Other virus and virus-like diseases

Contents - Previous - Next

Diseases transmitted by insect vectors

Besides greening disease, two other important diseases are vector-transmitted: tristeza (a virus disease) and stubborn (a disease due to the helical mycoplasma Spiroplasma citri).

Tristeza. No evidence for the presence of tristeza was seen in Saudi Arabia in 1981, 1982 or 1983. None of the many small-fruited acid lime trees observed showed leaf vein clearing or stem pitting, two symptoms specific to tristeza on lime trees. Furthermore, budwood samples were taken from several suspect trees, especially in the Najran area, and analysed for tristeza in Bordeaux using ELISA. They were all negative.

Tristeza virus is transmitted by various aphids. The tropical black aphid (Toxoptera citricida), a very efficient vector (Chapter 3, see drawing on p. 36), is not present in the Arabian Peninsula, but other aphid vectors, such as Aphis gossypii and Myzus persicae, are present in Saudi Arabia and could transmit various strains of the virus.

Stubborn disease. Typical symptoms of stub born disease were seen in 1981 in the Kassim area in one orchard (Saleh Abbad orchard, Unaizah area). Fruit samples from the affected Balady sweet orange trees were taken to Bordeaux. S. citri, the causal mycoplasmal agent of stubborn, was cultured from the fruit samples, confirming the presence of the stubborn disease pathogen in Saudi Arabia.

In California, S. citri is transmitted by the sugar beet leafhopper Neoaliturus (ex. Circulifer) tenellus. The major vector in the Mediterranean area is Neoaliturus haematoceps, also a sugar beet leafhopper. These two leafhoppers are present in Saudi Arabia in many areas (see Table 46 and Maps 9, 11 and 12) (Wittmer and Buttiker, 19791981). Salsola kali (Chenopodiaceae) is the preferred host plant of these two leafhoppers. The distribution of S. kali in Saudi Arabia is still to be determined.

Diseases not transmitted by insect vectors

Many cases of the following diseases have been observed in Saudi Arabia: psorosis young leaf symptoms, scaly bark psorosis, concave gum-blind pocket, gummy bark, impietratura and cachexia. These diseases are not transmitted by insect vectors, but they are spread by humans when taking budwood for propagation from affected trees. In addition, the cachexia viroid is mechanically transmissible.

Cachexia is widespread and has been observed in the following orchards on Willow-leaf mandarin scions (see Map 9): Saud Alabdali (Wadi Nahman, Mecca district); Maid Wazen (Turabah); Hedjera (Khurmah); Bakhit Addaussari (Figs 50 and 51) and Ben Kadassa (Figs 38 and 39) (Bishah); Ali Aboussak, Khaled Al Sudeyri (Figs 44 and 45), Mohamed Wabrane and Ali Abdel Jalil (Najran); in this last orchard, cachexia was also seen on sweet lime rootstock. Cachexia-affected mandarin trees were also present in the Brehim Khaibar area (Fig. 206).

MAP 11 Sites where Neoaliturus haematocepsand Neoaliturus tenellus have been found in the Riyadh area, Saudi Arabia (see Table 46)

TABLE 46 Sampling sites where Neoaliturus haematoceps and Neoaliturus tenellus have been found in Saudi Arabia

Vein flecking and oak-leaf pattern (psorosis) young leaf symptoms were seen in the Najran area (Figs 91 to 93 and 95) and north of Medina (Al Khelil district).

Psorosis-like scaly bark occurred on sweet orange scions in the Unaizah Agricultural Research Station (Figs 99 and 100). Concave gum depressions were seen in the Najran area in the Khaled Al Sudeyri orchard on sweet orange, and in the Ali Abdel Jalil orchard on mandarin.

Gummy bark of sweet orange was also noted quite frequently in the Turabah area (Narser Ben Sehmi orchard) near Bishah (Bakhit Addaussari and Ben Kadassa orchards) and near Najran (Khaled Al Sudeyri orchard). Clear-cut symptoms of impietratura were seen on sweet orange (Fig. 117) in the Emir Nasser Ben Sehmi orchard at Turabah.

It can be assumed that practically all the grafted old-line trees in Saudi Arabia are contaminated by one or more of these diseases. However, as no records have been kept of the origin of the planted trees and the nature (species) of their rootstocks, it is often difficult to correlate observed symptoms, such as stem pitting on the rootstock, with a given disease.

MAP 12 The southwestern region of Saudi Arabia

This situation is especially true in the Najran area.

