Technical Abstract: Cucurbit yellow stunting disorder virus (CYSDV) was identified in the fall of 2006 affecting cucurbit production in the Imperial Valley of California, the adjacent Yuma, AZ region, as well as nearby Sonora, Mexico. There was nearly universal infection of fall melon crops in 2006 and 2007, and late, limited infection of 2007 spring melons. Survival of CYSDV through the largely cucurbit-free winter months suggested the presence of weed or alternate crop hosts. Previous studies had suggested a very limited host range for CYSDV, limited primarily to members of the Cucurbitaceae. Subsequent research identified lettuce as an experimental host, although the virus had not been found to infect lettuce in the field. To determine potential reservoir hosts for CYSDV in desert production, weed and crop hosts were collected from throughout the region over a period of 24 months, and were tested for the presence of CYSDV by RT-PCR using CYSDV HSP70h- (3) and coat protein gene-specific primers. Most non-cucurbits collected from infected melon fields and nearby areas were symptomless and virus free, however, CYSDV was detected in snap bean (Phaseolus vulgaris), lettuce (Lactuca sativa), ground cherry (P. wrightii), alkali mallow (Sida hederacea), silverleaf nightshade (Solanum elaeagnifolium), lambsquarters (Chenopodium album), redroot pigweed (Amaranthus retroflexus) and London rocket (Sisymbrium irio), members of 7 taxonomic families. Typical crinivirus symptoms of interveinal yellowing and leaf brittleness were observed on infected snap bean and ground cherry, while lettuce and the other infected weed hosts were symptomless. Transmission tests demonstrated that lettuce, snap bean, alkali mallow, ground cherry and buffalo gourd could serve as virus reservoir hosts for transmission of CYSDV to melon and other cucurbits.