Start date: 2014 – 2016

At least 70 different tomato-infecting begomovirus species have been identified as naturally infecting tomato crops in different parts of the world. The most prominent and best studied of these is Tomato yellow leaf curl virus (TYLCV), which was first identified in the Middle East in the early 1960s but has since spread to many other regions of the New and Old World, often evolving through mutation and recombination in the process.

Begomoviruses are transmitted from plant to plant by Bemisia tabaci whiteflies. It has been recognized for some time now that B. tabaci is a complex of cryptic species.

More than five different major genes conferring tolerance-type resistance to begomovirus infection (designated Ty genes from Tomato yellow leaf curl resistance) have been identified from different tomato-relative species and have been successfully introgressed into cultivated varieties. However, the resistance genes were identified based on the host reaction to specific begomovirus species. The chromosome locations of several Ty genes have been determined and molecular markers linked to several major resistance genes (Ty-1, Ty-2, Ty-3, Ty-3a, ty-5 and Ty-6) (from S. peruvianum and S. habrochaites) are used for marker-assisted selection in breeding programs world-wide. However, the different Ty genes identified are not all equally effective against all species/strains of begomovirus, and there appears to be rapid breakdown of resistance in some locations. To counter this and to aid in the development of tomato lines suitable for a wider range of countries, the strategy being adopted is to pyramid several different Ty genes in the same tomato variety to achieve stronger and longer lasting resistance against a wide array of different begomovirus species/strains.

AVRDC and the Asia and Pacific Seed Association (APSA) members are conducting collaborative trials of tomato lines carrying different combinations of Ty genes in different seed company managed sites at different locations across Asia and the Pacific region where infection by begomovirus(es) is a major problem. The aim is to identify the predominant begomovirus species/strains at each trial site, identify the predominant whitefly genotype(s) at each trial site, and to produce sufficient seed of existing breeding lines and new tomato breeding lines.