For instance, in the Mohamed Wabrane orchard (Najran area) Willowleaf mandarin trees are grafted on unknown rootstocks. Some trees show typical and severe symptoms of cachexia (stem pitting, bark pegging and gumming) on the mandarin scion, but no symptoms on the rootstock (Figs 263 and 265). Other trees show stem pitting (without bark gumming) on the rootstock and no symptoms on the mandarin scion (Figs 264 and 265). A third category of trees show severe symptoms of cachexia on the mandarin scion and some stem pitting on the rootstock (Fig. 266). While the symptoms on the mandarin scion are those of cachexia, the stem pitting on the rootstock cannot be traced to a given disease because the rootstock is undetermined. Similarly, in several orchards in the Najran area the unrecorded rootstocks of stunted sweet orange trees (Fig. 267) show bark cracking (Figs 268 and 269) and severe stem pitting (Figs 269 and 270), while the scion is symptomless. Symptoms on the rootstock occur on young trees (Fig. 270). Again identification of rootstock is required for diagnosis. If the rootstocks were sweet lime, the stem pitting could be due to the cachexia agent and the bark cracking to the exocortis agent. In the Mohamed Wabrane orchard, sweet orange trees on unrecorded rootstock show bark scaling and cracking and mild stem pitting below the bud-union. Again, the cause of these symptoms cannot be determined.

In the Emir Meteb orchard (Buraidah region) severely stunted sweet orange trees (Fig. 271) show bud-union crease and wood staining below bud-union (Fig. 272). Once again, the rootstock is unfortunately unknown. Other trees on sweet lime rootstock show gum-impregnated bud-union lines (Fig. 132). However, bud-union crease is difficult to interpret when the rootstock of the affected tree is unrecorded (Fig. 135).

Finally, certain commercial citrus species or varieties are known to be tolerant of certain virus and virus-like diseases. This means that they show no symptoms of the disease even though the causal agent is present in the tree. For instance, sweet orange or lemon trees on sour orange rootstock can be fully infected by the viroid(s) of exocortis without showing any symptoms. Only indexing on sensitive indicator plants or other techniques will detect these agents.

For all these reasons, in Saudi Arabia, budwood for citrus propagation should not be taken from old-line trees. Only budwood from well-known and reliable sources, certified free of known diseases, should be propagated.

Phytophthora gummosis - a fungal disease

Many cases of phytophthora gummosis have been observed throughout Saudi Arabia, most of which could have been avoided by adequate horticultural practices. The following points are important. Only sufficiently large rootstocks should be grafted, so that the buds can be inserted on the rootstock seedlings more than 20 cm above soil level. Trees should not be planted too deep in order to keep the bud-union line well above soil level. They should not be planted in a hole but rather on a heap of soil. The worst situation occurs when trees are planted at the intersections of perpendicular irrigation furrows (Fig. 227).

Bacterial disease: Citrus canker

Citrus canker, a very important disease caused by the bacterium Xanthomonas campestris pv. citri, has been detected near Sabia, north of Jizan (see Map 10) on lime trees in an orchard of the Al Hagyriyh village, and in an orchard along the Wadi Damad road. Immediate burning and destruction of the trees in the affected orchards was strongly recommended the next day to the Ministry of Agriculture.

The trees with canker symptoms might have been brought from the northern part of Yemen. This is very likely, since an outbreak of citrus canker occurred in 1982 in the Tihama at the Al Garaba station, where the disease and the causal bacterium were very probably imported into this part of Yemen with citrus plants from India.

Bibliography

Bové, J.M. & Garnier, M. 1984. Citrus greening and psylla vectors of the disease in the Arabian Peninsula. In Proc. 9th Conf: IOCV, p. 109-114. Riverside, Univ. Calif.

Capoor, S.P., Rao, D.G. & Viswanath, S.M. 1967. Diaphorina citri Kuway., a vector of the greening disease of citrus in India. Indian J. Agric. Sci., 37: 572-576.

Celino, C.S., Salibe, A.A. & Cortez, R.E. 1966. Diaphorina citri Kuway., the insect vector for the leaf mottle virus of citrus in the Philippines. Manila, Bureau of Plant Industry.

FAO. 1972. Report to the Government of Saudi Arabia on Research in Plant Protection. Doc. No. AGP:TA/207. Rome.

Green, G.C. & Catling, H.S. 1971. Weather-induced mortality of the citrus psylla Trioza erytreae (Del Guercio) a vector of greening virus in some citrus-producing areas of Southern Africa. Agric. Meteorol., 8: 305-307.

Wittmer, W. & Buttiker, W. 1979-1981. Fauna of Saudi Arabia. 3 vols. Basle, Switzerland, Pro Entomologica, Nat. Hist. Mus.

Wooler, A., Badgham, D. & Arafat, A. 1973. Diaphorina citri on citrus in Saudi Arabia. FAO Plant Prot. Bull., 22: 93-94.

