Synchrony of planting and proportions of susceptible varieties affect ricetungro disease epidemics in the Philippines

Surveys were conducted in the Philippines from 1995 to 1997 to examine rela tionships between production environment variables (agroecosystem, synchron y of planting, and varieties planted) and the occurrence of rice tungro dis ease epidemics using correspondence analyses. The sites covered were Isabel a, Nueva Ecija, North Cotabato, and Bohol provinces as well as Bicol region . Tungro disease incidence in farmers' fields was assessed visually based o n typical symptoms. In addition, leaf samples were collected from each fiel d and indexed serologically by enzyme-linked immunosorbent assay for the pr esence of Rice tungro bacilliform (RTBV) and Rice tungro spherical (RTSV) v iruses. Thus, relationships between the production environment variables an d four disease variables - visual incidence and double RTBV and RTSV, singl e RTSV, and single RTBV infections - were examined. A higher association wa s observed between site and varieties planted as well as site and synchrony of planting than between site and agroecosystem or site and disease variab les (visual incidence, double RTBV and RTSV and single RTSV infections). Di sease variables depended on both varieties planted and synchrony of plantin g and correspondence analysis revealed that the low disease incidence in Nu eva Ecija was associated with synchronous planting while the high disease i ncidence in Isabela was associated with the planting of susceptible varieti es and asynchronous planting. Such findings suggest that the relationship b etween the last two factors at a given site is critical to predicting tungr o occurrence. Moreover, correspondence analysis of the relationship among d isease variables revealed that tungro incidence is associated with not only double RTBV and RTSV infections but also single RTSV infections. Implicati ons of these results on tungro epidemiology and management are discussed. ( C) 2001 Elsevier Science Ltd. All rights reserved.