Banana and citrus, two of the region's major fruit crops, are seriously threatened by virus diseases. Once a plant is infected with such diseases, it will remain infected until it dies, and so will its progeny. Insect and aphid vectors feeding on infected plants will go on to spread the disease further.
Some of the worst diseases of banana and citrus are transmitted through planting materials. New techniques are now available which can produce seedlings free of virus and other diseases. There are also new laboratory techniques for the diagnosis and indexing of virus diseases. However, problems still remain.
Indexing is expensive in relation to farm incomes in less industrialized countries. Such countries must develop systems of disease management which use laboratory tests as an occasional backup, not as a standard procedure for all orchards. Furthermore, new viruses are appearing every year, and new laboratory tests must be devised to identify them.
Distribution is another problem - often disease-free planting materials are not available in sufficient numbers when farmers need them, or else they are so expensive that farmers do not want them. Since disease-free seedlings are quite vulnerable to infection, another major problem is how to keep them free of disease after they are planted out in the field.
Major Disease Problems of Citrus
Almost 90% of citrus trees in Asia are infected with tristeza, a virus disease transmitted by several species of aphid. Many strains are mild ones which do little damage to the plant, but new virulent strains have arisen in recent decades which are a major threat to the citrus industry world-wide. The symptoms of tristeza vary according to the virus strain and the scion-rootstock combination. The use of resistant rootstock such as mandarin or trifoliate orange is the main method of control, plus the certification of budstock. Pre-immunization, whereby plants are inoculated with mild strains of virus to protect them from severe strains, has sometimes also proved useful.
Greening is the most destructive citrus disease in tropical Asia. It is caused by a fastidious bacteria, and is spread by the citrus psyllid. Control of this disease is based on early detection, which in turn depends on accurate diagnosis, and on the early removal of infected shoots and trees. This includes the detection of greening organisms in symptomless trees.
Control of greening disease also depends on control of the psyllid vector. Psyllids prefer to lay their eggs on new shoots. For this reason, the titer of the greening organism tends to rise with the spring flush. It is important to synchronize chemical sprays with the growth of new shoots. Growers applying chemical pesticides should also try and protect the natural enemies of the psyllid. One natural enemy, the parasitoid wasp Tamarixia radiata, is mass-produced in Taiwan for release in citrus orchards.
Major Disease Problems of Banana
All banana viruses are transmitted in infected planted materials. After planting, the viruses are usually spread further by aphids.
Banana Bunchytop (BBTV)
This is the most serious virus disease affecting banana and plantain in Asia. It has been present in the region since the last century, and probably much earlier than this. The virus has a limited range of vectors, and is found in only a few species. Apart from abaca, the main spread is from banana to banana. This means that the eradication of infected plants is an effective control measure.
Cucumber Mosaic Virus (CMV)
Unlike banana bunchytop, both the virus and its vectors occur on a wide range of plant species. It is found in common crops such as bean, cucumber, pepper and tomato. Eliminating the sources of virus outside the crop is important in controlling this disease.
Banana Streak (BSV)
This virus was only identified recently, being first described in Africa in 1974. The late diagnosis is probably because it is a highly variable virus, which makes detection and indexing difficult. The virus is widespread, and has probably existed for a very long time. Symptoms tend to be more severe in poorly managed plantations.
A startling recent discovery has been that all banana and plantain species contain segments of the DNA of this virus. When virus-free varieties of banana are propagated by tissue culture, the integrated DNA of the virus in the DNA of the plant sometimes becomes activated. As a result, the new plants are infected with BSV.
Detection of Virus Diseases
The reliability of virus indexing depends on the detection method used, the serological diversity of the virus, and the sampling method followed.
Viruses are generally not evenly distributed throughout the plant at a uniform rate. Concentrations of BBTV, for example, are much higher in younger leaves than in older ones, and are higher in the midrib than in the rest of the leaf. In contrast, BSV is present in higher concentration in old leaves than in young ones. The distribution of CMV is very uneven, and can vary greatly between different leaves of the same plant, or even different parts of the same leaf. There seems to be no relationship between the amount of CMV virus in the plant, and the severity of the symptoms.
The distribution of virus particles in the plant must be taken into account when collecting samples for testing. Otherwise, an infected plant may test negative for virus.
How to Detect Virus
Just looking at the plant is the easiest way of diagnosing a virus, but it is not very reliable. Mild strains of virus disease, or virus disease in its early stages, may have no symptoms. Mixed infections with several viruses are common, while the same virus may produce different symptoms in different plants. Laboratory tests are needed for an accurate diagnosis. There are two kinds of laboratory tests, PCR and serological assay.
This is based on antibodies produced by animals (usually rats or rabbits). When suitable antibodies are available, the most sensitive and efficient practical assay for plants is enzyme-linked immunosorbent assay (ELISA). In most cases, ELISA is the indexing method of choice. It is sensitive, easy to use, and needs minimal equipment.
Polymerase Chain Reaction (PCR) is a very sensitive assay, and can be used to test for 3-4 viruses at a time. There are two main problems in using it. The reagents used are expensive, and a tedious process of preparation is needed for each sample. It is used mainly for foundation stock, or in other situations where accuracy is very important.
