Changes in Cultivar Reactions to Tungro Due to Changes in 'Virulence' of the Leafhopper Vector

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Abstract
Populations of green leafhopper (GLH), Nephotettix virescens, were collected from five locations in the Philippines from August 1986 to January 1988 and reared on GLH-susceptible rice cultivar Taichung Native 1 (TN1). First or second generation adults that had fed on rice plants infected with rice tungro bacilliform virus (RTBV) and rice tungro spherical virus (RTSV) were tested for their feeding behavior by the honeydew-based bromocresol-green test during a 1-day inoculation access feeding on seedlings of IR26 and IR36 which were moderately resistant to GLH; IR54, IR62, and IR64 which were resistant to GLH; and susceptible TN1. A colony maintained on TN1 in a greenhouse served as the control. Inoculated seedlings were indexed by latex serology. Third or four generations were tested for GLH preference, population increase, and nymphal mortality. Populations collected in 1987 from Bicol; Koronadal, South Cotabato; Maligaya, Nueva Ecija; Leyte; and Los Banos, Laguna, fed more from phloem and transmitted RTBV and RTSV together more efficiently on GLH-resistant rest cultivars than did the greenhouse colony. On these cultivars, the field populations showed higher preference, less nymphal mortality, and greater population buildup than did the greenhouse colony, indicating higher virulence of the field populations to the resistant cultivars. GLH populations collected at Los Banos in 1986 fed largely from xylem and transmitted mainly RTBV alone on IR54, whereas those collected in 1987 and 1988 fed more from phloem and efficiently transmitted RTBV and RTSV together on IR54. At Los Banos, RTBV and RTSV incidences on IR54 and other releated GLH-resistant cultivars were negligible in 1985 and 1986; the incidences were very high in 1987 and 1988. Apparently, IR54 and other related GLH-resistant cultivars were severely infected in 1987 due to a shift of GLH "virulence" to these cutlivars. The "virulence" of a field population to the GLH-resistant cultivars was reduced drastically when the population was continuously reared on TN1 for seven or more generations.