YELLOWING DISEASES OF CUCURBITS- A MAJOR THREAT TO LEBANESE AGRICULTURE Y. ABOU JAWDAH 1 , H. SOBH 1 , A. FAYAD 1 , AND H. LE COQ2 1 American University of Beirut, FAFS, Beirut, Lebanon. 2 INRA, Station de Pathologie Vegetale, BP 94, 84143 Montfavet Cedex, France Background and Objectives Virus diseases are the major cause of economic losses in commercial cucurbit production in Lebanon. On the Lebanese coast most open field production of summer and fall cucurbits has been abandoned, while the yield loss in fall greenhouse cucurbits is estimated at 40 - 60%. The objective of this study was to identify the major disease causal agents in order to develop appropriate disease management strategies. Materials and Methods The survey covered the major cucurbit growing areas. 345 samples were tested by DAS-ELISA for the presence of seven viruses. For identification of the causal agent of the new yellowing disorder of greenhouse-grown cucurbits the following tests were performed: Transmission tests using the vector Bemisia tabaci to five differential hosts in ffisect-proof cages, and analysis by agarose gel electrophoresis of ds-RNA extracts and of RT-PCR products from total RNA extracts. Results and Conclusions This survey revealed that zucchini yellow mosaic potyvirus (ZYMV) and cucurbit aphid-bome yellows luteovirus (CABYV) are the most important viruses of open field grown cucurbits, they are followed by watermelon mosaic virus 2 potyvirus (WMV2), papaya ringspot potyvirus (PRSV) and to a lesser extent cucumber mosaic cucumovirs (CMV). Greenhouse-grown cucumber and melon, showed severe yellowing symptoms that started on the older leaves and moved upward. The incidence in fall crops reached 100% in several locations along the Lebanese coast. ELISA tests revealed the presence of CABYV in only 15% of the cucumber samples showing yellowing symptoms, suggesting the presence of another virus. The presence of cucurbit yellow stunting disorder virus (CYSDV) was confirmed by the following tests.. The transmission of the yellowing symptoms by Bemisia tabaci to cucumber, melon and squash but not to lettuce nor toPhysalis sp; the presence in infected plants of two ds RNAs of sizes about 8 and 9 Kbp similar to those reported for closteroviruses; and the amplification by RT-PCR, using CYSDV specific primers, of a DNA fragment of about 460 bp as reported for CYSDV. This is the first report of CYSDV in Lebanon and its coexistence in greenhouses with CABYV. References 1. Celix, A, Lopez-see A, Almarza N, Guillamon ML, Rodriguez-Cerezo E, 1996. Phytopathology 86:1370-1376.