Watermelon chlorotic stunt and Squash leaf curl begomoviruses-New threats to cucurbit crops in the Middle East

During the year 2002, two new diseases with unknown etiologies were detected in cucurbit crops in Israel. One disease was detected in squash fields throughout the country, while the second appeared in a single watermelon plot in the south, just outside the city of Elat. The infected watermelon plot was eradicated, but nonetheless the new disease spread throughout the country and today it is present in all the watermelon production areas. Both diseases were associated with elevated whitefly populations. Indeed, it was found that both are transmitted only by whiteflies, and are incited by two begomoviruses. Both viruses were cloned and sequenced, and were identified as Watermelon chlorotic stunt virus (WmCSV) and Squash leaf curl virus (SLCV). The host range of the Israeli WmCSV isolate (WmCSV-IL) was determined and resembles the host range of the three other known WmCSV isolates (from Yemen, Sudan, and Iran), but with a few differences. Although no genetic resistance to WmCSV was observed in cultivated watermelon [Citrullus lanatus (Thunb.) Matsum. & Nakai], resistance to the virus was found in a wild relative, colocynth (Citrullus colocynthis (L) Schrader). Nucleotide sequence comparisons revealed that WmCSV-IL is highly homologous to other WmCSV isolates, with the highest homology (nearly identical) to the isolate from Sudan. SLCV-IL host range was determined as well, and was also found to be similar to other SLCV isolates. However, following genome sequencing, it was found that due to two separate point mutations, two viral open reading frames (ORF) were altered. The AC2 ORF was extended by 129 nucleotides, while the BV1 ORF was reduced by 99 nucleotides.