Virus and virus-like diseases

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Citrus orchards in Libya are known to have been affected by virus and virus-like diseases for nearly 20 years. The incidence of various forms of psorosis, cachexia, gummy bark of sweet orange, woody gall, impietratura and exocortis, and of a few trees with tristeza virus has been reported by Chapot (1975), Nour-Eldin and Fudl-Allah (1976) and Fudl-Allah (1977). Most of these disorders are widespread in many Mediterranean countries (Bové, 1966). Since most citrus cultivars currently used in Libya were probably imported from Italy, the country formerly known as Palestine, Spain and neighbouring countries, it must be expected that many intracellular pathogens were inadvertently also introduced at the same time.

TABLE 35 Statistics for major citrus species in the Libyan Arab Jamahiriya in 1980

Scaly bark psorosis (psorosis A) and concave gum-blind pocket

Scaly bark psorosis has a worldwide distribution and is found in most Mediterranean orchards. The disease has a long incubation period, i.e. it is slow acting, requiring several years to produce visible bark scaling symptoms. Bark scaling rarely occurs before a tree is six years old.

Concave gum-blind pocket is generally less destructive. Affected trees show concavities of various forms in large limbs and trunks.

Psorosis A is one of the major factors preventing crops from achieving high yields in the older orchards of Libya. Many trees exhibit typical trunk symptoms of psorosis A bark scaling and concave gum-blind pocket.

Bark scaling is more frequent in orange trees. Concave gum-blind pocket has been found in trees of Sukkari, Abu Surra, Shamouti and Jaffa oranges as well as on trees of Kinya mandarin. Psorosis young leaf symptoms (oak-leaf patterns) were observed in many orchards of Shamouti orange. In older orchards, trees exhibiting concave gum and blind pocket were as numerous as those with psorosis bark scaling.

Exclusion of psorosis can easily be achieved by using budwood from healthy mother trees in the production of nursery plants. Nucellar clones of some varieties, but not of all commercial citrus, are already available in the country. Attention should be paid to seed transmission of the psorosis virus, which has been found to occur in certain rootstock types such as Poncirus trifoliata and some of its hybrids.

Cachexia-xyloporosis

Cachexia-xyloporosis is widespread in the mandarin orchards of the Mediterranean basin, and the plantations of the Libyan citrus belt are no exception. Typical symptoms of the disease were observed on the trunk, above the bud-union, of Kinya and Clementine mandarin trees. However, the number of diseased trees was not very high, and in most orchards inspected trees with and without cachexia-xyloporosis symptoms coexist, indicating that more than one clone of those varieties was used in the formation of the nurseries. The incidence of this disease in orange and lemon orchards requires indexing on sensitive varieties, since they are symptom-less carriers.

No insect transmission of cachexia is known to occur. Therefore, the use of cachexia-free budwood is the recommended method of preventing the introduction of this disease into new orchards.

Stubborn

Trees with some abnormalities resembling those caused by stubborn were observed, but no trees were seen with all symptoms of the disease. However, previous reports indicate stubborn disease to be widespread in the orchards of Libya. Nour-Eldin showed typical stubborn-affected trees to Bové in 1975.

Exocortis

Since no exocortis-susceptible rootstocks are used in Libya, no symptoms were expected to be found in commercial orchards. However, on the basis of evidence obtained in most citrus-growing areas of the world, exocortis viroid is the most widely distributed intracellular pathogen of citrus and is probably infecting many trees in Libya.

The real destructive capacity of exocortis would become apparent only if susceptible rootstocks, such as Poncirus trifoliata, Troyer citrange and other trifoliate hybrids, and Rangpur lime, come into use. Indexing for exocortis is necessary to determine the distribution of the viroid in plantations and to select healthy mother trees.

Tristeza

Apparently, the early cases of tristeza disease found in the citrus areas of the Mediterranean basin can be traced back to the introduction of infected budwood from abroad. All countries that have introduced Meyer lemon trees have also introduced tristeza, and these include Algeria, Israel, Italy, Morocco and Tunisia (Bové, 1966). Other varieties imported from Australia, Japan, South Africa and the United States of America have also been reported to have introduced tristeza into Mediterranean countries.

In 1976, Nour-Eldin and Fudl-Allah reported the occurrence of tristeza in a number of trees in Libya. They found 30 sweet orange trees grafted on sour orange rootstock that were stunted and exhibited characteristic bark pitting and honeycombing of the rootstock bark. Indexing of these trees on Mexican lime seedlings revealed vein clearing but no stem pitting.

However, Salibe found no indication that tristeza virus was present in Libya. Inspection of many field seedlings of small-fruited acid lime (Balady) in the germplasm collection at the Agricultural Research Centre (ARC) revealed no symptoms of tristeza. However, the virus represents a permanent threat to the entire citrus industry of Libya, and the problem requires constant attention.