Use of Tissue-Cultured Seedlings
Plantlets produced by tissue culture have the advantage, not only of being free of disease, but of being relatively uniform. In general, they give higher quality fruit, and production costs are lower. However, there are also some drawbacks. Banana plantlets grown by tissue culture are more susceptible to CMV than suckers. They are also vulnerable to herbicide damage.
Plantlets produced by tissue culture are fairly expensive. Banana farmers who use suckers get their planting materials free, as do citrus producers who produce their own seedlings by grafting or marcotting. Growers should buy disease-free planting materials only in areas which are relatively free of both virus and vectors. It is a waste of money to use these materials in areas where the virus still persists.
Management of Disease-Free Orchards
Protecting seedlings from infection in the field involves spraying with insecticide to control the vectors, disinfection of pruning tools, and in the case of citrus, the use of resistant rootstock. Cultural practices which can help keep orchards and plantations free of disease include windbreaks to reduce the number of windborne vectors, and the use of catch crops such as curryleaf. Both curryleaf and jasmine orange are preferred hosts for citrus psyllids, and can be grown around citrus orchards as trap plants.
Techniques of indexing diseases and diagnosing virus diseases are improving rapidly. Virus diseases can be identified and indexed with a speed and precision that would have been impossible a decade ago. Techniques of producing virus-free planting material have also developed rapidly. However, there are still many difficulties in applying these techniques in a cost effective way, and in keeping seedlings free of disease after they are planted out in the field.
Since virus diseases and citrus greening are easily transmitted by insect vectors, early detection and early removal of infected plants are critical. However in practice early detection is often difficult, since there may be few or no symptoms. Even where plants are definitely diagnosed with virus disease or greening, eradication may be difficult. Infected plants may still give a profitable yield which farmers are unwilling to sacrifice, particularly if the plants show only minor symptoms.
FFTC International Workshop Disease Management for Banana and Citrus: The Use and Management of Disease-Free Planting Materials
Location: Davao City, Philippines
Date: October 14-16 1998
No. Participating Countries: 9 (Australia, Indonesia, Japan, Malaysia, Philippines, Taiwan ROC, Thailand, USA, Vietnam)
No. Papers: 18
No. Participants: 75
Co-sponsors:International Network for the Improvement of Banana and Plantain - Asia and Pacific Network (INIBAP-ASPNET)
Philippine Council for Agriculture, Forestry and Natural Resources Research and Development (PCARRD), Department of Science and Technology
Davao National Crop Research and Development Center, Bureau of Plant Industry, Department of Agriculture, Philippines
List of Papers
1. Epidemiological review on citrus greening and virus diseases of citrus and banana, with special reference to disease-free nursery system
- Hong-Ji Su
2. Production and cultivation of virus-free banana tissue-culture plantlets in Taiwan
- Shin-Chuan Hwang
3. Viruses of banana and methods for their detection
- John Thomas
4. Virus and virus-like diseases of banana and citrus in Malaysia: Status and control strategies
- Ching-Ang Ong
5. Disease management in citrus orchards planted with disease-free seedlings in Thailand
- Suchat Vichitrananda
6. Recent progress in the research on citrus greening in Asia, including serological diagnosis
- Yoshihiro Ohtsu
7. Ecology of the insect vectors of citrus systemic diseases and their control in Taiwan
- Chiou-Nan Chen
8. Citrus greening control project in Okinawa, Japan
- Shinji Kawano
9. Management of viral streak in banana and plantain: Understanding a new challenge
- Ben Lockhart
10. Pathological and molecular characterization of BBTV strains in Asia
- Hong-Ji Su
11. The impact of tissue culture plants in the ongoing eradication and rehabilitation program in the Philippines
- Lydia Magnay
12. Epidemiology and integrated management of Abaca bunchytop in the Philippines
- Avelino Raymundo
13. Rehabilitation of BBTV affected areas in the Philippines: Experiences and problems
- Rene Rafael Espino
14. Status of disease management of citrus in the Philippines
- Ceferino Baniqued
15. Management of disease-free citrus seedlings in southern Vietnam
- Le Thi Thu Hong
16. Management of disease-free citrus seedlings in the North of Vietnam
- Ha Minh Trung
17. Establishment of disease-free foundation stock and nursery for controlling greening disease and citrus tristeza virus: The Sarawak experience
- Chan Hock Teo
18. Status of disease management of virus diseases of banana and citrus in Indonesia
- A. Nurhadi
Index of Images
Figure 1 Leaf Symptoms of Pummelo with Greening Disease (Yellowing and Mottling). a Healthy Leaf Is Shown on the Right.
Figure 2 Citrus Tree with Tristeza Virus, Showing Dieback and Poor Fruit Set.
Figure 3 Citrus Brown Aphid (Toxoptera Citricida Kircaldy) the Primary Vector for Citrus Tristeza Virus
Figure 4 Banana Plant with Bunchytop. Note the Small Size of the Hand.