Rehabilitating Asia's Citrus Industry through Pathogen-Free Seedlings and Rapid Disease-Indexing Techniques
Fighting the spread of Huanglongbing and other citrus virus diseases
A major survey project initiated by FFTC in the last decade revealed that many serious virus diseases have a much wider distribution in Asia than had been previously believed. Of these, HLB or citrus greening is the most destructive especially in tropical and subtropical regions like southeast and east Asia. The survey, which launched FFTC's long-term commitment to rehabilitate the region's citrus industry, was followed by a series of workshops and training courses. Information about the diagnosis and indexing of plant viruses using new molecular techniques developed by the National Taiwan University (NTU) was disseminated. DNA primers and antibodies were provided to national laboratories in the region, and a demonstration project in Vietnam on the management of disease-free citrus orchards was set up.
While considerable gains have been achieved under these past initiatives, FFTC believes that efforts must continue progressively in terms of wiping out citrus diseases that are considered a truly limiting factor for citrus production in Asia. Hence, two of the Center's recent special projects involved the development of an integrated management system primarily involving quick detection of any infections that may become established in the orchards, and the use of pathogen-free foundation seedlings.
Tracing the Evolution of Citrus Greening Pathogen
Basic information on the evolution of HLB strains in terms of pathogenicity and disease resistance is of primary importance in formulating adequate measures for controlling the HLB disease. In view of this, FFTC recently completed a two-year international collaboration project on Evolution of citrus greening pathogen (HLB) strains in ASPAC in partnership with the National Institute of Fruit Tree Science (NIFTS) of Japan. The project focused on the investigation of pathological and epidemiological natures of HLB pathogen strains in Taiwan and in other countries in the Asian and Pacific (ASPAC) region. Under this collaborative project, the following researches were performed at the Virus Laboratory of the Department of Plant Pathology, NTU:
- Pathogenicity identification of citrus greening (HLB) pathogen strains collected from Taiwan and other southeast Asian countries;
- Distribution of jasmine orange (Murraya paniculata), and their susceptibility test to HLB pathogen; and
- Disease resistance of important citrus cultivars against HLB virulent common strains.
During the project's first year of implementation, it was established that Taiwan cultivars of Murraya paniculata showed immunity to HLB. Most HLB isolates collected from important citrus cultivars were identified to belong to strain II. Meanwhile, the wild strain of HLB which causes typical greening symptom on mandarin and orange seedlings was seldom found in most of the important cultivars in the island.
A survey was also conducted in the citrus growing areas of northern and central Vietnam where dried citrus samples were collected and confirmed to have HLB infection by PCR analysis. It was found that as in Taiwan, strain II attacking mandarin and pummelo plants has evolved and dominated in the northern and central Vietnam.
During its second year project implementation, the survey team visited several citrus-growing areas in Vietnam and Cambodia to gather citrus samples for disease indexing, which will serve as benchmark information in developing integrated pest management (IPM) to control the citrus HLB problem. In both countries, the need to establish indexing laboratory, as well as insect-proof screenhouses for the production of pathogen-free citrus foundation seedlings were raised. The survey team also donated some CTV/HLB diagnostic reagents and iodine kits for rapid diagnosis of HLB and CTV to the Royal University of Agriculture (RUA) of Cambodia.
Responding to the Need for Pathogen-Free Germplasm Repository
Systemic diseases can only be effectively controlled by integrated measures of disease management involving:
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 the management of pathogen-free (PF) nursery system through health certification of PF seedlings; 3) establishment of shoot-tip micrografting technique for obtaining PF germplasm foundation; and 4) health management to prevent 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 a three-year special project entitled Establishment of pathogen-free citrus germplasm repository for the improvement of the citrus industry in ASPAC, an advanced Taiwanese technology package will be transferred first to Cambodia, and then to other countries in the Asian region. This three-year (2006-2008) project will be carried out in Taiwan ROC, Cambodia, Vietnam, Laos, and other ASPAC countries, and is co-sponsored by the National Taiwan University (NTU) and the Rural Development Foundation (RDF), Taiwan ROC.
The technology package to be promoted under this project 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:
- Survey of citrus greening and virus disease incidence in ASPAC countries;
- Conduct of technical consultation among growers on health management in the citrus-growing areas of the region;
- Training-workshop on virus indexing and shoot-tip micrografting technology;
- 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
- Establishment and management of pathogen-free citrus nursery system in the ASPAC region.
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 citrus diseases. HLB and other virus diseases have been ravaging citrus production in the country, and have 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 project objectives specific to Cambodia for year 1 are as follows:
- Establishment of HLB-virus indexing techniques and laboratory at the Royal University of Agriculture (RUA), Cambodia;
- Construction of insect-proof citrus foundation repository for the production of pathogen-free citrus seedlings; and
- Demonstration of pathogen-free citrus seedlings in the field to promote citrus production.
In August 2006, a survey team visited citrus areas in Battambang Province in Cambodia. Most of the orchards in the area were seriously affected by the greening disease. Systemic HLB and CTV diseases were found to be 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.
As part of the project objectives, initial preparations are now ongoing for the establishment of a disease indexing laboratory in RUA. The instruments for the laboratory will be acquired through the assistance of RDF, while the diagnostic kit/probes (virus-antibodies and PCR-primers) and reagents will be provided by NTU. A land next to the laboratory has been 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.
Promoting Technical Cooperation
Systemic greening and virus diseases have had catastrophic impacts on citrus production in the ASPAC region. Thousands of orchards have been badly damaged, and many have been almost wiped out by these devastating diseases. Hence, for the past years, FFTC and its partner organizations have been implementing programs and activities aimed at providing technological solutions to combat these diseases, particularly through the production of pathogen-free seedlings and the adoption of disease indexing techniques.
Over the year's, FFTC's commitment in rehabilitating Asia's citrus industry considerably promoted the sharing of technology on virus detection and diagnoses, which are basic information to ensure that foundation stocks are healthy, and so that planting materials can be certified as disease-free. Scientific knowledge on the production of seedlings by tissue culture from disease-free foundation stock, and various techniques to prevent outbreaks of virus diseases in the field have been exchanged among participating countries. In essence, the significant results of the above projects serve as a proof of the value of sharing technical information for the mutual benefit of developed and developing countries alike in the ASPAC region.
Index of Images
Figure 1 Visit to the pathogen-free citrus foundation screenhouse at the Plant Protection Research Institute (PPRI) in Hanoi, Vietnam which was donated by FFTC/RDF in 1996.
Figure 2 FFTC consultant and project coordinator Prof. Hong-Ji Su demonstrates HLB rapid detection using iodine test kit to PPRI laboratory staff.
Figure 3 Positive reaction of HLB-infected leaf of Xa doi tree using the iodine test kit at the Van Giang Experiment Station in Northern Vietnam.
Figure 4 The survey team visited citrus orchards seriously affected by the greening disease in Battambang Province, Cambodia.