Other virus and virus-like diseases

Rumple. This disease of lemons was observed on a few fruits of a Femminello-type lemon tree that had been top-worked on a previous orange tree in the Zawia area. Rumple is suspected to be of a virus nature, but no definite proof has been obtained as yet. It is recommended that no budwood for propagation be taken from diseased trees.

Impietratura. This virus problem affects citrus fruits in many areas of the Mediterranean basin, and is also present in Libya. A few fruits of a blood orange variety showed gum pockets in the albedo, characteristic of impietratura disease. Indexing should be carried out in order to avoid possible confusion between impietratura symptoms and those induced by boron deficiency.

Gummy bark of sweet orange. Nour-Eldin and Fudl-Allah (1976) found that about 30 percent of sweet orange trees were affected by this disease. Symptoms in sweet orange are similar to those induced by cachexia-xyloporosis in mandarin. Scraping some moderately stunted orange trees above the bud-union revealed the presence of gummy bark symptoms, but the incidence of the problem in the various commercial citrus varieties has stir] to be determined.

Woody gall of sweet orange. This disorder has been observed to affect many orange trees in Libya. According to Nour-Eldin (1975), the disease was confined to a local blood orange variety named Demi-sweet orange. However, it seems that other varieties are also affected by woody galls. Round galls were found to affect trunks and main branches of orange trees, rarely going down to the sour orange rootstock. In one orange orchard, of about 20 years old, in the Swami area, 8 percent of the trees had galls. Many of the diseased trees showed severe psorosis leaf symptoms. Further work will have to be done to determine the cause of the galls. It seems that they are not caused by the vein enation-woody gall virus (Nour-Eldin and Fudl-Allah, 1976).

Virus and virus-like diseases that were not observed by Salibe in his survey were cristacortis, greening, vein enation-woody gall, satsuma dwarf, leaf curl, yellow vein, multiple sprouting, citrus tatterleaf, citrange stunt, bud-union crease and gum pocket. Local specialists must make themselves familiar with the symptoms of these and other foreign diseases in order to eradicate them promptly should they appear in the country.

Other disease problems

Autumn leaf drop. This is considered by growers and research workers to be one of the most important problems affecting citrus trees in Libya. Abnormal leaf drop appears during the autumn and winter months, and affects trees of various ages and of old and new clones. The problem was observed in most orchards visited. Initial symptoms may appear as mesophyll collapse or resemble blade discoloration, as if the leaf had been treated with hot water (it looks cooked). The disease mainly affects branches at the top of the tree. The affected leaves wilt and drop, leaving the petiole attached to the twig for some time. The petioles drop later and severe dieback of the twigs takes place (Fudl-Allah, 1978). High populations of spider mites were observed in trees of many orchards affected by autumn leaf drop, in association with mesophyll collapse. Natural spread of the problem has been indicated by some authors (Nour-Eldin and Fudl-Allah, 1976).

Autumn leaf drop is probably a physiological disorder resulting from cold morning temperatures (7-8°C). The problem may be enhanced by a number of factors, among which are: mite damage to leaves, late fertilization inducing potash deficiency, deep ploughing weakening the trees, use of cold-sensitive varieties, and incorrect irrigation. Autumn leaf drop also occurs in other countries and was observed in Morocco and Tunisia.

Fungal and bacterial diseases. Phytophthora bark rot and penicillium fruit rot were other disease problems observed during visits to various orchards. Black pit induced by Pseudomonas syringae, reported to occur in the country by Chapot (1975), was not encountered by Salibe.

Other observations

The ARC germplasm collection

The ARC maintains a fairly large bank of citrus germplasm in Tripoli. Varieties were collected within the country and many were imported from abroad several years ago. They are practically all old-line cultivars around 15 years of age, budded on sour orange rootstock, except for a few that are budded on Palestine sweet lime.

Several varieties of sweet orange are represented in the collection, including Shamouti (Jaffa), Abu Surra (Washington navel), Valencia and local blood and non-blood oranges, a few mandarin varieties including Clementine and Kinya (identical to Youssef Effendi, Avana and Willowleaf), some lemons, grapefruits and shaddocks.

Rootstock varieties represented in the collection include sour orange, Cleopatra mandarin, Troyer citrange and rough lemon. There is also a line of seedlings of Balady (Mexican) lime.

Inspection of many of the trees in this orchard revealed a number of abnormalities indicative of virus and virus-like pathogens. A few orange trees of Spanish varieties showed psorosis A bark scaling on the trunk. Others exhibited exocortis-like scaling on the rootstock, which are probably Palestine sweet lime. Trees of Shamouti orange displayed trunk overgrowth above the bud-union, flat branches and trunk "bumps" or galls with some gum exudation.

