Plant Protection Centre (NPPC) proposed that a detail laboratory work could be initiated in 2019-2020 as part of the collaborative activities between ARDC-Samtenling and NPPC. Thus, the current work on identification of causal agents of ginger rhizome rot was carried out.

Samples consisting of both soil and plant were collected from symptomatic (yellowing & wilting) ginger fields in Taraythang Gewog, Sarpang Dzongkhag. A total of 14 rhizome and 16 soil samples were collected from four fields across three chiwogs from 29 October to 1 November 2019.

Plant samples were examined in the Pathology Laboratory of NPPC. All rhizome samples exhibited similar symptoms showing internal tunneling, brown and mushy disintegrated tissue masses turning straw like when dry and some appearing hollow. Majority of the samples also harboured larvae (maggots) and pupae.

A fly fitting the morphological description of what is commonly known as ‘rhizome fly’ in the genus Mimegralla (Diptera: Micropezidae) emerged when representative samples with maggots and pupae were reared. Further work is underway for proper identification of the fly and determining whether the fly is the primary causal agent of rhizome rot in Sarpang and other areas in Bhutan. Future work includes detection surveys to record the infested areas; monitoring fly emergence under field condition possibly using sticky traps or other methods; and determining management options using neem oil and other alternatives against this fly.

Soil and plant samples were also processed for detection of fungi and fungi-like organisms. Isolates suspected to be that of Pythium sp. were obtained. Isolates are being stored in the NPPC for future work on pathogenicity provided disease free rhizomes can be obtained.

Contributors:

Ratu Kinley – ARDC-Samtenling

Sangay Chophel, Pema Tobgay, Tshomo and Namgay Om – National Plant Protection Centre