Citrus is today one of the most important fruit crops in Asia, especially for millions of the region’s rural poor. However, the spread of Huanglongbing (HLB), also known as citrus greening, and other virus diseases such as citrus tristeza closterovirus (CTV), citrus tatter leaf capillovirus (CTLV), and citrus exocortis viroid (CEV) have long been seriously threatening the region’s citrus industry. These systemic diseases can only be controlled effectively through an integrated management approach involving: precise and rapid disease indexing; cultivating pathogen-free seedlings and establishing pathogen-free nursery system; eliminating inoculum sources; and preventing secondary spread by vectors.
A major survey project initiated by FFTC in the last decade revealed that many serious systemic diseases have a much wider distribution in Asia than had been previously reported. 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 was established in Vietnam on the management of disease-free citrus orchard.
Continuingcommitment to Rehabilitate Asia’s Citrus Industry
While considerable gains have been achieved under FFTC’s initiatives in the past, the Center believes that efforts must continue progressively if the goal is to wipe out major citrus diseases that are considered a truly limiting factor for citrus production in Asia. Hence, FFTC organized the international workshop on the Management of Citrus Greening and Virus Diseases for the Rehabilitation of the Citrus Industry in Asia held in Hanoi, Vietnam on September 8-12, 2008, in coordination with the Plant Protection Research Institute (PPRI), Vietnam and the National Institute of Fruit Tree Science (NIFTS), Japan.
The workshop was participated in by 15 speakers from 11 countries (Australia, Brazil, Cambodia, Indonesia, Japan, Malaysia, Philippines, Taiwan, Thailand, USA, and Vietnam) and about 30 local participants. Primarily, the activity aimed to provide a forum among scientists and extension workers to: gain a better understanding of the current status of citrus greening and virus disease occurrence in the region and the corresponding management initiatives in each participating country; and collect and exchange new technological information to help scientists and extension workers develop a holistic management approach for citrus greening and other virus diseases.
Key Components of a Holistic Approach to Combat Citrus Greening
Some countries are more advanced in terms of implementing a holistic management approach to combat citrus greening, while some are still in the level of consolidating a mix of best practices aimed at coming up with sustainable and effective measures against the disease. Four key steps, however, are common among the participating countries: a) Rapid detection and removal of infected trees; b) Controlling the vector, Diaphorina citri; c) Mass production/cultivation of HLB-free seedlings and establishment of pathogen-free seedling nurseries; and d) Health management of pathogen-free seedlings.
Rapid detection is the first important step, and can be done through such methods as: visual (field) detection of infected trees; rapid and accurate quick field test; and laboratory analysis for indexing and detection of causal agents (PCR, ICAN, LAMP, etc.). Removal of infected trees involves cutting down of infected trees, pesticide/herbicide application of trees and root system, and follow-up surveys to continuously monitor the presence of the disease. Controlling the vector, Diaphorina citri, constitutes systematic field surveys to gather ecological information for effective timing of control, and effective pesticide application. Finally, mass production and cultivation of HLB-free seedlings and establishment of pathogen-free seedling nurseries are essential in sustaining efforts to rehabilitate citrus orchards. This can be achieved through the production of pathogen-free seedlings by micro-grafting, and the establishment of screen house for mass production of pathogen-free seedlings to ensure stable supply for the growers.
Recommendations and Future Perspectives
In controlling the spread of HLB and in rehabilitating disease-infected orchards, strong government support is a critical factor, together with the full cooperation of all stakeholders in the citrus industry, especially the growers/farmers. Scientific efforts in terms of diagnosis, investigation, field survey, and laboratory analysis to establish control measures against HLB must be pursued, as well as a strong education and training campaign among growers, together with action programs and cohesive coordination of efforts by the national and local
governments. A management control package that is both economically and environmentally sustainable must also be initiated by the government, together with funding and logistics support for the mass production of pathogen-free seedlings with a certification system.
Likewise, a strong government regulation on the movement of host plants must be in place. Meanwhile, the scientific community must continue to address and explore the following areas: prevention of new infections through pesticide use; effective insecticide application as compared to symptomatic tree elimination; developing rapid and cost-efficient field diagnosis; and understanding other factors affecting the spread of vectors/diseases (temperature, wind, etc.).
Over the years, FFTC’s international technical cooperation program aimed at rehabilitating Asia’s citrus industry considerably promoted the transfer/sharing of technology on HLB detection and diagnosis, production of pathogen-free seedlings, and various techniques to prevent outbreaks of citrus greening. In essence, the significant results of these initiatives serve as a proof of the value of, and the need to continue, international technical cooperation for the mutual benefit of developed and developing countries in the Asian region.