General observation of the trees indicated the presence of autumn leaf drop and twig dieback, high mite infestation (red spider), leaf chlorosis, zinc and manganese deficiency (soil pH around 8), deep planting of many trees, and cracks in fruit indicating irregular irrigation.

Fruit-fly control is very efficiently carried out in the orchard by applying full tree sprays instead of a bait spray as is the usual practice elsewhere. Some scale problems were observed on certain trees, especially on lemon trees. Productivity of the trees was extremely low, sometimes only 5 to 10 kg of fruit per tree.

It is recommended that all citrus varieties represented in this germplasm collection should be indexed and cleansed of possible intracellular pathogens by micrografting. A new gene bank of healthy citrus germplasm, including these and other varieties of potential value, should be established by the ARC to provide superior propagative material to nurseries and citrus growers.

Experiment station of the Faculty of Agriculture

Trees of a small collection were inspected, and most of the Kinya and Clementine mandarin trees were found to show severe cachexia-xyloporosis symptoms. Some diseased trees had double tops, with branches of orange and mandarin, indicating top-working.

An outstanding young orchard of nucellar lines of several varieties budded on Rangpur lime rootstock was visited. Unfortunately, trees were severely pruned and the tops removed. Since nucellar lines show juvenile characteristics and ageing is directly related to the amount of cell division, the top of the tree is the best source of budwood for propagation.

A large rootstock trial with two orange scions of nucellar origin has been established at the Faculty of Agriculture and the trees are outstanding. Autumn leaf drop was seen in the tops of most of them. Rootstocks tested in this trial include Troyer and Uvalde citranges, Swingle citrumelo, Orlando tangelo, Citrus volkameriana and Citrus macrophylla.

Tajoura Agricultural Experiment Station

A visit was made to the Tajoura Agricultural Experiment Station of the ARC and four citrus orchards were examined for symptoms of virus and virus-like diseases. Trees were well cared for, vigorous and productive. Balady orange trees were superior in vigour and fruit production to those of Washington navel. Varieties grown in the plots visited included various orange varieties, Kinya mandarin and some lemons of Italian origin, of which one named Camary showed continuous blooming.

Some old trees of Kinya mandarin showed trunk depressions characteristic of concave gum-blind pocket, but they were producing a fairly good crop. Stubborn was suspected to be present in some orange trees, on the basis of leaf symptoms.

Other observations included heavy infestations of the fruit-fly, C. capitata, which was causing high fruit losses, and severe autumn leaf drop and dieback on orange, mandarin and lemon trees.

Cultural practices

Poor management of citrus orchards rather than the presence of infectious diseases is responsible for low productivity. Excessive pruning, deep ploughing periodically destroying a large proportion of the root system, imbalanced fertilization, incorrect timing and application of fertilizers and inadequate pest control programmes were typical of the poor practices noted. Leaf and fruit burning was observed in some orchards, caused by excessive and uneven spraying.

A strong and well-planned extension service to aid citrus growers is urgently needed, as a first step to increasing productivity in existing orchards. Bulletins explaining the various aspects of citrus growing, such as that of Abu-Daba and Abu-Ziada (1978), should be brought to the attention of growers.

Bibliography

Abu-Daba, N.M. & Abu-Ziada, l. 1978. Citrus in Libya [in Arabic]. Tripoli, Extension Service, Ministry of Agriculture. 108 pp.

Bové, J.M. 1966. Citrus virus diseases in the Mediterranean area. Report presented at the meeting on Phytiatry and Phytopharmacy, Marseilles (France), 1965, updated for the 4th Conf. IOCV. 44 pp. (mimeo) Chapot, H. 1975. A study of research requirements in citriculture. Consultant Series Bull., No. 4. Libya, ARC.

Fudl-Allah, A.E.-S.A. 1977. Present status of stubborn disease of citrus. Libyan J. Agric., 6(2): 55-65.

Fudl-Allah, A.E.-S.A. 1978. Observations on citrus autumn leaf drop in the Libyan Jamahiriya. Libyan J. Agric., 7: 125- 127. Nour-Eldin, F. 1975. Woody gall of sweet orange. Libyan J. Agric., 4: 101 - I 10.

Nour-Eldin, F. & Fudl-Allah, A.E.-S.A. 1976. Citrus virus and virus-like diseases in Libya. Libyan J. Agric., 5: 101-110.

Chapter 15: Morocco

Citrus growing in Morocco
Virus and virus-like diseases
Control of virus and virus-like diseases
Other observations
Recommendations
Bibliography

Morocco is among the ten largest producers and exporters of citrus in the world. During the last decade production of citrus fruits has ranged from 500 000 to 1 070 000 tonnes per year and the volume of exports between 460 000 and 720 000 tonnes. Exports represent around 70 percent of production and place Morocco third among Mediterranean citrus-exporting countries. In fact, citrus exports constitute the third largest source of foreign revenue in Morocco, after phosphates and manual labourers working abroad. Citrus orchards cover an estimated area of about 70 000 ha of fertile soil and constitute one of the most important sectors of national agriculture. They ensure work for more than 50 000 labourers in 8 000 orchards, packing stations (stations de conditionnement), ports and other related workplaces.

