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Citrus is one of the most important commercial fruit crops grown in all continents of the world, and areas under citrus are in continuous expansion. While citrus fruit contributes to the nourishment and refreshment of the people, citrus products and by-products provide the basis for local agricultural industries, generate employment, raise income and, in many cases, constitute an important source of foreign revenue to developing and developed countries.
As with any commercial crop, citrus is subject to various pest problems. These include insects, mites, fungi, bacteria, nematodes, viruses, viroids etc. that can cause varying degrees of damage and affect the quality and quantity of the produce on the one hand and the vigour and longevity of the trees on the other. Most of these pest problems can be solved in the field through conventional control measures, i.e. cultural practices, field sanitation, chemotherapy etc. Exceptions are problems posed by virus and virus-like diseases for which the only known cure is to provide healthy (virus-free) planting material, usually within the framework of a phytosanitation programme based on diagnosis, detection and elimination of the causal agent(s) and the maintenance and distribution of healthy stocks.
Detection of virus and virus-like diseases of citrus has significantly developed over the last half-century. At present, a wide variety of indicator plants exists for the detection of almost all known virus and virus-like diseases within a reasonably short time, provided that appropriate indexing facilities are available. In addition to biological indicators, many laboratory methods, ranging from culturing of the causal agent to molecular biologybased techniques, have also been developed during the last two decades. Such rapid detection methods are now used for quarantine purposes and large-scale surveys as well as for sanitation programmes in conjunction with biological assays.
When virus and virus-like diseases have been diagnosed by any of the above methods, and no source of healthy planting material is available, the only solution is to eliminate the infection from the diseased material. To accomplish this, various methods have been developed, including the production of nucellars, thermotherapy and shoot-tip micrografting. Such methods, used singly or in combination, have proved effective in eliminating virus and virus-like diseases, thus enabling the reutilization of valuable, sanitized resources.
The first attempt to collate information on detection techniques of citrus virus diseases was made over 20 years ago, when the United States Department of Agriculture published Indexing procedures for 15 virus diseases of citrus trees. This was prepared by the Committee on Indexing Procedures, Diagnosis and Nomenclature, established by the International Organization of Citrus Virologists (IOCV). With the continuing flow of knowledge and new findings on virus and virus-like diseases, the need for an up-to-date publication on detection techniques for citrus graft-transmissible diseases was expressed on various occasions. Such a publication was needed to assist in the detection of quarantine-significant citrus diseases before any introduction of citrus planting material; to help in conducting regular surveys to detect and avoid the potential spread of citrus vector transmitted diseases such as tristeza, greening and stubborn; and, more important, to aid the various national and regional citrus sanitation programmes in the diagnosis and detection of citrus virus and virus-like diseases.
Realizing the importance of such a publication, FAO and IOCV cooperated in the preparation of this handbook on detection of citrus graft-transmissible pathogens. It was written somewhat in the form of a "recipe book" wherein all available knowledge on detection techniques is presented in the simplest and most straightforward way.
The handbook has been prepared by C.N. Roistacher, Emeritus Plant Pathologist of the University of California, Riverside, United States of America, who has devoted almost 40 years to the detection of citrus viruses and has played a significant role in the development of the California citrus clonal improvement programme. Other contributors to the handbook are scientists acknowledged as world authorities on the various detection techniques presented here. The author and most of the contributors are also members of the International Organization of Citrus Virologists. Without the dedication and generosity of all those who have voluntarily agreed to give their time and share their experience, this handbook would not have been written.
It is worth mentioning here that, although Part III of the handbook deals with laboratory detection techniques of citrus graft-transmissible pathogens, some of these techniques are also applicable to other viruses and virus-like pathogens of various field crops, vegetables, fruit trees and ornamentals.
As a follow-up to this handbook, and in order to make available to all interested parties the current technology on production of healthy citrus planting material, another publication on the elimination of citrus graft-transmissible pathogens is being prepared and will be published by FAO in due course.
Finally, it is hoped that this handbook will help to boost existing citrus phytosanitation programmes and will encourage the development of new citrus phytosanitation activities.
Lukas Brader
Plant Production and Protection Division
FAO, Rome