Key words: Greening, tristeza, citrus industry in Vietnam, pathogen-free citrus nursery, citrus disease control
Vietnam stretches from north to south at 15° N latitude. This geographical diversity allows the country to grow any kind of fruit, be it tropical, subtropical, or even low-chill temperate. Citrus is one of the most important fruits of Vietnam. This fruit crop is grown at an altitude of at least 900 m above sea level in mountainous regions in the north, down to sea level in the Mekong River Delta. Citrus is grown in 107,380 ha with a total production of 731,203 t.
Major citrus-growing areas are in the northeast (Ha Giang, Tuyen Quang, Phu Tho, Lang Son), north central coast (Nghe An, Ha Tinh), and the Mekong River Delta (Tien Giang, Vinh Long, Dong Thap, Can Tho, Ben Tre and Hau Giang). The Mekong River Delta is Vietnam's largest citrus-growing area with an acreage of 60,204 ha or 56.1% of the total area ( Table 1).
Greening and tristeza are major factors causing the decline of citrus production in Vietnam. A development program to rehabilitate the citrus industry was initiated by the National Institute for Plant Protection (NIPP) and Southern Fruit Research Institute (SOFRI) in the early 1990s. This program, strongly supported by MARD, received a fruitful cooperation with the international research community (FFTC, NTU, NCHU, CIRAD, ACIAR, AUF, and JIRCAS, among other institutions).
This program included the following:
- Survey on the distribution of greening, tristeza, and other virus diseases and its damages;
- Establishment of a disease-free citrus nursery system; and
- Demonstration of integrated orchard management.
This paper presents some of the major activities that have been undertaken by SOFRI and NIPP during the program's implementation.
Survey on Greening and Tristeza
The first disease survey was conducted by FFTC and NIPP in 1992. The areas surveyed were Dong Nai and Khanh Hoa, Hoa Binh, Ha Tay, and Hanoi outskirts. The second survey was undertaken by CIRAD, SOFRI, and NIPP in Tien Giang, Vinh Long, Can Tho, Dong Nai, Hoa Binh, Ha Tay, and Hanoi outskirts.
Citrus greening and tristeza were found in all surveyed areas. During the two surveys, molecular indexing technique was first used for disease diagnosis by Prof. Hong Ji Su (NTU) and Prof. Bove of the University of Bordeaux. Insect vectors and natural enemies were identified by Prof. Y.I.Chu (NTU) and Dr. M.Miyazaki from NARC.
Aside from greening and tristeza, other diseases were also found. These were exocortis, Phytophthora gummosis, and citrus canker found in the south and the north, as well as cristacortis, which was found only in the south.
Citrus greening disease is of high economic importance. It has been found on Citrus sinensis, Citrus reticulata, Citrus grandis, Citrus arantifolia, and Citrus microcarpa. The life span, fruit yield, and quality of citrus in greening-infected orchards are remarkably reduced ( Table 2).
A similar situation was observed in the Mekong delta, southern Vietnam, where the average yield losses of sweet orange, King mandarin, and sweet mandarin were 15.67 t/ha, 13.49 t/ha, and 9.88 t/ha, respectively.
The vein-clearing symptom caused by the citrus tristeza closterovirus (CTV) was found in all samples having positive reaction to the ELISA tests. This finding showed that the strain of CTV was a mild one.
However stem pitting and dwarfing symptoms recently became more common. This finding showed that the CTV strain was more virulent, making tristeza a disease of economic importance.
Diaphorina citri, the insect vector of citrus greening, was found in all surveyed spots. Their natural enemies, Tamarixia radiata and Diaphrencyrtus aligahensia, were available only in some surveyed citrus orchards.
Three aphid species were found as insect vectors of CTV. They were Aphis gossypii, Myzus persicae, and Toxoptera citricidus. None of their natural enemies were found. This was probably because the surveys were conducted in August (1992), rainy season, when the aphid population was usually low.
Surveys on the disease occurrence and nursery management were also undertaken in the state citrus nursery Xuan Mai, 40 km north of Hanoi in 1992 as well as in some private nurseries in the province of Ben Tre in 1994. All the surveyed citrus nurseries had very poor management. Planting materials, mother trees, and root stocks were grown outdoors without any insect protection net. Seedlings were severely infected by greening, tristeza, bacterial canker, and other diseases. It must be noted that Xuan Mai and Ben Tre are the two biggest citrus nurseries providing seedlings for the citrus industry in Vietnam.
Establishment of a Disease-Free Nursery System and Extension Program
Greening, tristeza, and viral diseases of citrus are mostly transmitted by insect vectors and planting materials. Main components of the disease management program are as follows:
- Cultivation of healthy citrus seedlings;
- Elimination of insect vectors and their host plants; and
- Integrated crop management to protect healthy orchards from being re-infected.
Establishment of a disease-free nursery system is of primary importance in a greening management program. Molecular detection techniques for plant health control are an indispensable component of this system. Material and technical basis of the above system includes laboratories for micro grafting (STG) and molecular indexing, insect-proof greenhouses with metallic net for long-term use (10-12 years), and a network of temporary nurseries with plastic net for short-term use (2-3 years) ( Table 3).
NIPP and SOFRI staff members have received short-term training on micro grafting and indexing techniques from FFTC and CIRAD. A funding support from MARD, FFTC, RDF, and CIRAD was provided to establish micro grafting and disease detection laboratories as well as initial insect-proof screen houses in SOFRI (1995) and NIPP (1997). The valuable support and fruitful international cooperation have enabled Vietnam to create a first material-technical base for a national pathogen-free citrus nursery system.