Citrus fruits have been grown in Morocco for centuries and some believe they were introduced into the country by the Romans. It appears that it was in Morocco, on the banks of the River Qued Lukus, that the famous Garden of the Hesperides was situated. For centuries, however, citrus was grown in the country as scattered trees for local consumption.

At the beginning of this century the total number of citrus trees was estimated at around 250 000. Production was insufficient to meet local demand and some imports were made to satisfy requirements. Extensive commercial orchards were first planted around 1930 and by 1933/34 production had risen to about 14 000 tonnes, of which 1300 tonnes were exported. The availability of water for irrigation from new dams (barrages) constructed at that time was a major factor in the expansion of citrus culture in the country. The growth in production allowed an increase in the volume of citrus exported, mainly to western Europe. In the last decade Morocco also started to export citrus to the Near East, mainly to the Islamic Republic of Iran, Kuwait and Saudi Arabia.

Citrus growers in Morocco are grouped in the Association de Producteurs d'Agrumes au Maroc (ASPAM), which comprises 12 sections, each representing a producing region. The association has an excellent extension service and, together with the Office de Commercialisation et d'Exportation (OCE), sponsored the creation of the Société Agricole de Services au Maroc (SASMA), an organization devoted to advising growers and exporters on recent advances and techniques for better orchard and packing station management.

In this way the most important aspects of technological advancement have been incorporated into the routine cultural practices of citrus orchards. However, fruit production per unit area is relatively low, since it is said to be between 12 and 15 tonnes per hectare. The future of the citrus industry in Morocco is essentially dependent upon revenues obtained by the growers.

Citrus growing in Morocco

Citrus in Morocco is grown mainly in the coastal areas of the Mediterranean Sea and Atlantic Ocean, where the climate is mild and rainfall abundant. The citrus orchards cover about 70 000 ha, which represents only I percent of the total agricultural area of the country and 16 percent of the area dedicated to fruit crops. Major citrus-producing regions and their areas and relative percentages according to a survey made in 1977 are shown in Table 36 (see also Map 4 in Chapter 7).

Total production during 1976/77 was 769 633 tonnes, of which 76.8 percent, or 590 787 tonnes, was exported. Available data indicate production ranging from 900 000 to one million tonnes, with exports for the 1982 and 1983 seasons of 601 226 tonnes and 521 873 tonnes respectively. There are 75 packing stations in the country and five juice factories which annually process about 150 000 to 200 000 tonnes.

Commercially grown varieties are: early ripening types - Clementine mandarin and navel orange (mainly Washington and Navelina); mid-season types - these include Salustiana orange, Washington-Sanguine orange and blood oranges; and late ripening types - Valencia late orange, locally named "Maroc late", and Vernia orange. Lemons are also grown, mainly for local consumption. Growers are advised to plant only five of the better varieties for export, namely Clementine mandarin, navel, Salustiana, Washington-Sanguine and Maroc late oranges. Production and export, by varieties, in the 1976/77 season are shown in Table 37.

Wilking mandarin was widely grown until 1973, with production estimated at 40000 tonnes per year. However, since this variety was planted near Clementine mandarin trees and induced, by pollination, the appearance of seeds which thus affected the export quality of the Clementines, all Wilking mandarin trees have been eliminated, by order Dahir portant loi No. 1-73-172 du 8 Moharam 1393 [22 February 1973], which required the destruction or top-working of all Wilking mandarin trees.

Sour orange, Citrus aurantium L., is practically the only rootstock used in the country. Some Troyer citrange was used but it does not produce a high performance tree in calcareous soils and is being abandoned.

Trees below ten years of age at present constitute 35.57 percent, those from ten to 29 years 51.23 percent, and older trees (30 or more years of age) 13.40 percent of all citrus trees in the country. Many of these trees will need replacement before long.

About four million nursery trees are produced every year in 416 nurseries spread over the country. Citrus account for 9 percent of all nursery trees produced and about 80 percent of these come from only four nurseries those of the Société de Développement Agricole (SODEA), SASMA, Domaine Royal and Institut National de Recherche Agricole (INRA). In 1989, SODEA produced one million certified citrus trees in the Agadir nursery.

These data clearly indicate that Morocco has the capacity to increase its citrus output rapidly provided that export prices are maintained at a favourable level and that water is available for the further increase of the growing area.

TABLE 36 Production of major citrus areas in Morocco

Source: Squalli, 1978.