Chapter 19: Somalia

Mogadishu region: Citrus cultivars and climatic data
Fungal diseases of citrus in the Mogadishu region
Virus and virus-like diseases in the Mogadishu region
Exocortis
Important diseases probably not present in the Mogadishu region
Diseases of citrus in the Mogadishu region: A summary
Hargeisa region: General information and climatic data
Greening
Tristeza in the Hargeisa region
Other virus and virus-like diseases
Fungal diseases
Salt toxicity
Insect pests
Conclusions and recommendations
Bibliography

A survey of virus and virus-like diseases of citrus was conducted in two regions of Somalia (see Map 13): the area around Mogadishu from Giohar to Goluin (see Map 14), along the River Scebeli; and the area around Hargeisa (see Maps 15 and 16) in the northwestern part of the country.

Mogadishu region: Citrus cultivars and climatic data

Grapefruit is the major citrus variety grown in the Mogadishu region. The oldest trees are 20 to 30 years old and were planted by Italian citrus growers. These trees are all grafted on sour orange rootstock, and the scions represent old-line cultivars.

In the 1970s, new plantations of grapefruit trees were established. The cultivars used comprised not only old varieties but also new ones such as Marsh seedless, Ruby, Redblush, etc. The rootstock was mainly sour orange, but rough lemon also began to be used. The grapefruit trees cover approximately 1 000 ha. The best orchards have a yield of 20 tonnes per hectare, but many produce less. Normally, under good management, grapefruit orchards should give at least 50 tonnes per hectare.

Since 1980, the Genale nursery has distributed about 150 000 grapefruit trees. This number should be sufficient to plant about 500 ha of new citrus orchards. This brings the total area of production of grapefruit trees to about 1 500 ha.

Besides grapefruit, small-fruited acid lime is the second most important citrus variety grown in the Mogadishu region. Lime trees cover approximately 500 ha, but receive very little care.

The mean monthly maximum temperatures at Genale (latitude 1°50' North, altitude 69 m), fluctuated in 1965 between 33.5°C and 28.6°C, the mean monthly minimum temperature between 23.6°C and 21°C, and the mean monthly temperature between 28.3°C (April) and 24.9°C (July). The absolute maximum temperatures per month were between 39°C in March-April and 34°C in July, the absolute minimum temperatures between 20°C in April and 13°C in January. These data indicate that the Mogadishu region has a uniformly warm climate. Relative humidity is always high and varies from 74 percent to 82 percent. Rainfall is relatively low and reached 471 mm in 1965. There are two rainy and two dry seasons. Gigal is the major dry season and lasts from December to March inclusive, with a total of 32 mm of rain in 1965. Gigal is followed by Gu, the major rainy season, with 230 mm of rainfall from April to June inclusive. Hagal is the second dry season (100 mm for July and August) and Der the second rainy season with 107 mm of rain in September, October and November.

Citrus trees flower during the first rainy season (Gu) from April to June, with fruiting reaching maturity from December to February (20 percent of annual crop). The major blossom period occurs during the second rainy season (Der) from September to November with fruits ripening from May to September (80 percent of annual crop).

MAP 13 Somalia

During Gigal, the dry season from December to March, the Scebeli River is low, irrigation water is pumped from wells and salinity can be high. From April to November, irrigation water comes from the river.

The wind regime follows two monsoons. From April to October, winds (3.2 km/see and above) come from the southwest and south. From November to March, they blow from the northeast and east (less than 2.4 km/see).

Field research on citrus is carried out at the Central Agricultural Research Station (CARS) at Afgoi on approximately 6 ha of land. Several citrus varieties were introduced at CARS in the period 1966 to 1969. The introductions were from California and Sicily as well as from Somalia. Some of these cultivars and especially those from Italy and Somalia were probably not free of virus and virus-like diseases since, for instance, concave gum symptoms were observed on Avana mandarin from Palermo.

These introductions have been used in various trials, including variety screening and rootstock trials. In view of the very poor condition of the trees in these trials, it is doubtful whether much information can be gained from the experiments.

New budwood introductions (Ruby and Reed grapefruits; Carter, CRC and Frost navel sweet oranges) were made in 1978 and used for rootstock trials planted in 1979, comprising Brazilian sour orange, rough lemon, Citrus macrophylla, Rangpur lime, Carrizo citrange, citrumello 4475, Citrus taiwanica and Citrus amblycarpa. Impietratura-like symptoms and severe symptoms of stubborn have been seen on Frost navel sweet orange. All sweet orange trees on Carrizo citrange rootstock have died.

