The only way to remedy the situation is to renew diseased orchards by planting healthy seedlings.
At present, disease is so widespread that many of the seedlings being bought by farmers come from infected stock, and are already infected themselves. New orchards hardly have time to come into production before the trees die.
Several large citrus development programs in the region have failed because the citrus seedlings used, although thought to be healthy, were in fact infected with virus disease or citrus greening.
The growth of citrus seedlings to maturity, as well the progress of these diseases, is relatively slow, while citrus seedlings several years to come into production. Farmers who accidentally plant diseased seedlings in their orchards will suffer a considerable loss. It may well take several years for them to realize that the trees are infected, at which point there is no remedy but to remove the trees and try again with new plants. During all this time, and while waiting for the first harvest of the replacement trees, farmers will not have received any income from their orchards.
In response to this need, the Center has begun a three-year project for the establishment of nursery systems for disease free seedlings. The project is financially supported by the Chung-Cheng Foundation. The Project began work this year in Vietnam, where the National Institute of Plant Protection played a leading role. Later, the Center hopes to extend this project to other countries in the region.
The project includes four types of activity.
In 1997, an FFTC team surveyed the production of citrus seedlings in Thailand and Malaysia, and began a program for the production of disease-free seedlings in Vietnam. The treatments used for indexing diseases and producing disease-free citrus seedlings were developed at the Department of Plant Pathology and Entomology of National Taiwan University (NTU), which has made a major contribution to this program.
Virus can infect a plant long before any symptoms appear. Just because a plant seems to be healthy is no guarantee that it is free of virus. If mother plants are infected with virus disease, any cuttings taken from them for propagation will also be infected. Indexing (i.e. testing whether virus is present and what kind it is) is an indispensable technique to monitor the reinfection of either foundation or propagation screenhouses. Indexing techniques using monoclonal antibodies and DNA probes can give an accurate diagnosis in a way that was impossible ten years ago. Training courses and seminars in indexing techniques, and provision of the necessary primers and DNA probes etc. will be an essential part of the program.
In Thailand, two private nurseries are successfully producing disease-free seedlings by the NTU method, under the supervision of Thailand's Department of Agriculture and Cooperatives. Seedlings are sold with a certificate from the Department, which supplies the mother scion plants and carries out periodic monitoring for any re-infection during propagation.
At the moment, the Department of Agriculture does not have the techniques to eliminate citrus greening or citrus tristeza from elite cultivars. Further assistance from FFTC is needed, particularly the PCR protocol for indexing and provision of disease-free foundation plants.
In Malaysia, disease-free citrus seedling production is being carried in two states, Sarawak and Terengganu. In Sarawak, disease-free foundation plants are kept in screenhouses under the strict supervision of the state experiment station. Mass production is carried out in screenhouses belonging to the farmers' cooperative movement. Although the seedlings produced are of good quality, the production efficiency is too low to satisfy the urgent demand of fruit growers for seedlings.
In both Thailand and Sarawak, a major problem is how to speed up the production of seedlings, to cope with the urgent demand for the seedlings among fruit growers. Since the seedling propagation cycle requires only six months, the FFTC team offered a number of technical suggestions on how the present system might be improved. These included improving the success rate of grafting to 90% or more through the use of active young buds, soft banding tapes and improved cutting methods. Seedbed management might also be improved by using soil with a high nutrient content and water-holding capacity, by shading the beds before and germination, and by improved pest control, particularly of mites. Another recommendation was to lower the temperature in the plant nursery by shading, ventilation and fans, to avoid summer dormancy. It is also important to use appropriate root stock, if possible with resistance to phytophthera root rot and citrus tristeza virus.
In Terengganu, seedling production is more difficult, since the state has neither disease-free foundation stock nor screenhouses. This means that scions have been collected from non-certified plants, and all the seedlings under propagation are subjected to re-infection.
After discussion with the FFTC survey team, the Malaysian Agricultural Research and Development Institute (MARDI) agreed to organize a disease-free nursery system which would serve the whole of Malaysia. MARDI will be responsible for maintaining primary foundation plants, and supplying secondary foundation plants to State Experiment Stations. It will also monitor the secondary foundation plants and seedlings in propagating nurseries, to check that they have not been reinfected. FFTC will support MARDI in this program by the provision of current indexing techniques and disease-free foundation plants, plus technical assistance if required.
State Experiment Stations will serve as local centers responsible for the maintenance of secondary foundation plants, and the production of scions to be supplied to propagating nurseries. They will also provide technical guidance for nursery workers, and carry out periodic monitoring of plant nurseries.
Vietnam stretches a thousand kilometers from north to south, while orchards range in altitude from close to sea level in the Mekong delta, to 800m in mountainous regions in the north and center. This geographical diversity allows Vietnam to grow any kind of fruit, whether tropical or temperate. Banana and citrus, the target crops of the present project, are the first and second most important fruit crops, respectively, in terms of planted area.
The potential for banana production in Vietnam is high. In the south, the soil in the Mekong River Delta is fertile enough to give excellent yields, while banana is much more tolerant than citrus to the high water table. In the north, the rather cool climatic conditions over the winter months give banana a rich aroma and top eating quality. Some banana are already being exported to Mainland China. With better quality control, there are good prospects for expanded markets in Hong Kong or Japan.
