IRRI's achievements & successes

Wide hybridization used to fight a serious rice disease

IR64, a popular and widely grown rice variety in South and Southeast Asia, has become very susceptible to Rice tungro (photo), the most serious virus disease in the region. Caused by the rice tungro bacilliform virus (RTBV) and the rice tungro spherical virus (RTSV), the green leafhopper is the principal vector.

Unfortunately, genetic variability is limited for resistance to RTBV, which is the main cause of tungro symptoms. So, IRRI breeders made wide (or interspecific) crosses between IR64 and three accessions of the wild rice species, Oryza rufipogon, which is tolerant of tungro. Backcrosses were produced with the recurrent rice parent, IR64. The backcross progenies were field-screened in "hot spot" tungro areas. After four backcrosses, uniform progenies resembling the recurrent parent and resistant to tungro disease were selected.

One of the elite breeding lines (IR73885-1-4-3-2-1-6) resistant to tungro has been released as a stop-gap variety for cultivation in tungro-prone areas of the Philippines. Molecular markers have been identified in this line showing association with resistance to tungro. Attempts are now being made to pyramid genes for even better tungro resistance.

One of the IRRI breeding lines derived from the IR64 x O. rufipogon cross was released as a Vietnamese national variety in 2002. It has become quite popular with farmers because it has the added bonus of doing well in acid sulfate soils. Currently occupying more than 100,000 hectares in Vietnam, this variety matures 5–10 days earlier than IR64.