In 1981-82 the Genale nursery introduced seven grapefruit cultivars from the French Citrus Experiment Station in San Giuliano, Corsica. These cultivars, certified free of known virus and virus-like diseases, included white flesh varieties: Marsh seedless, David seedless and Little River; a pink variety: Thompson; and red varieties: Redblush, Ruby, Shambar.

Fungal diseases of citrus in the Mogadishu region

Rio Grande gummosis

The most important of all diseases of citrus in the Mogadishu region is Rio Grande gummosis. The disease has been seen in practically all grapefruit orchards. On grapefruit and orange trees, the symptoms -profuse gum production - appear high up on the trunk and out on the larger branches (Figs 237 to 239). The gum oozes out of vertical cracks in the bark (see Fig. 244) and runs down along the trunk or hangs down from the branches in stalactite manner (Figs 236, 239 and 240). At the time of initial gumming there is no scaling of the bark at the sites where the bark is split. However, the first stage in the healing-over process is the sloughing of thin scales of dead outer bark. Then follows the development of scar tissue generated by the bark. Repair is only temporary and healed-over lesions may again start gumming and enlarging (Figs 241 and 242). In this way, lesions may pass through repeated cycles of recovery and relapse, in the course of which they progressively enlarge and expose more and more wood. In old, inactive lesions the wood is exposed (Fig. 240). The gum pockets may be located deep in the wood (see Fig. 245) and the gum travels a considerable distance in and along the wood, so that gum pockets may exist far removed from the nearest active, gum-producing lesion.

MAP 14 The Mogadishu region, Somalia

The bark scaling associated with the healing-over process and the sloughing of scales of dead outer bark can be pronounced (Fig. 243) but should not be confused with that of scaly bark psorosis (see Figs 98 to 100), "popcorn" (Fig. 101) or phytophthora gummosis.

In Somalia, Rio Grande gummosis is widespread throughout the Mogadishu region from Giohar to Goluin. It mainly affects grapefruit trees. The first symptoms appear on seven- to eight-year-old trees. Older trees can be very severely affected, with dieback and leaf drop in addition to gumming. Rio Grande gummosis-affected trees seem to be more susceptible to attack by the larvae of the borer Macrotoma palmata (Coleoptera, Cerambycidae).

There seems to be a correlation between the appearance of the first symptoms of Rio Grande gummosis and the year when the grapefruit trees are pruned for the first time indeed, it has been pointed out by several citrus growers that the disease shows up soon after the trees have been first pruned. In general, trees are pruned at the age of six to seven years, and Rio Grande gummosis appears when the trees are seven to eight years old. These observations tally with the results obtained in California, where it was found that the causal fungal agent enters the tree through pruning wounds. If so, only very few signs of the disease should be present in orchards where trees are only slightly pruned, or not pruned at all. This is precisely the case in one of the orchards visited. Even though this gummosis-free, unpruned, grapefruit orchard is only nine years old, many other orchards of the same age show severe gummosis, and in all these orchards pruning is carried out.

Once infection invades the trunk, it is practically impossible to eliminate the disease by surgery. Control has to be by prevention. Pruning should therefore be reduced to a minimum and only small branches with a diameter of less than 25 mm should be cut. Pruning cuts should be disinfected and, when dry, covered with wound dressing or asphalt. In California, carbolineum with 2 percent phenols has been recommended as a disinfectant for pruning wounds and, for final coating on the disinfected wood, low melting-point asphalt mixed with an equal quantity of carbolineum.

Heat- and frost-injured areas are also points of entry for the Rio Grande gummosis agent. Obviously, only heat injuries must be considered in Somalia. Whitewashing of trunks and branches that might become exposed to sunlight (after heavy pruning) should be carried out with a zinc-copper-lime mix.

Phytophthora gummosis or footrot

The second most important disease in the Mogadishu region is footrot or phytophthora gummosis. Normally, Rio Grande gummosis does not kill affected trees, but footrot does. It is a truly destructive disease. The differences between the symptoms of footrot and Rio Grande gummosis are as follows. Phytophthora gummosis is essentially a disease of the hark; though a thin layer of wood under the affected bark may be stained brown: The entire bark is affected by the fungus, not only the outer layer as in Rio Grande gummosis or scaly bark psorosis. Contrary to Rio Grande gummosis, there are no pockets of gum in the wood. The disease often begins at the soil level when a susceptible citrus species (scion or rootstock) is directly in contact with the earth. Indeed, footrot is due to soil-borne fungi of the genus Phytophthora: P. citrophthora, P. nicotianae var. parasitica, P. syringae Kleb., or P. hibernalis Came. When phytophthora-resistant rootstocks are used, footrot lesions usually begin near the bud-union and expand rapidly in an upward direction (Figs 222 and 223). Trunk lesions rarely extend higher than 35-50 cm from the ground or the bud-union line.

