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S. citri in ornamental periwinkle in Nizwa
S. citri, the helical mycoplasmal agent of citrus stubborn disease, is naturally transmitted by leafhoppers. In the Mediterranean area and the Near East, the leafhopper Neoaliturus haematoceps is the main vector. In the Near East, Neoaliturus tenellus (the vector in California and Arizona) may also play a role.
Leafhoppers are responsible for S. citri infection of citrus as well as of many non-citrus plants including periwinkle. Ornamental periwinkles with symptoms of S. citri infection were detected at the Nizwa Hotel and samples of them were collected. Electron microscopic examination revealed the spiroplasma cells in the sieve tubes of an affected periwinkle, and S. citri was also cultured from the Omani periwinkle samples. These results show unambiguously that S. citri-infected leafhoppers exist that are able to transmit the spiroplasma to periwinkle, and very probably to other plants as well.
In the Rumais Research Centre, Washington navel trees of Egyptian origin showed symptoms of stubborn. Stubborn and S. citri should not be overlooked if Oman decides to extend sweet orange orchards.
Gummy bark
Gummy bark disease of sweet orange (see Figs 60 to 79) was detected on Valencia late sweet orange trees, not only at the Rumais Research Centre (Fig. 60) but also at the Qairun Hairati farm in the Salalah region. It is probably the same line of Valencia late sweet orange that shows the disease whether it is grown in Rumais or Qairun Hairati. Gummy bark was also seen at the Tanuf farm on a sweet orange line of Egyptian origin. At Qairun Hairati a sweet orange line of unknown variety showed gummy bark in addition to what appears to be scaly bark psorosis (psorosis A). Most of the cases of gummy bark observed are severe and the trees are stunted.
The only way to control gummy bark is to use disease-free citrus material. The pathogen is not transmitted by vectors, therefore trees free of gummy bark remain so. The presence of the disease in Oman is a result of haphazard introductions of budwood or trees from countries, such as Egypt, which at present cannot supply citrus material certified free of virus and virus-like diseases.
Cachexia-xyloporosis on mandarin
Willowleaf mandarin trees, severely affected by cachexia-xyloporosis, were detected at the Tanuf farm. Willowleaf mandarin trees showing symptoms of the disease at Rumais Research Centre were probably of the same line. A severe case of the disease was seen on mandarin trees in the farm of Abdulhafiz Bin Salem Bin Rajeb near Salalah.
Sweet lime is also susceptible to cachexia-xyloporosis. A mild form of the disease was seen on sweet lime at the Tanuf farm on a tree also affected by exocortis. The disease is controlled by using disease-free budwood. There is no transmission of the disease by vectors. Mechanical transmission is possible. Once again, citrus material for propagation should only be imported from reliable sources.
Impietratura on grapefruit
Impietratura fruit symptoms were seen on grapefruit at the Qairun Hairati farm. In addition, when this grapefruit line was taken to France and propagated, the young grapefruit leaves that developed showed very severe psorosis young leaf symptoms. These symptoms (see Figs 91 to 96) are not specific to impietratura, but are common to the following diseases: scaly bark psorosis (psorosis A), concave gum-blind pocket, cristacortis, crinkly leaf-infectious variegation and impietratura.
The psorosis young leaf symptoms require relatively low temperatures (about 18-22°C) for expression. In cool climates, the symptoms can easily be seen each year on the newly produced young leaves in the orchard. In Oman, temperatures are too high for young leaf symptoms to occur in the orchard though they can be obtained in temperature controlled greenhouses. Once again, control lies in using only disease-free planting material.
Scaly bark psorosis (psorosis A) on sweet orange
At the Qairun Hairati farm, a sweet orange tree showed symptoms similar to scaly bark psorosis (psorosis A) in addition to gummy bark. The only control is avoidance, by using disease-free planting material.
Concave gum on sweet orange
At the Rumais Research Centre, Washington navel sweet orange trees of Egyptian origin showed concave depressions typical of concave gum disease. Some of the trees were also thought to have stubborn disease. Once again the only control is avoidance, by using only disease-free planting material.
Exocortis on sweet lime
The exocortis viroid induces bark scaling on susceptible species such as Poncirus trifoliata (see Fig. 83), the citranges (hybrids of P. trifoliata x sweet orange), Rangpur lime, etc. mainly used as rootstocks. On sweet lime, the viroid induces vertical splits resembling growth cracks, as seen on a sweet lime tree at the Tanuf farm.
The exocortis viroid is mechanically transmissible by pruning knives, scissors, etc. Control of the disease is again by use of disease-free planting material but, in addition, pruning tools and budding knives must be disinfected with bleach solution (household bleach diluted to give a 5 percent solution of sodium hypochlorite), to prevent mechanical transmission from an infected tree to a healthy one.
