Citrus greening disease (HLB) and other virus diseases such as citrus tristeza closterovirus (CTV), citrus tatter leaf cappilovirus (CTLV) and citrus exocortis viroid (CEVd) have been causing considerable damage to fruit yield and quality, and have become serious constraints for the citrus industry in the Asian and Pacific (ASPAC) region in recent decades. These systemic diseases are effectively controlled by integrated measures of disease management such as: 1) establishment of virus-free citrus cultivar repository, which is of primary importance in preventing prevalence of the diseases; 2) precise and rapid disease indexing techniques indispensable for management of pathogen-free (PF) nursery system through health certificate of PF seedlings; 3) establishment of shoot-tip micrografting technique for obtaining PF germplasm foundation; and 4) performance of health management for preventing reinfection of PF trees in the field through IPM of vector insects. These integrated techniques have been well-developed and adopted for the improvement and development of the citrus industry in Taiwan. Hence, citrus growing countries in the ASPAC look forward to Taiwan's technological support to prevent growers' losses due to citrus greening and other virus diseases.

Under this three-year special project, to rehabilitate the citrus industry in ASPAC countries, an advanced Taiwanese technology package will be transferred first to Cambodia, and then to other countries in the region. The technology package shall include establishment and application of pathogen-free citrus foundation, and disease-indexing technique for controlling serious epidemic of citrus greening (HLB) and other virus diseases. Specifically, the project shall have the following acvitivities:

• 1) Survey of citrus greening and virus disease incidence in ASPAC countries;
• 2) Conduct of technical consultation among growers on health management in the citrus-growing areas of the region;
• 3) Training-workshop on virus indexing and shoot-tip micrografting technology;
• 4) Collection and virus-elimination of the most important citrus cultivars (more than 15) selected from participating countries in the region for the establishment of pathogen-free citrus germplasm repositories to be made available to every country as a way of promoting promote regional technical cooperation; and
• 5) Establishment and management of pathogen-free citrus nursery system in the ASPAC region.

Project Implementation in Cambodia (Y1)

Citrus greening (HLB) and other virus diseases have been devastating citrus production in Cambodia, and has become a serious epidemic, affecting over 80 percent in some orchards of Pursat orange. The HLB disease also infects pummelo trees, giving rise to low quality fruits. The first year implementation of this special project focused on the case of Cambodia as a major citrus-growing country in the region devastated by HLB and other virus diseases. The project objectives specific to Cambodia for year 1 were as follows:

• 1) Establishment of HLB-virus indexing techniques and laboratory at the Royal University of Agriculture (RUA), Cambodia;
• 2) Construction of insect-proof citrus foundation repository for the production of pathogen-free citrus seedlings; and
• 3) Demonstration of pathogen-free citrus seedlings in the field to promote citrus production.

Highlights of the Technical Survey

In August 2006, a survey team visited citrus areas in Battambang Province in Cambodia. The Pursat cultivar of sweet orange is being cultivated in Banon area of Battambang. The fruit quality of Pursat orange was acceptable to the people, and growth of the trees were well-adapted to the environment. However, most of the Pursat orchards were seriously affected by greening disease. The team noticed that the HLB disease has been seriously devastating sweet orange orchards in the area. Some pummelo trees showed greening symptoms, indicating that the strains of HLB pathogen in Cambodia have evolved into strain I, which mainly attacks mandarin and sweet oranges, and strain II, which has been infecting mandarin, sweet orange and pummelo cultivars. The team collected leaf tissues for disease indexing. They also visited the Siem Reap areas for citrus orchards and inland aquaculture on the north coast of Tonle Sap Lake. HLB-infected trees of Pursat and Mexican lime were identified in some citrus orchards in the suburban Siem Reap. Disease indexing of citrus materials from Cambodia were done, and PCR patterns were obtained.

The systemic HLB and CTV diseases are mainly transmitted through vegetatively propagated seedlings, and spread by vector insects in the field. Accordingly, establishment of indexing techniques, pathogen-free citrus foundation, and nursery system are of primary importance in combating the disease epidemics.

The Royal University of Agriculture (RUA) of Cambodia seeks to establish indexing laboratory and pathogen-free citrus foundation repository in the university in order to rehabilitate the citrus industry in the country from the epidemic devastation. Two scientists from Cambodia had the opportunity to learn basic techniques of disease indexing and shoot-tip micro-grafting at the Plant-Virus Laboratory of the National Taiwan University (NTU).

This international collaboration will be implemented for the second year, with the following specific objectives: establishment of HLB-virus indexing techniques, laboratory, and citrus foundation; construction of insect-proof screenhouse repository for citrus foundation, and production of pathogen-free foundation via shoot-tip micro-grafting; and production of pathogen-free citrus seedlings with excellent and potential cultivars selected from locally-adapted Pursat orange, tropical mandarin, and pummelo cultivars.

Initial preparations are now ongoing for the establishment of a disease indexing laboratory in RUA. The indispensable instruments for the activities mentioned above, will be prepared through the assistance of the Rural Development Foundation (RDF) of Taiwan ROC. Meanwhile, the diagnostic kit/probes (virus-antibodies and PCR-primers) and reagents shall be provided by NTU. A land next to the laboratory is secured for the construction of a screenhouse repository. Seedlings of citrus root-stock have already been grown in pots in a simple net-cover stand, in order to accelerate the production of pathogen-free citrus foundation and seedlings for field demonstration.

Establishment of Pathogen-free Citrus Germplasm Repository for the Improvement of the Citrus Industry in ASPAC (Year 1)

This three-year (2006-2008) special project will be carried out in Taiwan ROC, Cambodia, Vietnam, Laos, and other ASPAC countries.

Co-sponsors: National Taiwan University (NTU)

Rural Development Foundation (RDF), Taiwan ROC

• For further information, contact:
• Dr. Hong-Ji Su, FFTC Technical Consultant

Index of Images

• 

  Figure 1 The Survey Team Visited a Pursat Orange Orchard Seriously Affected by the Greening Disease in Battambang Province, Cambodia. <B>(Inset)</B> a 5-Year-Old Orange Tree Showing Greening Symptoms.

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  Figure 2 Visit to Pursat Orange Orchard Affected by Greening Disease in Siem Reap Area, Cambodia. <B>(Inset)</B> Diseased Pursat Branch Showing Typical Yellow-Mottling Symptoms.

• 

  Figure 3 Growing of Root-Stock Citrus Seedlings in Pe Pots Inside Simple Insect-Proof Net Covers at the Royal University of Agriculture (Rua), Cambodia.

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