In the following cases footrot can also occur below ground, and soil must be removed from around the tree to expose the lesions:

• The rootstock is phytophthora-susceptible. The order of susceptibility among rootstocks commonly used is starting with the most susceptible sweet lime, lemon, acid lime, sweet orange, rough lemon, Cleopatra mandarin, citranges, sour orange and Poncirus trifoliata. In Somalia only two rootstocks are used commercially: the phytophthora-susceptible rough lemon and the phytophthora-resistant sour orange. At the Government orchard at Balad, only the trees on rough lemon show footrot.
• The bud-union line is below soil-level (see Figs 228 and 230) and the rootstock trunk as well as part of the scion trunk are in the soil. Footrot lesions can develop not only on the rootstock but also on susceptible scions such as grapefruit and sweet orange.
• Soil is put around the trunk, and the bud-union line is covered (see Figs 224 to 226). Removal of the soil exposes phytophthora-footrot lesions (see Fig. 228).

In these cases, development of the pathogen requires the soil in contact with the phytophthora-susceptible trunk to be moist or wet. Soil moisture at or near saturation is most favourable for fungal growth, spore production and movement of zoospores. The fungus is very sensitive to moisture fluctuations; its activity ceases when soil dries out.

P. citrophthora grows fastest when the temperature is near 25°C. P. parasitica has a higher optimum, near 30°C. Such temperatures are characteristic of Somalia at various times of the year.

Finally, a pH of 6.0-7.5 favours the growth and multiplication of the fungus. Such pH levels are common in Somali soils.

Thus, Somalia has excellent conditions for the development of the soil-borne Phytophthora spp. Footrot can only be prevented by the use of resistant rootstocks and proper horticultural practices (Fig. 231). Unfortunately this is not often the case. Rough lemon is often the rootstock, trees are budded too low, with the bud-union close to or even buried in the soil, and soil is put around the trees.

Virus and virus-like diseases in the Mogadishu region

Cachexia-xyloporosis

On practically all mandarin trees encountered, cachexia-xyloporosis symptoms were seen, namely the presence of gum impregnations in the bark' pegs on the cambial side of the bark, and stem pitting, that is, pits in the wood matching the pegs on the bark. Gum impregnations in the bark of susceptible species are easily seen by scratching the trunk with a knife at the bud-union to remove the outer layer of bark, as shown in Figures 38, 40 and 42. When only the scion and not the rootstock is susceptible, gum impregnations will only be seen above the bud-union. This is precisely the case of mandarin (susceptible) on sour orange (tolerant) (see Figs 38,48 and 49). With sweet orange on sweet lime, Rangpur lime, tangelo or Citrus macrophylla, gum impregnation and/or stem pitting would only be seen below the bud-union as the sweet orange scion is tolerant but none of the four rootstocks are (see Fig. 57).

To diagnose cachexia-xyloporosis further, a piece of bark should be removed at the bud-union line as shown in Figures 38, 41 and 44. The piece of bark will show gum only in the susceptible part, the mandarin bark in the case of Figures 38 and 39, but not the sour orange bark. Similarly, only the cambial side of the mandarin bark will show pegs (see Figs 39, 41, 43 and 45). After removal of bark, the exposed wood shows stem pitting (see Figs 38, 41, 44 and 54).

Cachexia-xyloporosis is a bud-propagated disease. This fact is well illustrated in the Al Sabahyia orchards. On this farm there are three mandarin orchards. One is 30 years old, the second, 20 years old and the third, ten years old. Buds for the youngest orchard were taken from trees of the 20-year-old orchard which had, in turn, been taken from the oldest orchard.

In other words, the 30-year-old orchard is the source for both the 20- and the ten-year old orchards. The 30-year-old orchard happened to be contaminated by a mild strain of cachexia-xyloporosis (as well as concave gum). Both the 20- and the ten-year-old orchards also show the symptoms of cachexia-xyloporosis and concave gum.

Concave gum

The concave depressions (see Figs 102 to 105) induced on susceptible species (mandarins, tangelos, sweet oranges, grapefruits, etc.) by the concave gum agent (not yet identified) were seen on mandarin trees at CARS at Afgoi (Avana mandarin from Italy) and at Al Sabahyia orchards on trees which also bore cachexia-xyloporosis symptoms. This example shows that more than one pathogen can infect the same trees concomitantly.