Virus and virus-like diseases

Virus and mycoplasm diseases are known to have been affecting the efficiency of citrus trees in Morocco for more than 30 years (Wyss-Dumont, 1951; Perret, 1953; Chapot, 1956a, 1959; Chapot and Cassin, 1961; Cassin, 1962, 1963a, 1964). A number of attempts have been made to eliminate these pathogens from new orchards. However, they continue to be propagated through the use of infected plants and are a major factor restricting high yield.

Virus and virus-like diseases reported to occur in citrus trees in Morocco include psorosis A bark scaling, concave gum-blind pocket, cachexia-xyloporosis, exocortis, tristeza, impietratura, cristacortis, infectious variegation, gummy bark, stubborn, infectious variegation and orange pitting.

Scaly bark psorosis (psorosis A) and concave gum-blind pocket

Psorosis A and concave gum-blind pocket have a worldwide distribution and are found in practically all citrus-growing areas of the Mediterranean and the Near East.

In Morocco they have been reported in orange and mandarin trees (Cassin, 1962; Chapot and De Lucchi, 1964; Nhami and Kissi, 1978; Nhami and Bourge, 1979; Nhami and Zidane, 1984). Varieties found in the orchards showing symptoms of psorosis A bark scaling, according to the reports of these authors, include navel, Navelina, Salustiana, Hamlin, Cadenera, Jaffa (Shamouti), Valencia (Maroc late), Vernia, Grosse Sanguine, Sanguinelli and Tarocco oranges. Symptoms of concave gum and/or blind pocket were observed in trees of most of these orange varieties and also in common, Wilking and Clementine mandarins. Infectious variegation was encountered in trees of navel orange, lemon and sour orange.

Salibe visited a very limited number of citrus orchards owing to shortage of time. However, in one orchard of Maroc late orange in Larache, about 35 years of age, typical symptoms of psorosis A bark scaling, blind pocket and concave gum were observed. The number of diseased trees was small considering that most of them were probably carrying the psorosis virus. No psorosis A bark scaling was encountered in any of the Clementine mandarin orchards inspected. It must be emphasized that the presence of the pathogen, even when symptoms are not evident, has a debilitating effect on the citrus trees. The use of virus-free budwood is the recommended method of excluding the disease from new orchards.

TABLE 37 Production and export figures for 1976/77

Source: Squalli, 1978.

Cachexia-xyloporosis

Cachexia-xyloporosis is frequently found in mandarin orchards in most Mediterranean countries. In Morocco the disease was reported to be affecting mandarin trees by Chapot and Cassin (1961) and Cassin (1964). They reported that symptoms were sometimes severe, with pinholing in addition to the typical conoid wood pitting. More recently, Nhami and Kissi (1978) reported cachexia-xyloporosis in trees of Clementine and common (Youssef Effendi) mandarin, sweet lime, M'Guergueb and Rhobs El Arsa citron (cedratier) and in Rangpur lime.

Exocortis

Exocortis-susceptible rootstocks are not widely used in Morocco, but the viroid seems widespread nevertheless, according to various reports (Bové, 1966; Nhami and Kissi, 1978; Nhami and Bourge, 1979; Nhami and Zidane, 1984). These authors observed exocortis symptoms in trees budded on trifoliate orange, Rangpur lime and on M'Guergueb and Rhobs El Arsa citron.

Stubborn

Symptoms of stubborn were first described in Morocco by Wyss-Dumont (1951) and Perret (1953), and the presence of the disease was confirmed by several other specialists (Chapot, 1956a, 1956b, 1959; Schneider, 1966; Bové, 1978; Nhami and Kissi, 1978; Nhami and Bourge, 1979). The disease is present in all the country's citrus-growing regions, but it is especially severe in the Tadla and Souss areas. In the Tadla area, the number of badly affected trees was so high that whole orchards had to be removed. Spiroplasma citri, the stubborn agent, has been cultured from many trees in Morocco, and the reference strain R8A2 was obtained in 1970 from a tree in a severely affected orchard (Gontard) in the Tadla region (Saglio et al., 1973). Natural transmission of S. citri to periwinkle (Catharanthus roseus) plants has been demonstrated, and the leafhopper Neoaliturus haematoceps has been found to be infected with S. citri.