Three-quarters of Vietnam's commercial citrus production is found in the Mekong River Delta, where phytophthora rot is a serious disease in areas with poor drainage. The high water table also tends to prevent vigorous growth of citrus plants and consequently favors disease incidence. However, it is tristeza and citrus greening which are the major constraints to citrus production. They are probably the main reason for the relatively short life span of orchards. Most citrus trees live only 7-10 years, and the life of many orchards is even shorter. Since citrus seedlings take several years to come into production and begin earning income, it is a serious loss for the farmer if he soon has to replace them.
Citrus greening might be an important factor in the early death of trees. A pilot project for Valencia orange near Hanoi, which covered 200 ha, has been completely destroyed by greening disease after only 12 years. The orchard was established between 1985 and 1987, with the help of France, Cuba and Spain. Greening disease first appeared in 1992. By 1997, more than 90% of the trees had died and all the remaining trees were infected.
The first, and most important, step is to supply farmers with seedlings that are free of greening and virus disease. To do this, FFTC will help provide healthy mother trees that are certified disease free. The project will also help provide an insect-proof screenhouse, so the trees can be kept free of disease. Cuttings taken from these trees will be used as foundation stock, which will also be grown in screenhouses to supply healthy planting materials for Vietnam's citrus nurseries.
Both parent stock and seedlings will be constantly monitored, to make sure that they remain free of disease. Once the seedlings are planted out in the field, farmers will be encouraged to follow cultural practices which will keep the seedlings free of disease for as long as possible. This involves good sanitation - farmers must root out infected trees which are a potential source of the pathogen. They must also remove plants growing near the orchard which are alternate hosts of the insect vector. Finally, the population of vectors insects must be kept down to a minimum, by whatever means possible - barrier crops, insecticides and cultural practices. It may not be possible to protect the trees from virus disease indefinitely, but it is important to keep them free of disease as long as possible, especially when they are young. The younger a tree is when it becomes infected with virus, the more serious the disease symptoms are likely to be.
The most urgent need is a screenhouse for foundation stock. In 1997, a screenhouse was built at the National Institute of Plant Protection, with funds from the Rural Development Foundation. The screenhouse has double doors in an "airlock" system, and the walls and roof are made of fine mesh. This protects the plants inside, not only from insects, but also from fungal pathogens such as Phytophthera. The screenhouse was finished in November, and the propagation of disease-free seedlings will begin in 1998.
Virus indexing will be carried out on a regular basis, to check that the plants remain free of disease. Training in indexing techniques and other technical support will be provided under the Project. Initially, monitoring will be carried out in laboratories abroad, using dried tissue samples. Eventually, all the indexing will be done in Vietnam.
Over the past five years, the Long Dinh Fruit Research Center (LDFRC) and other research institutions in Vietnam have conducted surveys and collections of citrus varieties. Up to the present, 27 varieties belonging to six species and hybrids of mandarin have been collected, together with 15 varieties of orange, 11 varieties and 5 wild species of lemon, and 45 varieties of pomelo. Between 1994 and 1997, many varieties of mandarin and orange were introduced from abroad.
Southeast Asia is one of the centers of origin of citrus species. Tolerant and resistant germplasms are important for breeding cultivars with high pest tolerance and wide adaptability to different environments. As part of this program, FFTC has carried out a survey of excellent pomelo germplasm. The survey team collected samples for indexing, to be used for the production of disease-free mother trees (see following Report, Genetic Resources of Tropical Fruits and Root Crops). The Center plans to collect more information about indigenous citrus species and varieties, to support Vietnam in its citrus breeding programs.
FFTC will continue to provide technical support in the diagnosis of new diseases, the indexing of virus diseases, and the elimination of pathogenic agents from elite plants. After surveys establish the problems and difficulties, the Center will do its best to provide technical and other assistance.
In Thailand and Malaysia, which are already producing disease-free citrus plants, the urgent need is to speed up the production of disease-free seedlings, to meet the demand of growers. In both Malaysia and Thailand, the output of the present system could be increased with improved grafting techniques and seed bed management, shading and temperature control so that the plants are in a cooler environment, and the selection of appropriate root stock varieties.
In Vietnam, strong government efforts to promote production have led to a rapid expansion in the area planted in fruit trees. There is widespread demand for high-quality seedlings. It is vital that the seedlings planted in these new orchards are free of disease. There is particular danger from virus and virus-like diseases, which are difficult to detect and impossible to cure. The building of the screenhouse will support the government programs already in progress to maintain disease-free foundation stock. Training and technical support are very important, and in 1997, the Center held a training course on the management of a disease-free nursery system (See Training Courses: Technology for Virus-free Citrus Nursery Systems).
Location: Malaysia, Thailand, Vietnam,
Figure 1 Screenhouse for Citrus Foundation Stock, Vietnam. It Has Double Doors in an â€œAir-Lockâ€ System, to Keep Out Insects
Figure 2 Screenhouse for Citrus Foundation Stock. the Step Has an Overhang and Gravel, to Prevent Soil Splashes
Figure 3 Micrografting of Citrus
Figure 4 Close-up of Micrografting
Figure 5 Tiny Citrus Seedlings, One Produced by Side-Grafting (Left) and One by Top-Grafting (Right)