Disease Management for Banana and Citrus
Two of the region's most important economic crops are threatened by virus diseases
Virus Diseases of Banana and Citrus
Some of the worst diseases of banana and citrus are transmitted through planting materials. Plants infected with virus disease will remain infected until they die and so will their progeny. Meanwhile, insect and aphid vectors feeding on infected plants will spread the disease further.
In October 1998, FFTC held an international workshop in Mindanao, southern Philippines, on the production of disease free banana and citrus seedlings, and how to keep them free of disease once they are planted out in the field.
Citrus Diseases
Two of the most damaging diseases faced by Asian citrus farmers are tristeza virus and greening disease.
Tristeza
Almost 90% of citrus in Asia is infected with tristeza virus. Many strains are mild ones which do little damage. Recent years have seen the appearance of new, virulent strains which are a major threat to the region's citrus industry. The main method of control is to use resistant rootstock such as mandarin or trifoliate orange, plus the certification of budstock.
Greening Disease
Citrus greening is the most destructive disease of citrus trees in tropical Asia. It is caused by a fastidious bacteria which is spread by the citrus psyllid. Control of this disease is based on control of the psyllid vector.
Early detection of the disease is also important, so that infected trees can be destroyed before they infect others. This in turn depends on accurate diagnosis, which can only be done in a laboratory. At the early stages of infection, there may be few or no visible symptoms.
The citrus psyllid
Psyllids prefer to lay their eggs on new shoots. With the spring flush of new leaves, there is a rise in both the number of psyllids on the plant, and the titer of greening organisms in the plant. It is at this time that pesticides sprays are most effective.
In itself, the psyllid is a very minor nuisance. It is important only as a vector of disease. Fortunately, the Asian psyllid cannot tolerate low temperatures, so greening disease is not found in temperate countries like Korea. A cold-tolerant species of citrus psyllid does exist in South Africa, but so far Asia has remained free of it.
Biological control of psyllids
Scientists in Taiwan have found several parasites of psyllids, but these themselves have natural enemies which keep their numbers down. Also, natural predators have a slower population build-up than psyllids. They cannot keep down the population surge of psyllids in the spring.
A parasitic wasp, Tamarixia radiata, from Reunion Island in the Indian Ocean, was found to be quite effective in helping to keep down psyllid populations. It is now being mass produced in Taiwan for release in citrus orchards.
Banana Virus Diseases
All banana viruses are transmitted in infected planted materials. After planting, the viruses are usually spread further by aphids.
Banana Streak (BSV)
This is a variable virus, with many different strains and symptoms. One typical symptom is chlorotic or necrotic streaks and flecks on the leaves (see left). Some strains can cause the death of the heart leaf and pseudostem. Symptoms tend to be more severe in poorly managed plants. The disease may have little or no effect on yield if infected banana plants are given plenty of fertilizer and good management.
Cucumber Mosaic Virus (CMV)
Both the virus and its vectors occur on a wide range of plant species. Eliminating the sources of virus outside the crop may be more important than the use of virus-free planting material in controlling this disease.
A startling discovery has been that all banana and plantain species contain segments of the DNA of CMV virus. When virus free varieties of banana are propagated by tissue culture, the DNA of the virus can become integrated into the DNA of the plant and activated,. As a result, the new plants are infected with CMV virus.
Banana Bunchytop (BBTV)
Banana Bunchytop is the most serious virus disease of banana in Asia. It has been present in the region for some time. At an early stage of infection, the plant is dwarfed, and there are dark green streaks or dots around the midrib and lateral veins. At a later stage the leaves are erect with yellow, up-curled margins.
The virus is found in only a few species, and is spread mainly from one banana plant to another. Eradication of infected trees is an effective control measure.
Abaca belongs to the same genus as banana, but is grown for its fiber rather than its fruit. It suffers from a very similar disease, known as Abaca Bunchytop. The virus which causes Abaca Bunchytop is very similar to BBTV, and may well be the same virus. Unfortunately, control programs in most countries are commodity oriented. Eradication programs for banana and abaca are being carried out separately. This means that plantations of the two crops may well be exchanging virus and reinfecting each other.
Banana plants infected with BBTV have to bekilled, not just cut down, otherwise infected suckers will grow again. This can be done by digging out the roots, but it is very hard work.
In the Philippines, abaca plants infected with virus are killed after the stem has been cut by hollowing out the top of the stump into a bowl shape and filling it with herbicide.