Popcorn

Figure 101 illustrates a sweet orange trunk showing symptoms of popcorn at the Al Sabahyia orchard. Small scales or flakes of the outer bark loosen, break away and finally drop, leaving the small pustules uncovered. Although the pustules are reminiscent of the eruptive forms of scaly bark psorosis, they are smaller, rarely measuring more than 10 mm in diameter, and do not enlarge. Gum exudation may be frequently observed in this type of lesion. Symptoms of popcorn were observed by Childs (1953) on sweet orange trees in Florida and by Rossetti on sweet orange in Brazil and Portugal (Rossetti and Salibe, 1965).

Impietratura

Impietratura is an infectious disease affecting grapefruit, sweet orange, mandarins, lemon and Citrus volkameriana. Affected fruit is hard, like stone - hence the name impietratura -and there are gum pockets in the albedo (see Figs 1 16 and 1 17). Mild impietratura-like symptoms, i.e. gum in the albedo, have been seen on several occasions in Somalia on navel sweet orange. As boron deficiency also results in the presence of gum in the albedo, further work is required to assess the presence of the impietratura agent in the country.

The presence of impietratura in Somalia would not be surprising, however, since several citrus varieties, including navel orange, have been introduced from Sicily, where impietratura is widespread.

Cristacortis

Cristacortis affects many citrus species, and sour orange in particular. The disease is characterized by conspicuous pegs on the cambial side of the bark with corresponding pits in the wood (see Figs 109 to 114). In cristacortis there are no gum impregnations in the bark. Cristacortis-like symptoms have been seen on sour orange carrying a mandarin top. Graft-inoculation experiments are required to assess the presence of the cristacortis agent in Somalia. Like impietratura, cristacortis is well known in Sicily.

Stubborn

Stubborn disease is due to the helical mycoplasma Spiroplasma citri. Highly typical symptoms of stubborn, such as those seen in Figure 148, have been seen at CARS at Afgoi on navel sweet orange trees from California. Trees are stunted, with small, cupped leaves that also display mottle. Fruit peel is thick and coarse at the peduncular end but thin and smooth at the stylar end (see Fig. 165). Seeds are aborted and the fruit axis is curved, giving lopsided fruit (see Fig. 168).

Symptoms identical to those seen at CARS were also noticed on navel sweet orange trees in the Salah Ahmed Ali orchard at Jambalul. This is not surprising as budwood for these navel trees was obtained from CARS.

The navel sweet orange cultivar carrying stubborn disease is of Californian origin. Stubborn is widespread throughout California, and there are several cases where budwood imported from California was found to carry stubborn.

Miscellaneous

Grapefruit trees on sour orange rootstock with overlapping bud-unions have been noticed. This situation is sometimes encountered with tristeza-infected trees on sour orange rootstock. The tristeza-induced necrosis of the sour orange sieve tubes below the bud-union line prevents the rootstock from growing as fast as the scion and results in the scion overlapping the stock. Trees were analysed for the presence of CTV using ELISA on shoots taken to Bordeaux. The assays were CTV-negative.

Alga spots produced by the partly parasitic alga, Cephaleuros mycoida, have been noticed on small-fruited acid lime leaves.

Russeting due to the citrus rust mite, Phyllocoptruta oleivora, has been seen on fruit of grapefruit trees.

Exocortis

Symptoms of exocortis can only be seen when a susceptible rootstock, such as P. trifoliata, citrange, Rangpur lime or sweet lime is used (see Figs 83, 85 to 87). None of these rootstocks is used in the Mogadishu region, as sour orange is the major rootstock. Sour orange is tolerant of exocortis, as is grapefruit, the major scion species grown on sour orange in the Mogadishu area. Exocortis cannot, therefore, be observed on grapefruit trees grafted on sour orange rootstock even when such trees are infected with the exocortis viroid, as is probably the case in several orchards in the Mogadishu region. Only indexing, i.e. graft inoculation of bark from a candidate tree into a highly susceptible Etrog citron (Citrus medica) line, will show if the candidate tree is infected or not. Citron will react to graft inoculation of the exocortis agent by specific symptoms including leaf epinasty and curling, as shown in Figure 90 (see Chapter 1, Tables 2 and 3).

It is well known that many old-line citrus cultivars are infected with the exocortis agent. This is very probably the case with the old grapefruit lines that were imported into Somalia many years ago, from Italy or other countries, at a time when indexing for exocortis was not yet developed. Old sweet orange and mandarin lines are also known to be frequently infected with exocortis, even though, being tolerant, they do not show symptoms of the disease. It is, therefore, more than likely that many old lines of the major commercial citrus species carry exocortis in Somalia.

It is important to be aware of this situation. Indeed, many tristeza-tolerant rootstocks are susceptible to exocortis. Hence, it would be dangerous to graft old-line citrus cultivars on these rootstocks, as they will undoubtedly develop exocortis whenever the old-line cultivar carries the exocortis pathogen.

