Appearance and Expansion of TYLCV: a Historical Point of View

In 1959, the Israeli Ministry of Agriculture urged farmers in the Jordan Valley to replace the tasty but soft tomato “Marmande” with the long-shelf life variety “Money Maker,” which was more suitable for export. A month after transplanting (August), most of the tomato plants in the region were affected by a disease of unknown etiology. Symptoms included severe stunting of plant growth, erect shoots, and markedly smaller and misshaped leaflets. The leaflets that appeared immediately after infection were cupped down and inward, and subsequently developing leaves were strikingly chlorotic and showed an upward curling of the leaflet margins. When young plants were infected, they barely produced any marketable fruits (Cohen & Nitzany, 1960). The growers’ first reaction was to blame the change in tomato variety and they demanded compensation from the Ministry of Agriculture. Dr. F. E. Nitzany, head of the Virology Laboratory at the Volcani Center, Agricultural Research Organization (ARO), Israel, was asked to determine the causal agent of the disease and find solutions to the problem. A field survey revealed that most of the tomato plots in the area had been completely destroyed, and that the disease was accompanied by large populations of whiteflies. The whitefly population had built up in the nearby cotton fields, a crop which was being grown on a commercial scale for the first time in Israel. Soon enough, the suspicion that the whiteflies were the vector of the disease was confirmed, following controlled transmission experiments in the laboratory. Moreover, the “Marmande” tomato was found to be as susceptible as “Money Maker” to the disease, which was found to be viral in nature (Cohen & Nitzany, 1960). The virus was named Tomato yellow leaf curl virus (TYLCV) by the late Professor I. Harpaz of the Hebrew University (Cohen & Harpaz, 1964). Interestingly, similar disease symptoms had first been observed on tomatoes grown in the Jordan Valley as early as 1929, as well as in subsequent years (Avidov, 1944). The outbreaks of TYLCV disease were always accompanied by large populations of whiteflies (Cohen & Berlinger, 1986). However, the geminate shape of the viral capsid was first observed in 1980 (Russo et al., 1980), and it was only in 1988 that the virus was isolated (Czosnek et al., 1988). It took another 3 years to clone and sequence the virus, and to demonstrate that the genome of TYLCV is composed of only one singlestranded (ss) DNA molecule (Navot et al., 1991).

The first evidence of economic damage to vegetable crops caused by the whitefly Bemisia tabaci (Gennadius) in Israel was recorded in 1931 (Avidov, 1944). Since 1935, it has been a permanent pest, mainly in the Jordan Valley. Avidov concluded that the Bemisia whitefly can raise as many as 15 generations per year in the Jordan Valley, due to the favorable climate in the area (Avidov, 1944). The silvering of squashes caused by Bemisia, which was observed as early as 1963 (Baery & Kapoller, 1963), and the very wide host range of this insect indicate that the B (or silverleaf) biotype has been present in this region for a long time.