Dr. Paula Tennant

Virus diseases of fruit trees are responsible for severe crop losses, reduced commercial productivity and quality. Measures for controlling these diseases are generally based on prevention rather than treatment and as such focus on plant resistance through conventional breeding methods and more recently through biotechnology.

The Caribbean Health Leadership Institute
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CONVENTIONAL BREEDING METHODS

Traditionally, plant breeding involves the identification and selection of desirable traits in plants and combining these into one individual plant. Some traits, including virus disease resistance, few useful traits have been identified and only a limited number of commercial crop cultivars and rootstocks exhibit transferable characteristics. The development of crop plants through agricultural biotechnology has thus become an important strategy to implement effective and sustainable control measures in these instances.

BIOTECHNOLOGICAL METHODS

Development of crop plants through agricultural biotechnology involves the transfer of the genetic material responsible for a trait (that is, a gene) from one organism to another. The process is referred to as genetic engineering. Organisms modified in this way, are referred to as transgenic, genetically modified or simply as GMOs. Through this method, there can be the addition or alteration of an individual trait without the transfer of undesirable traits. Moreover, the plants need not be sexually compatible as is required by traditional methods of crop improvement.

UWI PRODUCING VIRUS DISEASE-RESISTANT PLANTS

Two projects with papaya and citrus that utilise both traditional and biotechnological methods for the development of new virus resistant varieties are in progress in the Department of Life Sciences and the Biotechnology Centre, in collaboration with the Departments of Biochemistry and Chemistry. Improved virus detection techniques and virus control programmes will enable farmers to find new solutions to problems of propagation and production.

PAPAYA

Papaya, an essential part of the diet in many tropical and subtropical regions, is a relatively new product on the international market. Commercial production of papaya in Jamaica since the mid 1980s has exceeded 20,000 metric tonnes. However, the industry is threatened by the papaya ringspot virus disease. As the name suggests, diseased trees produce fruits with ring-spotting blemishes and the leaves develop mosaic patterns resulting in a general decline in tree vigour and fruit production. Natural genetic resistance is not available in commercial papaya cultivars. Using biotechnological methods, a gene of the virus was transferred to papaya to confer virus resistance. Promising cultivars have undergone some four years of field-testing. They exhibit varying levels of resistance against virus infections. Moreover, selected nutritional components (for example, vitamins, minerals, acidity, and sugars) of the fruits are comparable to those of conventional papaya cultivars. Safety studies with animal models also show that there are no detrimental effects associated with the consumption of the transgenic papaya.

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CITRUS

Although relatively small in absolute trade and production volumes, the citrus industry in Jamaica plays an important role in the country’s national economy. Jamaica presently produces Sweet oranges, Ortaniques, Grapefruit and Ugli on some 30,000 hectares situated across all 14 parishes. Fresh fruit or processed fruit are exported. However, since the mid 1980s, there have been numerous reports of decline in fruit production.

CITRUS DISEASE

One of the prevalent thought responsible is tristeza (which means “sadness” in Spanish). The disease is caused by a virus, Citrus tristeza virus, which similar to Papaya ringspot virus, is spread by aphids but also by the use of infected scion materials at grafting. Two severe economical strains of Citrus tristeza virus cause decline syndromes and stem pitting disease. Given the complex biology of citrus, however, breeding by conventional methods is difficult and few cultivars are available.

Moreover, the virus has evolved a defense system that targets and overcomes resistance mechanisms in plants. Citrus tristeza virus represents the biggest threat to the Jamaican citrus industry because 90% of the citrus orchards are established on the susceptible sour orange rootstock and the insect vector of the virus is distributed throughout the island. A study of CTV strains in Jamaica has been initiated with the long-term aim of reducing losses in fruit quality and production. The widespread distribution of severe CTV strains was found, thus emphasizing the need for continued testing and removal of CTV-infected trees. Eventually, virus resistant varieties developed through agricultural biotechnology will be investigated as means of managing the pathogen.