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The Mediterranean and subtropical zones, situated approximately between latitudes 20° and 35°N and extending from Morocco in the west to Pakistan in the east, together represent one of the largest and most ancient citrus-growing regions of the world. Mediterranean countries also contribute to make this region one of the foremost commercial citrus production areas, and other countries in the region have recently begun to develop modern citrus industries or are ready to do so.
Diseases are among the major factors limiting plant production. Infectious, graft-transmissible diseases caused by viruses, viroids or phloem-restricted prokaryotes (mollicutes and bacteria) must receive special attention since plants, once contaminated by these virus and virus-like agents, cannot be cured. The only control measure is prevention. This is of particular importance in the case of woody perennial plants, such as citrus, which have a productive life extending over many years. If such plants are propagated from infected budwood, they stay infected throughout their life and eventually deteriorate owing to the presence of infectious agents (pathogens) over many years. If the effect is severe, the trees either die or have to be destroyed.
Several techniques are known for producing plants free of virus and virus-like agents. More than any other fruit, citrus has greatly benefited from these methods. One such technique is shoot-tip grafting, which was developed specifically for citrus and has been used with great success over the last ten years. Hence, large-scale production of citrus trees free of virus and virus-like agents is no longer a technical research problem. Programmes for the production of such trees are in progress in several countries, where thousands of healthy nursery trees have already been planted in the field.
Unfortunately, a young orchard planted with healthy trees will not necessarily stay free of infectious agents, some of which are transmitted by insect vectors. For instance, citrus tristeza virus (CTV) is spread by aphids; Spiroplasma citri, the mycoplasmal agent of stubborn disease, has leafhopper vectors; the bacterium associated with greening disease is transmitted by psyllids. Therefore, if both the vector and the disease agent occur in a given area, healthy trees may become infected by natural contamination. It is not enough to produce healthy trees. It is also important to see that the trees stay healthy by controlling the vectors as much as possible so as to keep natural contamination to a minimum. In the case of tristeza, this cannot be achieved but, fortunately, one can "live with" this disease by choosing tolerant rootstocks and scion varieties. Indeed, it is well known that, for a given disease, certain citrus species or varieties are tolerant while others are susceptible. A tolerant species or host plant is one in which the virus or virus-like agent is able to multiply but in which it does not induce symptoms. In a susceptible host, on the other hand, multiplication of the agent results in symptom development. In a resistant host, the agent cannot multiply and hence there can be no symptoms.
About 30 economically important virus and virus-like diseases of citrus are known. Some agents of these diseases have insect vectors. Not all of these agents are necessarily present in every citrus-growing area of the world, and this is especially true for insect-transmitted agents. While CTV is endemic in Asia and killed 20 million trees in South America between 1930 and 1950, it has not yet reached North Africa, even though it has been spreading in Spain since the 1950s.
It is thus essential to assess periodically the disease status of citrus in the various citrus-growing regions of the world. An accurate knowledge of the disease agents and/or vectors present or absent in a given area is a prerequisite for the development, maintenance and improvement of commercial citrus production.
In 1969-70, FAO consultant H. Chapot visited several countries in North Africa and the Near East to determine the nature of the problems, including diseases, affecting citrus. However, major developments have occurred since then. The agents of stubborn and greening have been characterized and shown to be entirely unrelated. Their respective insect vectors have been identified. Exocortis and cachexia have been shown to be viroids. Lethal diseases, such as blight, have assumed unprecedented importance. Entirely new diseases, such as witches' broom disease of lime trees in Oman, have appeared and are causing great concern. New techniques for the detection and characterization of the agents of tristeza, exocortis, cachexia, stubborn, greening, etc. have been developed. Finally, many countries in the Near East and the Arabian peninsula wish to produce more and better citrus.
For these reasons, a survey of virus and virus-like diseases of citrus was carried out by FAO in the Near East region. Surveys were carried out by the author and A. Salibe of Faculdade de Ciencias Agronômicas, Universidade Estadual Paulista Horticultura, Botucatu, Brazil.
The author surveyed the following countries: the Islamic Republic of Iran, Iraq, Jordan, Oman, Pakistan, Saudi Arabia, Somalia, the Sudan, the Syrian Arab Republic, the United Arab Emirates and Yemen. His experience with citrus in Algeria, Cyprus, Egypt, Morocco and Turkey was also taken into account. Prof. Salibe visited Algeria, Cyprus, Egypt, Jordan, the Libyan Arab Jamahiriya, Morocco, the Syrian Arab Republic, Tunisia and Turkey. In previous years he had visited France, Greece, Israel, Italy, Lebanon, Portugal and Spain. Mediterranean countries for which the citrus disease situation is already well documented were not included in the surveys.
Pakistan is particularly important because it is at the point where the Near East meets Asia and it is via Pakistan and Afghanistan that citrus diseases typical of Asia, such as tristeza and greening, may find their way to the Near East. Vice versa, stubborn disease might pass this way from the Near East to Asia. Moreover, being relatively close to Oman, Pakistan could have harboured the new, lethal witches' broom disease which is killing more and more lime trees in the former country.
Somalia was surveyed because of its proximity to the tristeza- and greening-infected countries, Ethiopia and Kenya. The surveys were carried out between 1981 and 1987 but additional data, obtained by the author from various colleagues at a later date, have also been included in this review.
The review includes the essential points of FAO reports on virus and virus-like diseases of citrus in the various countries surveyed by Bové and Salibe. In order to make it as complete and as useful as possible, the first seven chapters were prepared by the author to precede the chapters concerned more specifically with the diseases in the various countries. These supplementary chapters cover: "The nature of virus and virus-like disease agents of citrus" (Chapter I); "interactions between virus and virus-like pathogens and the host plant: susceptible, tolerant and resistant citrus species" (Chapter 2); "Transmission and spread of virus and virus-like pathogens" (Chapter 3); "Techniques for the detection and identification of virus and virus-like pathogens" (Chapter 4); "Production of citrus clones free of virus and virus-like pathogens" (Chapter 5); "Short description of major citrus diseases in the Near East" (Chapter 6); "Citrus stubborn disease in the Syrian Arab Republic and natural transmission of its causal agent, Spiroplasma citri" (Chapter 7). Prof. Bové also wrote the conclusion to the review and updated Tables 5, 6, 7 and 8 on host susceptibility and tolerance and the Annex Table on geographical distribution of citrus diseases, which first appeared in Bové and Vogel, 1980.
The preparation of this review was proposed by Dr M.M. Taher, FAO Regional Plant Protection Officer for the Near East and North Africa. Without his perseverance, it would never have been published. Many thanks to him.
Last but not least, Colette Bové (IRFA-INRA) has devoted a considerable amount of her time to getting everything into shape. Many thanks to her, also.
