CHARACTERISTICS OF RESISTANCE IN RICE TO RICE-HOJA-BLANCA-VIRUS (RHBV) AND ITS VECTOR, TAGOSODES ORIZICOLUS (MUIR)

This paper characterises resistance to rice hoja blanca virus (RHBV) w hich is transmitted by the planthopper Tagosodes orizicolus (Muir). Re sistance is expressed as decreased proportion of plants infected compa red to susceptible lines, although severity of symptom expression is s imilar in both types. This resistance is not due to differences in vec tor feeding behaviour. Vectors reared eight generations on resistant p lants showed no increased ability to transmit to resistant lines or de creased ability to transmit to susceptible ones. Longevity of vectors was similar when reared either on virus-resistant or susceptible plant s. Incubation period of the virus in resistant plants are significantl y longer than in susceptible plants. Resistance increased with plant a ge in resistant and susceptible cultivars. Increased virus dosage, as determined by increased number of viruliferous vectors per inoculated plant, caused an increase in transmission to both resistant and suscep tible cultivars. However, the ranking of resistant and susceptible rem ained the same across the experimental range of dosage and plant age. It is concluded that the resistance studied is to virus infection and there is little risk of ''breakdown'' occurring as a result of genetic or behavioural changes in the vector population. This will permit the use of economic thresholds to planthopper feeding damage with little risk of RHBV epidemic outbreak.