Charles E. Johnson, Ding Shih and Joey Quebedeaux
Strawberries are one of the most popular fruit crops grown in the world. Per capita consumption of fresh strawberries in the United States has increased in the past 10 years and is predicted to continue to rise in the foreseeable future. Most U.S. commer-cial production of strawberries is in California where the arid climate and low disease pressure make ideal growing conditions.
Most of Louisiana’s commercial strawberry production is in Livingston and Tangipahoa parishes. Louisiana growers set transplants in October and begin harvesting in late February. Harvesting continues through early May. Two of the major hazards to strawberry production in Louisiana are diseases that attack the plants and the cold winter weather that freezes the tender young fruit. Farmers routinely protect the crop from freezing with row covers and sprinkling systems. Diseases, however, are much harder to control because of Louisiana’s heavy rainfall and warm weather, an ideal growing condition for pathogenic fungi that attack both plants and fruit.
Growers in Louisiana must regularly spray fungicides to maintain healthy plants and marketable fruit. Fungicide sprays are applied weekly and sometimes twice weekly during harvest. A grower often will apply fungicides 15 times or more times during a growing season.
Because of all the spraying, disease control is a major expense in strawberry production for Louisiana growers. A more effective and less expensive method would be to develop strawberry varieties resistant to prevalent diseases. Traditional breeding methods involve making crosses and growing a large number of seedlings to select individual plants with desirable traits. The strawberry has a complex genetic system that requires a breeder to screen tens of thousands of seedlings to retrieve the desirable combination. This long-range commitment often takes 10 or more years to develop a new strawberry variety.
A serious strawberry disease in Louisiana and in the Gulf Coast region is anthracnose crown rot. This fungal disease can lead to heavy crop losses and even total crop loss under certain weather conditions. The primary fungal species that cause this disease are Colletotrichum fragariae and Colletotrichum acutatum. Currently, there is no fungicide registered for use on strawberry that reliably controls the anthracnose disease. The development of strawberry cultivars resistant to the anthracnose disease should provide a means for reliable, long-term disease control and offer economical benefits by reducing or even eliminating the need for costly spraying.
In the mid 1990s, LSU AgCenter scientists launched a project to develop strawberry lines resistant to anthracnose disease. In this project, scientists used the soil bacterium Agrobacterium tumefaciens to genetically transform Chandler strawberry plants into a stronger, more disease-resistant variety. This bacterium has the ability to transfer DNA to the host plant. When used as a gene vector, the Agrobacterium is altered in the laboratory to contain desirable genes.
The genes of interest are for production of two enzymes that enhance the defense mechanisms of the strawberry plant. Specifically, the Agrobacterium was altered to contain a gene from snap bean for the production of the enzyme chitinase and a gene from tobacco for production of the enzyme glucanase. These enzymes are found naturally in strawberry plants in various concentrations. When transferred into the strawberry from the Agrobacterium, these new genes could potentially increase the production of the naturally occurring defense agents in the strawberry.
Leaf strips of Chandler strawberry plants were inoculated with transformed Agrobacterium tumefaciens and then cultured on a regeneration medium containing antibiotics to control growth of Agrobacterium and to select out the nontransformed cells. Plants regenerated through this method were tested for the presence of the new genes by biochem-ical analysis. Transformed plants were transferred to peat pellets in a transition chamber and remained for 21 days before being transferred to a green-house. Transgenic lines were evaluated under field conditions for three years. Fruit size, shape and overall quality were evaluated along with commercial varieties. Plants from nine transgenic lines were inoculated under controlled conditions with a virulent isolate of Colletrotrichum, which is the causal agent of anthracnose. After evaluation, disease severity ratings indicated that the transgenic lines vary in their resistance to this prevalent pathogen. Several transgenic lines exhibited more resistance than the nontransgenic control.
The transgenic strawberry plants in this study varied considerably in fruit characteristics and disease resistance. The primary purpose of these experi-ments was to evaluate and develop methods for transferring genes to strawberry plants. One of the several challenges to the plant breeder in this process is the uncontrollable morphol-ogical and physiological changes that take place during the tissue culture procedure. The methods developed will enable scientists to produce large numbers of plants from which to choose the desired horticultural characteristics. This research has established basic methods from which scientists can integrate other useful genes into the strawberry genome.
Strawberry plants resistant to prevalent diseases would enable growers to use fewer pesticides in production. This reduces the cost of production and lessens the impact of pesticides on the environment. Evidence from these and other experiments indicate a good possibility of developing a new strawberry variety for Louisiana growers.