Tristeza
Tristeza is the most serious virus disease affecting lime trees. The author was therefore horrified to see clear-cut symptoms (vein clearing: see Fig. 128) of the disease at a nursery in Sohar on many young lime trees just imported from an unknown source in India. Tristeza is widespread in India and it is not surprising that lime trees from there carry the disease.
In addition to lime trees, grapefruit and mandarin trees were also imported from India with the same shipment. The grapefruit trees were also infected with tristeza virus and gave a highly positive ELISA reaction when analysed in Bordeaux. Young grapefruit trees do not show tristeza symptoms and so detection of the virus relies on the immunological ELISA reaction.
The consequences of releasing these tristeza-infected lime and grapefruit trees to the farmers would have been incalculable. The author requested that these trees be destroyed and burnt at once.
In addition to tristeza, the lime trees carried citrus canker - the most serious bacterial disease that can affect lime trees.
The author was told that the trees imported from India went through quarantine at the airport and so does not understand why lime trees with such obvious symptoms of tristeza and citrus canker were not stopped at the time of quarantine inspection. the affected trees from India were spotted by chance. It is impossible to say how many other nursery or farm owners have imported trees from this source.
In conclusion, without the destruction of the trees from India, the Sohar area and the whole Batinah would soon have become famous not only for witches' broom disease but also for tristeza and canker.
Virus and virus-like diseases: conclusions and recommendations
Of all virus and virus-like diseases, the two most widely seen in Oman are gummy bark on sweet orange and cachexia-xyloporosis on mandarin.
Identification of virus and virus-like diseases by visual symptoms is, however, insufficient as a means of assessing the true health status of citrus. For instance, it is impossible to determine from visual symptoms if sweet orange is infected by cachexia-xyloporosis and exocortis, or mandarin by gummy bark and exocortis, because sweet orange is tolerant to cachexia-xyloporosis and exocortis, and mandarin to gummy bark and exocortis. Therefore, initial diagnosis by visual symptoms must be completed by indexing on sensitive indicator plants or with laboratory techniques. However, at this stage of citrus development in Oman, this seems unnecessary because the citrus lines introduced already contain so many diseases that it would be meaningless to show that they have a few more.
The following straightforward recommendations can be made:
Defoliation and dieback of lemon and sweet lime trees at Nizwa
Mal secco is a dangerous disease caused by the fungus Deuterophoma tracheiphila (see Figs 232 to 235). It primarily affects lemon trees in many Mediterranean countries including Greece, Italy, the Syrian Arab Republic and Turkey. It is not yet known to be present in Iran, Iraq or Saudi Arabia.
Suspicious symptoms (defoliation, dieback) were seen on a few lemon and lime trees in the Nizwa region. Affected sweet lime shoots were analysed in Bordeaux. The wood of the shoots showed a brownish red, more or less localized staining. Small pieces of stained wood were surface-disinfected with calcium hypochlorite solution and placed in 15 percent agar malt medium. In 80 percent of cases, mycelial colonies were obtained which emitted a brownish pigment into the medium. The colonies carried asexual reproductive organs typical of the genus Phoma. The fungus was not further characterized.
At the Tanuf farm. sour orange seedling trees are growing next to affected lemon and lime trees. They showed no signs of the disease. As sour orange is very susceptible to mal secco, they would probably have been affected if D. tracheiphila was involved. For this reason, it is unlikely that mal secco occurs in the Nizwa area, but a careful watch should be kept.
Horticultural practices and phytophthora Gummosis
Several cases of phytophthora gummosis were observed, mainly due to improper irrigation and planting conditions. Trees are often planted too deep, with bud-union lines too close to the soil. They end up in cuvettes and are flooded instead of being irrigated, with the consequent danger of infection by the soil-borne pathogens of phytophthora gummosis.
In the Batinah and elsewhere, the lime "trees" are conducted on several branch-like trunks instead of having one, single, good trunk.
Miscellaneous disorders, injuries and inadvisable practices
At the Rumais Research Centre (Block D), old Washington navel sweet orange trees showed abundant gumming, the cause of which was not determined. Termites were also a problem. Some trees were being top-grafted, which is inadvisable when the viral status of the grafted partners is unknown.
Symptoms resembling autumn blast were seen on a few sweet orange trees. Autumn blast refers to twigs that suddenly die and dry up in situ (see Figs 252 to 254). Gummy material is conspicuous on parts of the stem, but the dried leaves do not fall off. There is a clear-cut margin between the upper, brownish, dead bark of the stem of the dried-up twig and the lower bark that is green and still alive. This twig dieback appears on the uppermost part of the canopy. In severe cases, lower branches also show dead twigs. A similar disorder was observed in the Libyan Arab Jamahiriya in 1974, in Iraq in 1978 (see Fig. 253), in Yemen in 1983 (see Fig. 254) and in the Islamic Republic of Iran in November 1985 (see Fig. 252). In Iraq, the symptoms start around mid-October. Mandarin and Clementine trees are most susceptible, followed by sweet orange and grapefruit trees. Lemon trees are much less affected and sour orange trees never.