At present, 230 pathogen-free mother trees (S0) from 35 varieties are maintained in SOFRI. The NIPP foundation stocks consist of mother trees from 40 varieties. Pathogen-free citrus nurseries in SOFRI and NIPP have served as the network extension core of the disease-free nursery system in Vietnam.
Extension Program for Disease Nursery System
To promote the rehabilitation of the citrus industry, MARD released decisions No. 4838 and No. 3574 to extend disease-free nurseries to provinces with commercial citrus production. NIPP established disease-free citrus nursery systems in Ha Giang, Tuyen Quang, Phu Tho, Nghe An, and Ha Tinh provinces. A state budget was provided for setting up this system ( Table 4).
In 2003-2004, these provincial pathogen-free nurseries distributed 510,000 healthy seedlings to citrus growers. Nine training-of-trainer and 45 farmer-training courses, with about 5,600 trainees, were organized.
Technology transfer of pathogen-free citrus nursery management and STG to the provinces Dong Thap, Tien Giang, Can Tho, Vinh Long, Ha Tinh, and Nghe An was undertaken by SOFRI from 1999 to 2002. A network of pathogen-free citrus nurseries was also set up in this province. A set of certified mother trees S1 of 12 major and common varieties was provided by SOFRI to the provincial nurseries.
So far, the national pathogen-free citrus nursery system has distributed about one million healthy seedlings to citrus growers. Of the total number of seedlings, 278,800 and 180,000 were produced by SOFRI and NIPP, respectively. The number was enough to set up a new citrus plantation of 2,000 ha at a planting rate of 500 seedlings/ha. This output, however, could not meet the demand for a full rehabilitation of the citrus industry by 2010. There is an urgent need to set up a local network of disease-free nursery systems to provide citrus growers with enough quantity of healthy seedlings. This local network includes simple net house nurseries established in major citrus-growing areas of each province. This net house will obtain healthy scion S2 derivate from the central and/or provincial disease-free citrus nurseries.
Establishment of Demonstration Orchards
This activity aimed to show citrus growers how to properly grow their young healthy seedlings so that they do not get re-infected when they are planted in the field.
Orchard management was based on an integrated crop health approach. It includes layout and design, proper soil and water management, good training and pruning, as well as integrated pest management with emphasis on control against insect vectors of citrus greening and CTV. Two demonstration orchards were set up by FFTC, NIPP, and SOFRI in 1999-2000.
The first demonstration orchard, with an acreage of 3 ha, was located in Bavi, Hatay province. The orchard has 702 orange trees belonging to four important local varieties. Its soil type is ferralistic hill soil. The orchard is drained by ditches and is provided with a drip irrigation system. Grass has been planted along the ditches and in between rows of trees to protect the orchard from erosion. The second orchard was set up in Thoi Son, Tien Giang province in the south. The pumelo variety is planted in lowland soil with a very high water table. Drainage is very important because the orchard is in a waterlogged plain. Ditches were dug between the rows of trees and filled with the trunks and leaves of longan trees, mixed with soil. A layer of coconut fiber or stones was laid over the longan trunks before the soil was replaced. This porous fill provides good drainage at a relatively low cost. Because the drains are underground they do not interfere with orchard operations.
Imidacloprid (Confider) was applied to prevent the invasion of insect vectors. Both demonstration orchards showed good performance of plant growth and IPM control. The low-cost technology is well suited to the small-scale citrus production in Vietnam.
Demonstration orchards with the use of integrated crop health management system was successfully extended to major citrus-growing areas in Vietnam such as Nghe An, Ha Giang, Tuyen Quang, Tien Giang, Ba Ria, and Vung Tau. Outputs obtained from the research and development activities presented in this report were a worthy contribution to the rehabilitation of citrus production of Vietnam.
Greening is the most common and destructive disease of citrus orchards in Vietnam. Recently, tristeza also became a disease of high economic importance. Because both diseases are transmitted by planting materials (e.g., budwood, marcot) and insect vectors, the setting up of a pathogen-free nursery system and practicing IPM are the most important components in Vietnam's citrus disease control.
Thanks to the fruitful international cooperation with FFTC, CIRAD, ACIAR, JIRCAS, and other overseas R&D institutions, as well as the help of MARD, the National Institute for Plant Protection and Southern Fruit Research Institute was able to set up the pathogen-free citrus nursery network. The network includes most of the major citrus-growing provinces with a production capacity of more than one million healthy seedlings per year.
With their use of integrated crop health management system, the demonstration orchards in Ba Vi, Hatay, and Thoi Son, Tien Giang, performed well and were successful in extending the system to somecommercial citrus-producing provinces such as Nghe An, Ha Giang, Tien Giang, Hau Giang, and Ba Ria, Vung Tau. The low-cost technologies of this system are well suited to the small-scale production in Vietnam.
To ensure the proper operation of a pathogen-free nursery network and the improvement of orchard management, we propose the following:
- Upgrade the knowledge and capacity of local staff involved in the pathogen-free citrus nursery network;
- Improve the material and technical bases of pathogen-free citrus nurseries and detection laboratories; and
- Strengthen the international cooperation for improving the indexing techniques; elucidating disease ecology and pathogen diversity; and detecting some potential dangerous diseases such as tatter leaf.
We extend our heartfelt gratitude to FFTC, RDF, CIRAD, ACIAR, NCHU, and JIRCAS for their fruitful cooperation and valuable support.
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Index of Images
Table 1 Major Citrus-Growing Areas in Vietnam
Table 2 Damage Caused by Citrus Greening in Northern Vietnam
Table 3 Disease-Free Citrus Nursery System
Table 4 Extension Program for Pathogen-Free Citrus Nursery System in Northern Vietnam.
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