Resistance to Rice Tungro-Associated Viruses in Rice Under Experimental and Natural Conditions

Abstract

Tungro is a composite disease resulting from infection with both rise tungro bacilliform virus (RTBV) and rise tungro spherical virus (RTSV). Both viruses are transmitted efficiently by the leafhopper Nephotettix virescens. Rice cultivars developed by the International Rice Research Institute with resistance to leafhoppers were evaluated under field and greenhouse conditions. In the field, moderately resistant cultivars had low tungro infection rates under low virus inoculum and leafhopper levels but high infection rates under high levels. Resistant cultivars had low infection rates regardless of inoculum or leafhopper levels. Resistance in cultivars was due to both antibiosis and nonpreference to adult leafhoppers. In mass or test-tube inoculations with leafhoppers that had fed on plants infected with RTBV and RTSV, resistant cultivars showed increasing rates of infection with increasing numbers of leafhoppers. When latex serological tests were used to index these inoculated plants, resistant cultivars had increasing RTBV infection rates, whereas moderately resistant cultivars had increasing RTBV and RTSV infection rates. Susceptible cultivars had high RTBV and RTSV infection rates irrespective of leafhopper number. When resistant cultivars were inoculated with RTSV alone, fairly high infection rates occurred. Low field infection rates in the resistant cultivars can be explained by their resistance to the leafhopper.