Recently (October 1990) the Bordeaux group (Bové, Fos, Saillard and Vignault) and the SODEA group (Nhami, Alaoui Ismaïli, Belaadel, Jabri, Karmoun Lamarti and Zidane) carried out a survey for Salsola kali, N. haematoceps and S. citri-infected plants. This survey was a continuation of the work carried out in 1978-80 (see the section on Morocco in Chapter 7, p. 95). The 1990 survey yielded new results. In particular S. kali, the preferred host plant of N. haematoceps and N. tenellus, the two leafhopper vectors of S. citri, was found at many sites along the coast from Agadir in the south to Mehdiya in the north (see Map 4 in Chapter 7). The Neo-aliturus spp. leafhoppers collected on S. kali varied with the geographical locations. Only N. tenellus was collected on the sandy beaches north and south of Agadir. On the beaches near El Jadida, 100 km south of Casablanca, both N. tenellus and N. haematoceps were found. At Rabat and north of Rabat only N. haematoceps was captured. N. tenellus had been reported to be present in Agadir by Frazier in 1953. By focusing on the preferred host plant, S. kali, the 1990 survey obtained, for the first time, high numbers of N. tenellus. Exceptionally high numbers of N. haematoceps were found in Rabat on S. kali plants growing along the River Bouregreg (Fig. 322). This site is close to where a S. citri-infected periwinkle plant was discovered in 1978 (Bové et al., 1978, 1979). Three additional S. citri-infected periwinkle plants were found in the same area during the October 1990 survey.

Tristeza

The number of citrus trees at present existing worldwide is around 500-600 million, and more than half carry the tristeza virus. Of the healthy trees, over 100 million are susceptible to tristeza because they are grafted on sour orange rootstock. Tristeza represents a permanent threat to all these trees.

All known cases of tristeza in Morocco are related to the introduction of infected budwood from abroad. Cassin (1963a) reported tristeza in Meyer lemon and in eight other trees - five Owari satsuma, two Valencia orange and one Washington navel orange.

Meyer lemon was propagated in some areas of Morocco, mainly in Marrakech, where homogeneous plantations existed until 1967. At that time, an official regulation made the eradication of all Meyer lemon trees in the country obligatory. However, it is said that a number of trees of Meyer lemon, doubtless harbouring tristeza virus, still exist in the country (Nhami and Kissi, 1978; Bové, 1978). This is an extremely dangerous situation as it is now known that the tristeza virus is capable of mutating, and possibly becoming more easily carried by aphids. Furthermore, the proximity of Morocco to Spain, where tristeza virus is spreading, makes the threat of this virus to the flourishing Moroccan citrus industry now more than ever a matter for concern. Recently, a large-scale tristeza indexing programme based on ELISA has been started in Morocco. Several tristeza virus infected trees have been detected.

Impietratura

Impietratura was reported to be affecting citrus fruits in Morocco as early as 1934. According to Nhami and Kissi (1978), impietratura was found by the Plant Protection Service in Casablanca orchards. Chapot (1961) also found impietratura in Morocco. Since then, symptoms of impietratura have been observed in several orchards in navel, Navelina, Hamlin, Salustiana, Vernia and Valencia late orange, and also grapefruit. During 1977, considerable amounts of diseased fruits occurred in orchards in the area of Larache. Nhami and Kissi (1978) reported, in one orchard of Washington navel oranges in that area, that 20 percent of trees were producing fruits with impietratura.

It has been suggested that the high incidence of trees with such fruits in the Larache area and in the lower Sebou valley results from insect transmission or some other means of diffusion (Devaux, 1978). Whether or not this is so remains to be established.

Varieties producing fruits with impietratura in Morocco are: navel, Navelina, Salustiana, Hamlin, Cadenera, Maroc late and Vernia sweet oranges, and grapefruit (Nhami and Kissi, 1978). The problem is serious and requires urgent attention. Budwood for propagation should not be taken from diseased orchards.

Other virus and virus-like diseases

A number of other abnormalities caused by virus or virus-like pathogens have been observed in Morocco in a very limited number of trees (Nhami and Kissi, 1978; Bourge and Nhami, 1979). The following problems deserve mention.

Cristacortis. This was found in four trees, one Tarocco orange in Marrakech, another Tarocco orange in Gharb, one Valencia late orange in Beni-Mellal and a Sanguine Double-Fine orange at Berkane. It is worth mentioning that the Tarocco sweet orange trees on which cristacortis was first observed by Vogel and Bové in 1963 in Corsica were from Morocco.

Gummy bark. Symptoms were found in only one tree of Valencia late orange, of about 35 years of age.

Infectious variegation-crinkly leaf. This was found in some Washington navel orange trees in Rabat in 1963 (Cassin, 1963b) and symptoms were later observed in lemon, sour orange and navels in the areas of Rabat and Larache.

Orange pitting (Nhami disease). This apparently previously unreported abnormality was found in Valencia orange trees in Beni-Mellal in 1969 and in Agadir in 1975. The disorder was also observed in one tree of Sanguine orange at the Souihla Experiment Station in 1977. Transmission experiments have so far proved negative.

Rusk citrange stem pitting. A disorder of unknown origin was shown to Salibe in the El Menzeh Citrus Experiment Station. Symptoms on old trees and young seedlings of Rusk citrange very much resemble those induced by tristeza virus. The problem is apparently seed-transmitted at a rate of 3 to 6 percent.