Important diseases probably not present in the Mogadishu region

Tristeza

For three reasons, tristeza is probably not present in the Mogadishu region.

• None of the many small-fruited acid lime trees examined showed any symptoms of CTV infection. Neither leaf vein clearing nor stem pitting could be observed. From the absence of these symptoms, it is probable that CTV is not spreading in the Mogadishu region.
• None of the many trees grafted on sour orange rootstock that were observed during the survey showed any signs of a tristeza-like decline.
• Most African strains of CTV, such as the Kenya strains, for instance, cause stem pitting on grapefruit. No stem pitting was observed on grapefruit.

It is essential that the virus be kept out of the area, for at least three reasons:

• Even though Toxoptera citricida, the black tropical citrus aphid vector of CTV, is apparently not present in the Mogadishu area, another CTV aphid vector is - Aphis gossypli, the cotton aphid. This aphid spreads CTV in Israel.
• Most trees are on sour orange and with this rootstock form a scion-rootstock combination that is susceptible to CTV.
• Most trees are grapefruit trees and this species is itself susceptible to certain CTV strains, regardless of rootstocks.

Greening

No evidence for citrus greening disease was seen in the Mogadishu region. The psyllid vector of the disease, Trioza erytreae, has never been reported, no signs of its presence were found and the climate of the Mogadishu region is considered too hot for T. erytreae.

MAP 15 The Hargeisa region, Somalia

Citrus canker

No evidence for citrus canker, caused by the bacterium Xanthomonas campestris pv. citri, was found in the Mogadishu region.

Diseases of citrus in the Mogadishu region: A summary

The two major diseases of citrus - which is mostly grapefruit on sour orange rootstock in the Mogadishu region are fungal diseases: Rio Grande gummosis and phytophthora gummosis or footrot. Cachexia-xyloporosis, a viroid disease, was seen on most mandarin trees examined. Exocortis, also a viroid disease, could not be observed, as all grapefruit, sweet orange and mandarin trees on sour orange rootstock are tolerant to the exocortis agent. It is, however, very likely that many old-line grapefruit, sweet orange, mandarin and lemon trees carry the exocortis viroid. Similarly, the grapefruit and sweet orange (but not mandarin) trees on sour orange are tolerant to the cachexia-xyloporosis viroid. Old-line grapefruit and sweet orange cultivars may very well carry the cachexia-xyloporosis agent without showing symptoms of the disease.

No evidence for the presence of tristeza, greening or citrus canker was found in the Mogadishu region. A. gossypii, a vector of tristeza virus, is present in the region. T. erytreae, a psyllid vector of greening, is not present.

Hargeisa region: General information and climatic data

The information given below has been taken from the main report by SOGREAH on the Northwest Region Agricultural Development Project.

The northwestern region has a population of 450 000, of whom 300 000 live in the Hargeisa area (altitude I 300 m). Irrigated land covers 760 ha, of which 490 ha are watered permanently by pumping water from wells. In addition, a further 3 000 ha are suitable for irrigation.

The major agricultural problems are: inadequacy of supply of agricultural equipment and inputs (often no fuel is available to operate pumps for irrigation), lack of qualified extension personnel, poorly organized marketing and insufficient agricultural credit.

With an average rainfall of 400 mm per year, the northwestern region is classified as a semi-arid zone. Rainfall varies from year to year. The northeast monsoon lasts from October to May, and the southwest monsoon from June to September. The major humid season (Gu) -with 60 percent of rainfall - extends from April to June; the second humid season (Der or Karan), from July to October. The main dry season is from October-November to February-March.

In the highlands, the absolute maximum temperatures never go above 33°C, but they are above or close to 30°C from March to October. The absolute minimum temperatures vary between 10°C and 19°C; they are above 15°C from March to September. The mean relative humidity is between 55 percent and 65 percent.

Monthly average meteorological data

Cultivated land is composed mainly of recent alluvial soils, and the soil pH varies from 7.7 to 8.7. The pH of water is between 7.4 and 7.9. Water often has a high chloride content (10 to 700 mg/l) and this explains the numerous cases of salt injury encountered in citrus.

MAP 16 Northwest Region Agricultural Development Project, Somalia

TABLE 47 Agricultural production in the Hargeisa region, Somalia

Production levels for 1986 and 1988 are given in Table 47.

Citrus covers a total of 270 ha of land irrigated by pumped water. Total production reaches 2 700 tonnes, with an average production of 10 tonnes a year per hectare.