Diseased suckers regenerating from banana plants which have been cut down are a major source of BBTV infection. The other is abandoned or poorly managed banana plantations.
How Do You Know If a Plant Has Virus?
Just looking at the plant is the easiest way of diagnosing a virus disease, but it is not very reliable. Virus disease in its early stages, or mild strains, may have no symptoms. To make things more difficult, mixed infections with several viruses are common, while the same virus can produce different symptoms in different plants. Even experts need to carry out laboratory tests before they can be sure of a diagnosis. Unfortunately, these can be expensive unless they are subsidized by the government.
There are two main types of laboratory test, PCR and Serological Assay. PCR is a very sensitive assay, and can be used to test for 3-4 viruses at a time, but the reagents used are expensive, and each sample needs a long process of preparation. It is used when accuracy is very important, as when testing foundation stock.
Serological assay is based on antibodies produced by animals (usually rats or rabbits). When suitable antisera are available, the most sensitive and efficient practical assay for plant virus indexing is enzyme-linked immunosorbent assay (ELISA). ELISA is the testing method most widely used, since it is sensitive, easy to use, and doesn't need expensive equipment.
FFTC has been holding several training courses on testing for plant viruses, and using these tests in a production system for disease-free seedlings. One was held in the Philiipines in October, directly after the workshop. It was organized with the help of two scientists from the Philippines who had attended an earlier FFTC training course
Taking Samples for Virus Testing
Different viruses have different patterns of distribution in the plant. Some kinds of virus may be evenly distributed throughout the plant at a uniform rate, but others are not. Samples of leaves or stalks used to test for a particular virus should be taken from the correct part of the plant. Otherwise, an infected plant may give a negative result.
Fast-growing shoot tips or buds are usually free of virus, because the virus hasn't had time to colonize them. They should not be used for virus testing.
Concentrations of Banana Bunchytop Virus are much higher in younger leaves than in older ones, and are higher in the midrib than in the rest of the leaf.
Greening disease, on the other hand, is spread very unevenly through the plant. The same is true of two important banana viruses, Banana Streak Virus and Citrus Mosaic Virus (in spite of its name, this virus infects banana too!) The amount of virus can vary in different leaves of the same plant, or even in different parts of the same leaf! Some leaves from infected plants may show no pathogens at all. When testing for this kind of virus, you may need to take more samples from any one plant to make sure it is free of disease.
If you are getting your orchard tested for virus, try and find out first how many samples are needed, and what part of the plant is best for testing.
Conclusion
Over the last ten years, there have been great strides in controlling virus diseases of plants. We can now mass produce young citrus and banana seedlings that are certified free of virus. There are also new techniques which can diagnose virus diseases accurately and quickly. The main problem still remaining is how to keep seedlings free of disease after they are planted. Virus diseases and citrus greening are easily transmitted by insects and aphids. Early detection and early removal of infected plants are critical.
In practice, this is difficult. Early detection needs expensive laboratory tests. Even when a diagnosis is made, eradication programs are unpopular. Newly infected plants have only minor symptoms and may still give a profitable yield. Farmers are unwilling to sacrifice trees which are still giving fruit. Most Asian countries are reluctant to make eradication compulsory, and rely on persuasion rather than coercion. Governments are often asked to introduce a compensation scheme for farmers, but even wealthy Japan has not done this yet.
Disease-free seedlings produced by tissue culture are expensive. Farmers should spend their money on them only in areas which are relatively free of both virus and vectors.
Once disease-free seedlings are planted out in the field, growers should do their best to protect them from infection. The seedlings should be given timely sprays of insecticide to control insect vectors. Windbreaks may help to keep away flying insects and aphids. If plants do become infected, they should be removed early. Farmers should also eradicate nearby plants that act as alternate hosts for the vector or the virus.
In the case of citrus, pruning tools should be quickly disinfected after use on each tree. Resistant rootstock should be used. Sometimes cross protection is possible, inoculating plants with a mild strain of virus to protect them from a virulent strain.
Index of Images
Figure 1 Fruit and Leaves from Tree with Greening Disease (Left) Compared to Those from Healthy Tree (Right)
Figure 2 The Citrus Psyllid, Vector of Greening Disease
Figure 3 Banana Streak Virus
Figure 4 A Banana Plant with Banana Streak Virus Has Been Cut down. However, the Regenerating Sucker Is Still Infected with the Virus