Rootstock trunk shelling. Salibe visited an orchard in the Larache area where a disorder of unknown nature is affecting trees of Valencia orange. Symptoms resemble those of lemon dry bark rot, affecting only the sour orange portion of the trunk. The problem deserves careful attention.

Control of virus and virus-like diseases

Production of nucellar clones

The programme of production and selection of nucellar clones of commercial citrus varieties in Morocco started in 1964 at the El Menzeh Citrus Experiment Station. According to available information (Nadori, Quammou and Quaicha, 1984; Nadori, Nhami and Tourkmani, 1984), nucellar clones of 31 varieties have been produced including orange, mandarin, grapefruit and lemon. At present 1 600 nucellar trees are being studied for their performance and fruit quality. Among varieties of major importance are Valencia late orange (50 trees), Washington navel orange (28 trees), Washington-Sanguine orange (12 trees), Salustiana orange (four trees), Maltaise ovale orange (four trees) and Murcott tangor (one tree).

Large amounts of budwood of nucellar clones of Valencia orange are being released to nursery workers.

Selection of virus-free trees by indexing

Indexing for intracellular pathogens of citrus trees is currently under way in Morocco for the following diseases: tristeza, using Mexican lime as indicator; psorosis, using Hamlin orange; cachexia-xyloporosis, using Parson's Special mandarin; exocortis, using Etrog citron 60-13; and stubborn, using Madame Vinous orange (Nhami and Bourge, 1979; Nhami and Zidane, 1984; Nadori, Nhami and Tourkmani, 1984). Indexing has been carried out for more than 1 000 trees selected in the commercial orchards for their superior performance and apparent freedom from disease symptoms. Trees of nucellar clones were also selected for the indexing programme. Results already available indicate that 32 trees were positive for psorosis out of 121 tested (26 percent), eight trees were positive for cachexia out of 21 (38 percent), 74 trees were positive for exocortis out of 257 (29 percent) and 48 trees were positive for stubborn out of 236 tested (20 percent). The presence of one or more pathogens varied from 40 percent for Clementine mandarin trees to 100 percent for Navelina, Salustiana and Washington-Sanguine oranges. All nucellar trees tested were found to be free from virus and mycoplasmas at this point.

No tristeza was found in any tree tested.

Shoot-tip grafting

Shoot-tip grafting was introduced into the improvement programme of citrus varieties in Morocco in 1983 (Nhami and Zidane, 1984). Propagations already made by this method include eight clones of Clementine mandarin, one clone of Ortanique tangor and various clones of Navelina, navel, Navelate, Washington-Sanguine and Salustiana orange. At present, 14 clones are being indexed for verification of freedom from viruses after passing through the shoot-tip grafting process.

Release of virus-free budwood

Superior virus-free budwood from selected commercial varieties is being propagated in the nurseries of SODEA. Over one million certified trees were produced in the Agadir nursery in 1989.

The large nurseries of SASMA are using the nucellar clones produced and selected at the El Menzeh Citrus Experiment Station for their propagations.

According to present regulations, certified nursery plants will be available only from those agencies having all indexing facilities, as is the case for SODEA. All other nurseries will have to use selected material from the Citrus Experiment Station or will continue to propagate virus-infected budwood. According to information available, only SASMA uses nucellar clones for their propagations. All the rest (more than 400 nurseries of all sizes) use budwood of unknown origin.

The nurseries of SODEA and SASMA are propagating basic material under controlled conditions (plastic tunnels) and then using budwood from these young trees. This method is questionable, since it may allow massive propagation of undesirable mutations. However, for the time being, it is acceptable since no large volume of healthy budwood is available elsewhere for propagation.

The present regulations for the production of certified nursery plants of citrus have been established by a number of official resolutions, basically:

• Dahir No. 1-69-169 du 10 Joumada I, 1389 [25 July 1969], which established regulations for the production and commercialization of seeds and plants, and
• Dahir portant loi No. 1-76-472 du Chaoual 1397 [19 September 1977], which modified and complemented the first law

Other complementary recommendations and discussions of the programme were made by Nadori, Nhami and Tourkmani (1984). However, it is the author's opinion that the benefits of the certification programme should be extended to all citrus nursery workers and growers in the country.

Other observations

Visit to the El Menzeh Citrus Experiment Station

This research station was created in 1960 to work on the problems faced by the citrus growers of the country. The original area of 10 ha in 1960 was extended to 52 ha in 1963. It is situated 9 km north of Kenitra and 12 km from the Atlantic Ocean, in an area with an altitude ranging from 30 to 50 m above sea level.

The soil of the station is sandy loam, with a pH neutral to slightly acid. Average rainfall is 560 mm (350 to 750 mm), and orchards require some irrigation, mainly during the dry summer (April to October). Temperatures range from a minimum of -5°C to a maximum of 45°C.