Citrus was introduced into Hargeisa in 1930 from Cyprus and Australia. A citrus nursery was started at Arapsiyo in 1945 with new citrus introductions, but there is no record of their origin. The Arapsiyo nursery was transferred to Geed Deeble (see Map 16) in 1951, but had to be closed later because of lack of water. From 1951 to 1962 the two nurseries functioned together. The Arapsiyo nursery made trees available to all farmers, while the Geed Deeble nursery worked only for the government. New citrus introductions were made in 1959 from various countries including Ethiopia, the United States of America, East Africa (Kenya) and Zanzibar (now part of the United Republic of Tanzania). According to Bové, the trees found to be affected by greening and tristeza (see below) were very probably imported from Kenya, where the two diseases are known to be widely distributed; tristeza is present everywhere, in both the highlands and the coastal lowlands, while greening is present only in the highlands above 700 m. Greening is also present in Ethiopia.

Greening

Greening in the Hargeisa region

Greening was observed in the Arapsiyo area on 26-year-old navel (Figs 177 and 179) and Jaffa (Fig. 178) sweet orange trees, probably grafted on rough lemon rootstock. The disease in this orchard also affected mandarin trees. The trees affected by greening also carry tristeza.

The diagnosis of greening in this orchard is based on field symptoms and detection of the greening organism by electron microscopy in Bordeaux. Symptoms include yellow, sparse foliage with zinc-deficiency patterns (Fig. 179); off-season flowering (Fig. 179); fruit of various sizes with aborted seeds; and leaves with severe leaf mottling and vein corking (Fig. 180). Electron microscopy revealed the presence of the greening organism in the sieve tubes of the affected trees (Fig. 19).

Other orchards in the Arapsiyo area also had greening-affected trees. In the Osman Ashour Hassan orchard, many navel and non-navel sweet orange trees showed typical symptoms of greening, with severe leaf mottle and seed abortion. Electron microscopy confirmed the presence of greening (Fig. 20). One tree in the Aden Mohamad Olod orchard showed greening-like symptoms, with fruit of various sizes, aborted seeds and off-season flowers. However, the trees in this orchard also suffered from severe drought. The Ahmed Dahir orchard has many beautiful sweet orange trees, but one had very typical and severe symptoms of greening except for a single branch which was still symptomless. It is well known that greening-affected trees often show signs of the disease only on part of the canopy, especially when infection is still recent (see Fig. 217).

MAP 17 Citrus growing areas of Ethiopia

In the Awbarkadle area (see Map 16), east of Hargeisa, trees infected with tristeza virus, and probably also with the greening bacterium, were seen in the orchard of Mohamad Abdullah Aboussita. A mandarin tree and a sweet orange tree in this orchard were found to be infected by tristeza as shown by ELISA in Bordeaux. The trees came from Kenya. It is likely that some of the trees are also affected by greening.

The situation regarding psyllid vectors of greening in the region

Only the African psyllid vector (T. erytreae) of greening is present on the African continent, and in particular in Kenya and Ethiopia. T. erytreae was first reported as being present in Ethiopia by Schmutterer (1971) and, since then, has been reported from all areas where typical greening symptoms have been observed (see Map 17 - greening was found in all citrus areas except Gode, Lower Awash and Dilla/Yirga Alem). It has also been found on a rutaceous bush, Clausena anisata (Willd.) Beneth., known from South Africa to be a host of T. erytreae (Schwarz, 1976). Vepris undulata is another wild rutaceous host of T. erytreae.

The presence of T. erytreae in a citrus-growing area is revealed by the highly typical concave depressions that psyllid nymphs produce on the lower side of citrus leaves (see Figs 189 and 192). Each depression is, so to speak, a nest in which a nymph has developed. The depressions on the lower side of the leaf correspond to an equal number of "bumps" on the upper side of the leaf (see Figs 190 and 193). Diaphorina citri, the Asian psyllid vector of greening, does not produce such depressions and bumps.

No adults or nymphs of T. erytreae were seen during the survey in the Hargeisa region. Furthermore, no bumps or depressions could be found either on young leaves or on adult leaves of any age. This indicates that T. erytreae has probably been absent from the Hargeisa region for the last three or four years. Whether it is absent, or present only in very low numbers, is difficult to say on the basis of these observations. Further surveys must be made at various times of the year and especially when new growth flushes are produced, since the psyllid eggs are deposited on very young citrus leaves. However, knowing that T. erytreae is present in Ethiopia at Erer Gota and Urso in the vicinity of Dire Dawa, a town located only 100 km east of Hargeisa at a similar altitude (1200 m) to that of Hargeisa (I 300 m), it is fairly likely that T. erytreae is present, at certain times at least, in the Hargeisa region.

Contents - Previous - Next