Relationship Study Between the Brown Planthopper Population and the Intensity of Rice ragged stunt virus and Rice grassy stunt virus, as well as the Inoculum sources

The brown planthopper (BPH) and stunt virus are the main causes of rice crop damage in the field. Rice crops infected by viruliferous BPHs in high populations will end up with yield loss and a disease epidemic. BPH presence in the field occurred when the rice crops age was 1 week after planting (WAP). The highest population average of BPH reached 77.33 BPH/rice hills in 6 WAP, whereas the population declined on 7 to 9 WAP. Stunt disease incidence started appearing on 4 WAP and increased until 7 WAP. The increase of the BPH population was followed by increased stunt disease incidence rate in rice crops. The viruliferous BPHs found in 9 regencies in Bali carried two viruses, Rice grassy stunt virus (RGSV) and Rice ragged stunt virus (RRSV). The proportion of a BPH carrying two viruses (RGSV and RRSV) reached 20%. The proportion of BPH carrying in 1 virus for RGSV ranges around 15-44%, while the proportion of viruliferous BPHs carrying RRSV ranges around 19.04-42.85%. Other than vector insects as the cause of stunt virus spreading in the field, several weed species around rice crops are also alternative hosts of stunt virus. Weed species naturally infected by RGSV are Axonopus compressus, Eleusine indica, Echinochloa colona, and Monochoria vaginalis. RRSV is found in several weeds around rice crops, which are A. compressus, E. indica, and E. colona. Meanwhile, Paspalum distichum is uninfected by the two viruses. The main causes of both viral infections are the vector population and the presence of weeds always found in the field, making a suitable control strategy essential to be implemented.