A large proportion of Alabama's fresh market tomato production occurs in Blount and St.Clair continues in North Alabama and each year since 1992 tomato growers in these and surrounding counties have suffered severe losses due to an epidemic of the plant virus cucumber mosaic virus (CMV). Recent AAES and Alabama Cooperative Extension System (ACES) research indicates that genetically engineered tomato plants are less susceptible to CMV, giving growers a viable management option.

Tomatoes are typically transplanted to the field at two-week intervals beginning in April and continuing through July. The severity of disease caused from infection by CMV varies with the age of the plant at the time of infection. The younger the plant at the time of infection, the more severe the disease and the greater the loss in production of marketable fruit. Cultural practices used by growers in North Alabama, such as introduction of young tomato transplants every two weeks, provides a continuous supply of young, highly susceptible plants. As a result, each new setting of tomato plants becomes severely affected by disease, produces little or no marketable fruit, and provides CMV inoculum for subsequent plantings.

CMV is a difficult plant pathogen to manage under natural conditions because this virus can infect more than 800 plant species and is transmitted from plant to plant by anyone of 75 different aphid species. These two factors make CMV widespread in nature, particularly in Alabama's mild climate. To make matters worse, an aphid can acquire the virus by simply probing an infected cell with its stylet (as briefly as five seconds) and in a similar manner transmit the virus to another plant. This type of transmission of a virus by its aphid vector makes it impossible to manage the virus with aphid-controlling insecticides. In fact, research has shown that insecticides make aphids hyperactive, thereby enhancing plant-to-plant spread of virus.

CMV3 plant (top) free of symptoms and a Vec plant with severe CMV symptoms.

Management strategies for CMV depend on availability of commercially acceptable, genetically resistant varieties. Unfortunately, there are no fresh market tomato varieties available to growers that are genetically resistant to CMV. AAES and ACES scientists pursued an alternative approach to manage the extreme levels of CMV that occur in North Alabama. Tomato plants genetically engineered to express the capsid protein gene of CMV referred to as transgenic plants) were valuated in a grower's field in Blount County in 1995 and 1996 for their ability to withstand the extreme levels of disease that occur in North Alabama.

Four genetically engineered tomato lines—CMVl, CMV2, CMV3, and Vec—were evaluated. The three CMV lines expressed the capsid protein gene of CMV, while Vec represented an engineered susceptible control line. Experiments were carried out twice in 1995, early and late in the season, and once in 1996.

In each experiment, plants were evaluated at one, two, and four weeks after transplanting for development of CMV-like symptoms (stunting, yellowing of leaves, and deformation of young leaves). All plants were tested for the presence of CMV by an enzyme-linked immunosorbent assay (ELISA) test at the time of transplant and at four weeks after transplant (see the table). A similar trend was observed between experiments regarding development of CMV symptoms and accumulation of virus in plants. The data presented is representative of each of the trials.

Within the susceptible control treatment, Vec, 15% of the plants developed symptoms one week after transplanting with almost all plants (96%) showing symptoms by four weeks. In striking contrast to Vec plants, none of the CMV2 and CMV3 plants developed symptoms and only 9% of CMVI plants showed symptoms after four weeks.

Evaluation of CMV infection correlated well with symptom data in Vec plants; however, this was not the case for CMVl, CMV2, or CMV3 plants. All Vec plants were infected with CMV while 42%, 68%, and 67% of the plants in treatments CMVl, CMV2, and CMV3, respectively, were shown to be infected even though they were free of symptoms. Interestingly, even though many of the plants in treatments CMVl-3 were infected, the amount of CMV in these plants was significantly less than in Vec plants (see the figure), which might explain their apparent lack of symptoms.

These results show that infection of tomato with CMV under natural conditions of extreme disease pressure can be reduced through the use of genetically engineered plants. The transgeic plants apparently were not fully resistant to infection by CMV since a fairly high percentage of the plants became infected. However, the amount of CMV that accumulated in the transgenic plants was significantly less than in plants in the control treatment. Thus, even under the extreme disease pressure that occurs in North Alabama, these plants resisted CMV and generally remained free of symptoms. Genetically engineered virus resistance may serve as an alternative practical approach to tomato production.