A Characterization of Rice Tungro Epidemics in The Philippines from Historical Survey Data. Serge Savary, Plant Pathologist and ORSTOM Visiting Scientist, International Rice Research Institute, P.O. Box 933, 1099 Manila, The Philippines. Nestor Fabellar, Emmanuel R. Tiongco, and Paul S. Teng. Visiting Scientist, Research Assistant, Assistant Scientist, and Plant Pathologist, International Rice Research Institute, P.O. Box 933, 1099 Manila, The Philippines. Plant Dis. 77:376-382. Accepted for publication 12 November 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0376.

We used historical survey data to compare patterns of rice tungro epidemics in two endemic areas and one nonendemic area in The Philippines. Four categorized variables—planting date, cropping season, vector population, and proportion of viruliferous vectors—were tested for their ability to characterize the variation in tungro incidence. Correspondence analyses indicated that high tungro incidence was associated with intermediate planting dates, whereas absence of tungro was associated with very early or very late planting dates. In the two endemic areas (North Cotabato and Sultan Kudarat, Mindanao; data from nine cropping seasons), increasingly higher tungro incidence was associated with increasing vector population and proportion of viruliferous vectors, but the relationships among the three variables differed in the two areas. In Sultan Kudarat, a high proportion of viruliferous vectors compensated for a moderate vector population size to produce high tungro incidence. In the nonendemic area (Central Luzon, 16 cropping seasons), moderate to high tungro incidence was associated with the presence of viruliferous vectors and moderate to very large vector populations. Further analysis indicated that epidemic years in Central Luzon are primarily associated with the occurrence of viruliferous vectors, with the size of the vector population playing a secondary role only. The analyses suggest that tungro outbreaks are more responsive to inoculum (represented by viruliferous vectors), when present, in the nonendemic area (Central Luzon) than in the endemic areas (North Cotabato and Sultan Kudarat).

Keyword(s): categorical data, endemicity.