In banana and citrus, as with other vegetatively propagated crops, virus diseases are often transmitted through planting materials. Since 1997, the Center has conducted a three-year survey to study the incidence of virus diseases, and the feasibility of carrying out virus testing in the laboratories of various southeast Asian countries.
Once a plant is infected with virus or greening, it remains infected until it dies. This may take several years, during which time the yield declines and the quality of fruit is poor. Even more important, during this time it is a source of infection which endangers healthy plants. Systemic diseases of this kind can only be controlled by planting healthy seedlings produced from pathogen-free mother stock. This in turn depends on a nursery system which uses modern techniques such as tissue culture (for banana) or micrografting (of citrus) to produce healthy plants.
Plants used as mother stock must be carefully and continuously monitored to ensure that they remain free of virus. Young orchards and new plantations must then be carefully managed to minimize the chances of virus infection. This involves monitoring the insect vector, and controlling outbreaks of the vector with carefully timed pesticide applications.
The Plant Virus Laboratory of National Taiwan University has developed a method of rapid diagnosis, using molecular biotechnology to test for citrus greening, and for the viruses which cause tristeza and tatter leaf in citrus, and banana bunchytop, banana streak and banana mosaic. The staff of the Virus Laboratory and the Center have collaborated in holding a number of training courses on the laboratory techniques used to index these plant viruses and to produce pathogen-free seedlings.
In 1999, the Center continued to collect information on the spread of virus diseases in citrus and banana in the region. A survey of citrus orchards was carried out in Cheju Island, Korea. Citrus is the most important crop on the island, where there are thousands of small citrus orchards, most of them producing Satsuma mandarins. Fortunately the island is free of greening disease, since the citrus psyllid which transmits the disease cannot survive the cold winters. However, tests showed that tristeza virus is widespread. Tatter leaf virus was also found in some orchards, although it was not very common.
Both viruses can be present in infected foundation stock, but tatter leaf is fairly easy to control once it has been identified. Whereas tristeza is vectored by aphids, tatterleaf is transmitted mechanically from tree to tree on e.g. grafting knives or pruning shears.
Staff of the Cheju Citrus Research Institute will be traineduring the coming year (2000) in techniques of virus indexing, and will be provided with diagnostic probes. These will enable them to establish protocols for indexing and detecting virus diseases, and for monitoring the health of foundation stock and citrus nurseries.
A survey in West and East Java, Indonesia, found that all major cultivars of calamondin (a small acid fruit with a loose skin) and Valencia sweet orange were severely infected by virus. Samples collected by the survey team all tested positive for citrus greening and citrus tristeza. Even some foundation stock were infected with citrus greening.
Samples taken of young banana plants in a nursery all tested positive for banana streak, and one also tested positive for banana bunchytop. Fortunately, Indonesia has several laboratories which have excellent facilities for virus indexing. The Center could provide staff in these laboratories with training and diagnostic probes.
The survey team also discussed with Indonesian experts how the Center might provide technical support for national programs to produce disease-free planting materials.
Two main types of laboratory testing are used, PCR and serological assay. For several years, the Center has been providing a number of government laboratories in the region with the PCR primers used to index citrus greening, and the monoclonal antibodies and enzyme-conjugate antibodies used in ELISA testing to diagnose virus diseases. Primers and antibodies were provided free of charge in 1999 to laboratories in the Philippines, Malaysia and Vietnam, as well as Indonesia. Staff working in these laboratories were trained in their use.
Countries surveyed: Indonesia and Korea
Members of the survey team were:
Dr. Hong-Ji Su, National Taiwan University
Dr. Woo-Nang Chang, FFTC Project Coordinator
Figure 1 Citrus Orchard in Japan with Overhead Sprinkler System for Irrigation and Pesticide Sprays
Figure 2 Citrus Brown Aphid (Toxoptera Citricida), Vector of Citrus Tristeza Virus