A large number of the problems faced by the citrus growers are being studied there. Major attention has been given to the production of nucellar clones, the search for new rootstocks -concentrating on tolerance to tristeza and resistance to drought and footrot, the selection of superior clones of Clementine mandarin and other studies. Mention should be made of the fine research work carried out to solve the problem of alternate bearing and low productivity in Clementine mandarin trees, which led to the development of bending (Merle and Nadori, 1978).

The station produced excellent work for a time, but was then partially abandoned.

Recently it has received a new, dynamic management team and a group of young, outstanding research workers.

Salibe visited the laboratories of virology, which are equipped to conduct shoot-tip grafting and ELISA testing for tristeza. He also inspected the greenhouses with controlled temperature for the indexing of viral and mycoplasmal diseases, and the plastic tunnels for the multiplication of basic material to be released to growers and nursery workers. Further financial support is required, with adequate salaries for the research workers, so that the full potential for research may be realized.

Visit to SODEA

SODEA was established by royal decree on 30 October 1972 to manage the farms previously owned by Europeans. It is presided over by the prime minister. The board of directors includes eight ministers and the director general of OCE. The organization is a major producer of citrus, grapes and various other fruits and annual crops, has large nurseries, exports products (about 30 percent of all citrus fruits exported by the country) and carries out a number of other activities in the fields of agriculture, industrialization and exports. During the four-year period from 1976 to 1980, the nurseries of SODEA produced over 12 million plants, of which 217 000 were citrus.

SODEA decided to produce certified citrus plants and, with this aim, established a laboratory for indexing purposes plus greenhouses and nurseries in Temara, near Rabat. These facilities were visited by Salibe who considered them a model for many other citrus-growing areas in the world. The construction in 1980 of this outstanding centre was the result of Bové's report of his 1978 survey, in which he emphasized the need for rehabilitating the old, virus-infected citrus orchards of Morocco(Bové, 1978).

Salibe visited SODEA nurseries in the Rabat and Agadir regions and the germplasm collection - field gene bank - established in Ouled Taima. All nursery trees are produced in pots under plastic tunnels. The use of containers and microbudding techniques, plus the controlled environment in the tunnels, has allowed them to obtain plants that are ready to deliver in a period of 12 to 18 months instead of the 30 to 36 months of the traditional system used in the country.

The germplasm collection of healthy mother trees established by SODEA holds clones of Maroc late orange; navel orange (shoot-tip grafted, nucellar and old-line clones); Clementine mandarin (four clones); Salustiana orange (nucellar); Maltaise ovale orange; Eureka lemon; and Lisbon lemon. Each of the 12 clones is represented by 24 trees budded on four rootstocks (minimum two rootstocks per variety).

The rootstock block is composed of seedling trees (planted in June 1984) of Cleopatra mandarin; Troyer citrange; Carrizo citrange; Rangpur lime; Citrumela Swingle (4475); Volkamer lemon; Citrus macrophylla; common sour orange; and bitter-sweet orange.

SODEA maintains 2 000 ha in the Agadir region of which 1 200 ha are citrus orchards. They are vegetatively excellent orchards, but iron chlorosis is a general problem.

Visit to SASMA

SASMA was created under the auspices of ASPAM and OCE and is devoted to advising growers and exporters on the production and export aspects of citriculture.

Salibe visited the society's excellent laboratories in Casablanca and the nurseries in Agadir and Kenitra, and was highly impressed with the work of the organization. In Casablanca there were laboratories for soil and leaf analysis and water and fertilizer analysis among other activities. Annually, about 5 000 analyses are made of soil samples, another 5 000 of leaf samples and about 2 000 analyses of water and fertilizers used in citrus and vegetable crops. A biochemistry laboratory performs analyses on chemical residues in fruits, and conducts studies on post-harvest, export and fruit quality problems.

The SASMA nursery in the Plain of Souss (43 km south of Agadir) produces about 50 000 to 60 000 budded citrus plants every year. Another of SASMA's nurseries in the Plain of Gharb produced 130 000 budded citrus plants in 1983 and planned to produce 200 000 in 1984. Nurseries run by the society are outstanding - all trees are produced in containers under plastic tunnels using nucellar budwood provided by INRA. All nursery plants produced by SASMA are sold at cost price to growers of ASPAM.

Other visits

Salibe visited several laboratories of INRA in Rabat and of the Institut Agronomique et Vétérinaire (IAV) Hassan II in Rabat and Agadir. In total, Morocco has at present three laboratories for shoot-tip grafting (SODEA, El Menzeh Citrus Experiment Station and IAV Hassan II) and six laboratories equipped for ELISA testing for tristeza virus (two DPVC, two INRA and two IAV Hassan II at Rabat and Agadir). This represents obvious progress in the control of virus and virus-like diseases of citrus paralleled in only a few other developed countries in the world.

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