SUMMARY

Virus diseases are the major cause of economic losses in commercial cucurbit production in Lebanon. Besides mosaic inducing viruses, which have been noted for several years, greenhouse-grown cucumber and melon were often observed with severe yellowing symptoms that started as interveinal mottle on the older leaves and developed into severe yellowing symptoms as the leaves became older. The incidence, in fall crops reached up to 100 % in several locations along the Lebanese coast. The presence of Cucurbit yellow stunting disorder Crinivirus (CYSDV) was confirmed by the following tests: transmission by Bemisia tabaci to cucumber, mel-on and squash but not to lettuce or to Physalis floridana; presence in infected plants of two dsRNAs of about 8 and 9 kbp similar to those reported for criniviruses; observation of a few flexuous particles in leaf dip preparations; cytological alterations observed in infected plants, typical for Crinivirus and Clos-terovirus infections; and amplification by RT-PCR, us-ing CYSDV-specific primers, of a DNA fragment that showed 99 % nucleotide sequence identity with that of a CYSDV isolate from Spain. This is the first report o