Control of bacterial wilt by agricultural biotechnology

• University of the Philippines at Los Banos, Philippines
• Asian Vegetable Research and Development Centre, Taiwan
• Bogor Research Institute for Food Crops Biotechnology, Indonesia
• University of Queensland, Australia
• University of Adelaide, Australia

Bacterial wilt (BW) causes significant loss of production in bananas, potatoes, eggplant, peppers, tomatoes, peanuts and tobacco in tropical parts of Africa, Asia, South and Central America and India. The disease is a major problem because it hits the staple crops that provide income to small farmers, and limits the range of crops that may be grown successfully.
The bacterium Ralstonia solanacearum (RS) that causes bacterial wilt seems genetically variable, apparently out-manoeuvring normal plant breeding solutions. For example, resistant cultivars of tomatoes have been bred in Taiwan, but after one or two years' cultivation in the Philippines or Indonesia they again become susceptible to wilt for reasons unknown. However, cultivars of certain crops show some resistance. Control can also be achieved by crop rotations that include rice.
Integrated disease management (IDM) is a possible solution to bacterial wilt. This has been successfully implemented in Australia and a few other countries. Controls include the provision of uninfected seed (especially of potatoes), breeding resistant cultivars, systems of crop rotation, and soil amendment. This project contributed to the breeding of resistant cultivars. In addition scientists attempted to control the bacterium in potatoes and tomatoes by using weakened strains of RS.

The main objectives of this project were: 1) to apply molecular genetics as a component of the IDM of bacterial wilt, with a focus on tomatoes and potatoes; 2) to refine molecular genetic techniques for the identification of R. solanacearum and make them suitable for laboratory and field conditions existing in Asian countries; 3) to apply molecular genetic techniques for the grouping of field strains of R. solanacearum infecting tomatoes - such groupings to be denoted ecotypes; 4) to develop a molecular genetic method for quantitative measurement of RS in soil; 5) to extend the range of DNA probes available for detection of RS in plant tissue and soil, and for classification of isolates of RS; 6) to use molecular genetics to characterise virulence of RS to provide information relevant to IDM for BW, particularly for plant breeding components.

The major activity of the project was to develop a modified breeding strategy for tomatoes based on additional knowledge about the pathogen, its variation and its disease producing properties. There is a complex interaction of host, pathogen and environment that differs with each geographical location. A new concept, that of the ecotype, would be developed, an ecotype being a group of RS strains which have evolved in a specific ecological region and which can be distinguished by molecular genetic and other criteria. Procedures were set up to isolate and characterise a collection of strains from all the geographical areas of the participants in the project. These strains would be sent as live cultures to the University of Queensland for storage.
Given the lack of success in breeding BW-resistant potatoes, the most promising avenue for disease control lies in RS-free potato seed. Scientists attempted to achieve this by establishing a molecular genetic test that could detect a small number (10 or less) of bacteria in a given volume of potato tissue. The availability of a robust, sensitive and precise test for detecting RS in soil would be a valuable accessory tool.
The project team established field trials on the Atherton Tablelands in Queensland to address the following issues as they related to potato production and BW: 1) the survival of RS in the soil following a non-host crop; 2) the effects of crop rotation, biofumigation and soil amendments on the severity of BW in potatoes; 3) the potential of biological control agents as one of the components of an integrated approach for the control of BW of potatoes; 4) the detection of latent infection with potato tubers and the detection of RS in soil samples with a history of BW.

The project scientists developed a highly specific, robust molecular genetic test for detection of RS in plant, soil and laboratory material. This test has proven effective in many laboratories in many countries. They also demonstrated the role of molecular genetic techniques in strategic proposals to improve breeding for resistance to BW in tomatoes.
As well, a unique collection of strains of RS from Indonesia, Taiwan, Philippines, Vietnam and Australia was assembled at the University of Queensland. These strains have been extensively characterised by a variety of molecular genetic tests, for aggressiveness and for other features. Given the difficulties of transporting strains of this organism between countries, this collection represents a unique research resource for future work on this organism and this disease. Scientists also constructed the first physical/genetic whole genome map of RS.
Participants in the project from Indonesia and the Philippines have upgraded their molecular genetic, research management, training capacity and team-building skills and experience. As well The Crawford Fund Master Class program had its beginnings in ACIAR projects PN1990/015 and PN1994/052.
The project made a significant contribution towards the solution of an important but difficult and intractable regional problem. It has made great advances in relation to new knowledge related to the population and molecular genetics of RS and in promoting technology transfer and cooperation between Australia and developing countries. In the long term, the scientific approach utilised in this project, which is based on a thorough understanding of the genetic characteristics of the causative organism and how it relates to its pathogenicity and virulence, is the only basis for developing a rational strategy for the control of bacterial wilt. The project should serve as a foundation for future research in targeted areas that will bring about the desired benefits.