A WORLDWIDE SURVEY OF TOMATO YELLOW LEAF CURL VIRUSES

The name tomato yellow leaf curl virus (TYLCV) has been given to sever al whitefly-transmitted geminiviruses affecting tomato cultures in man y tropical and subtropical regions. Hybridization tests with two DNA p robes derived from a cloned isolate of TYLCV from Israel (TYLCV-ISR) w ere used to assess the affinities of viruses in naturally infected tom ato plants with yellow leaf curl or leaf curl symptoms from 25 countri es. Probe A which included most of the intergenic region was expected to detect only isolates closely related to TYLCV-ISR, especially after high stringency washes. In contrast probe B, which included the full- length genome, was expected to detect a wide range of whitefly-transmi tted geminiviruses. Tomato samples from six countries in the Middle Ea st, from Cuba or the Dominican Republic proved to be closely related t o TYLCV-ISR and probably were infected by strains of the same virus. S amples from Senegal and Cape Verde Islands were also related to the Mi ddle Eastern virus. Samples from nine other countries in the western M editerranean area, Africa, or South-East Asia were more distantly rela ted and probably represent one or more additional geminivirus species. Samples from five countries in Africa, Central or South America gave hybridization signals with the full-length viral genome, only after lo w stringency wash, indicating that these samples were infected by remo te viruses. These results were supported by DNA and protein sequence c omparison, which indicate that tomato geminiviruses fall into three ma in clusters representing viruses from 1) the Mediterranean/Middle East /African region, 2) India, the Far East and Australia, and 3) the Amer icas. Within the first cluster, two sub-clusters of viruses from the w estern Mediterranean or from the Middle East/Caribbean Islands were di stinguished. The incidence of tomato yellow leaf curl diseases has inc reased considerably between 1990 and 1996.