Received for publication 16 Dec. 1985. We thank Antonio V. Duran, Joseph A. Principe, Hector S. Garcia, and Daniel A. Rashid for assistance in the field tests; Janet A. Foreman and Jay R.S. Schwed for asistance in greenhouse tests and laboratory assays; Steve Castle for assistance in collecting whitefly data; and J.E. Duffus and H.Y. Liu for virus isolates and whiteflies. This research was supported in part by the California Iceberg Lettuce Research Program. Mention of a trademark, propriety product, or vendor does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products or vendors that may also be suitable. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.