That savory slice of juicy tomato reserved for the top of a freshly grilled burger or a gently tossed salad has been spared from nature's short list. Cornell plant pathologists have found the gene that resists the Cucumber Mosaic Virus (CMV), a plant disease that severely threatens tomatoes. The breakthrough: they have genetically engineered the resistance by introducing a segment of the viral genome -- the coat protein gene -- into the tomato.

"It's a devastating disease, very severe in places like Italy, Spain, China, Japan and it can be severe in California," said Marc Fuchs, Ph.D., Cornell research associate in plant pathology at Cornell's Agricultural Experiment Station in Geneva, N.Y. "Viruses are very difficult to control, since there is no viricide to eradicate them. Once a field is diseased, there is nothing a farmer can do. The crop is ruined."

The economic losses for tomatoes due to CMV infections in many countries has been so profound that tomato production was abandoned in some areas. CMV- infected tomatoes suffer an oppressive stunting of the plant's growth and significant yield losses. While the tomatoes are edible, they are not marketable. "There are reduction and distortions in size," Fuchs said. "Eating tomatoes with this plant virus won't harm humans, but consumers in almost every country wouldn't buy them for the fruit's cosmetics." Fuchs collaborated with colleagues Rosario Provvidenti (sic) and Dennis Gonsalves, both Liberty Hyde Bailey professors of plant pathology at Cornell's Agricultural Experiment Station, and Jerry L. Slightom of the Upjohn Co., Kalamazoo, Mich. The study, "Evaluation of Transgenic Tomato Plants Expressing the Coat Protein Gene of Cucumber Mosaic Virus Strain WL under Field Conditions," was published recently in the American Phytopathological Society's journal Plant Disease, (Vol. 80, No. 3). The research was funded by the Asgrow Seed Co., Kalamazoo, Mich., and the U.S. Department of Agriculture.

Scientists had scoured the world looking for related plants to move from the wild world into the domesticated agricultural market. In the process, breeders found some wild tomato plants that resisted the virus, however they were not able to transfer this resistance into commercial varieties. The gene from the virus is spliced into the chromosome of the non-resistant tomato. The result is a tomato plant resistant to CMV, something that had previously eluded scientists.

Gonsalves said this technique has been commercially available for other fruits and vegetables. In fact, Gonsalves' lab has collaborated with Asgrow Seed Co. to develop virus resistant vegetables. Freedom II, the genetically engineered squash variety resistant to zucchini yellow mosaic virus and watermelon mosaic virus 2, was distributed for commercial sale last year.

But, have the scientists found a way to permanently reduce diseases? The Cornell researchers feel that this technique will help control a number of viruses. However, it is not the only answer. They said a range of approaches need to be taken to control plant viruses on a case-by-case basis.