by Roger Cabunagan and Il-Ryong Choi
International Rice Research Institute, Los Baños, Philippines
Why outbreaks of BPH-transmitted virus diseases occurred in south Vietnam during 2006-2007 and remains a problem until now (see figure below) is our main objective. Here we report on some preliminary work we carried out recently to get better understanding.
We were in south Vietnam in March 10-25, 2009 to investigate this question. A survey was conducted and plant samples from selected south Vietnam provinces (see figure below) were collected to determine the biological and genetic characteristics of the viruses infecting rice crops in south Vietnam and to determine the epidemiological characteristics of BPH vectors and the viruses they transmit.
An immunological assay (ELISA) for viruses in BPH and leaf samples collected during the trip and assays conducted by the Post Entry Quarantine Laboratory (Ho Chi Minh and Can Tho), and the Southern Regional Plant Protection Center (SRPPC) in Tien Giang, for samples from different provinces in south Vietnam revealed the prevalence of rice ragged stunt virus (RRSV) and rice grassy stunt virus (RGSV) in the affected areas. The plants were found often mix-infected with both RRSV and RGSV (see figures below).
Additional data on BPH and virus outbreaks in south Vietnam (from 1997-2009), and data on the 2006-2008 outbreaks, broken down by province, season and damage severity, were gathered from SRPPC. Also data on area planted per season in different provinces in south Vietnam for the years 2007 to 2008 and light trap data with regard to BPH broken down by province, district and month for years 2007 and 2008 were collected during the trip. These data are being analyzed to answer the question on why outbreaks of virus diseases occurred in south Vietnam.
Another question is why virus incidence is sometimes very high even at early stages (see picture above from Binh Phouc province in 2006) although only about 44% virus carriers were detected from BPH caught in the light trap and in the field (see figure). However, this could happen when very high population of BPH landed on a newly transplanted or seeded field and this needs to be further studied.