The cause of autumn blast is not known, although it is probably not due to parasitic micro-organisms or pests. It seems to reflect the failure of the terminal twigs to secure sufficient water. Factors that could be responsible for insufficient water supply in autumn in Iraq are the significant temperature difference between day and night, and relatively low soil temperatures contrasting with high air temperatures during the day. These temperature differences might induce twig dieback, especially if the root system is inadequate because of poor cultivation, root injury, attack by fungi such as Phytophthora spp., unbalanced water supply or overwatering on one side of the tree and poor supply of water on the other, due to the one-furrow irrigation system.
Obviously, twig dieback is more of a problem under extreme climatic conditions than in temperate climates. It is therefore important to maintain adequate moisture in the soil by proper irrigation.
Last, but not least, citrus canker was seen in Oman in two locations in 1986: the orchard of Said Ben Rachid Kilbani (including nearby courtyard trees) at Homania near Ibri (Figs 248, 249 and 251) and the orchard of Said
Bin Salem Al Wahebi in Salalah (Fig. 250). In both cases, typical canker symptoms were observed on both lime and grapefruit. In 1987 the disease was seen in the Sohar region on young acid lime seedling trees just imported from India. Immediate destruction of these seedlings was strongly recommended. Whether these recommendations were followed or not, citrus canker appeared in the Sohar region in 1990. It was observed by the author in November 1992 in Sohar.
Isolation and characterization of the citrus canker pathogen
During the 1986 survey, five citrus canker samples were taken for culture and identification of the causal agent, the bacterium Xanthomonas campestris pv. citri. The samples were lime leaves and twigs from Homania region lime leaves and twigs from Salalah region, and grapefruit leaves from Salalah region.
After surface sterilization with 95 percent ethanol, canker lesions from a given sample were collected and placed in a sterile Petri dish. They were macerated in 5 ml sterile distilled water with a lanceolate needle. After 15 minutes of incubation at room temperature, the homogenate was streaked on solid LPGA medium in a Petri dish (20 µl of homogenate per dish) and incubated at 28°C for 72 hours. LPGA medium contains, per litre, 7 g bactopeptone (DIFCO), 7 g yeast extract (DIFCO), 15 g agar (DIFCO), 50 ppm cycloheximide, and is at pH 7.2.
On solid LPGA medium, Xanthomonas sp. yields characteristic yellow colonies. All five samples from Oman gave such colonies. Colonies from each sample were further sub-cultured on solid medium in a tube. The subcultured isolates can be kept in tubes on LPGA medium and passaged every four days at 28°C, or every seven days at 16°C. They can be resuspended in sterile water and kept at -30~C.
Representative isolates from each Omani sample were further characterized as indicated in Table 40. All isolates were identified as Xanthomonas campestris.
X. campestris has several pathovars, according to the affected host. The Omani isolates come from citrus (acid lime and grapefruit), and hence they are almost certainly X. campestris pv. citri.
There are three major pathotypes of X. campestris pv. citri: A, B and C. Pathotype A is the most common and is endemic throughout Pakistan, India, Southeast Asia and Indonesia. It affects species of the genus Citrus, including small-fruited acid lime, grapefruit, sweet orange and Poncirus trifoliata, as well as wild rutaceous species. Pathotype B occurs mainly in Argentina, is less severe than A and does not affect sweet orange or grapefruit. Pathotype C has been reported from Brazil as a very severe type on acid lime, which also affects sour orange but no other citrus. On the basis of host range, the pathotype present in Oman is A, since the agent was not only found on acid lime but also on grapefruit. The pathotypes can be identified more precisely by their sensitivity or resistance to infection by various bacteriophages. These analyses have shown that the pathotype involved is definitely not B. The lack of one phage made it impossible to distinguish between A and C but, on the basis of all other data, A is very probably the pathotype present in Oman.
In conclusion, X. campestris pv. citri, very probably pathotype A, has been isolated and cultured from acid lime and grapefruit samples collected in Homania and Salalah. This demonstrates the presence of the causal agent of citrus canker in Oman.
Distribution
Citrus canker is a very serious disease of oriental origin. It is present in India, Japan, Southeast Asia, the Philippines, Indonesia, etc. Because of its severity, the disease has been eradicated in Florida, South Africa, Australia and New Zealand. In Florida, between 1914 and 1931, over 250 000 adult citrus trees and three million nursery trees were eradicated and destroyed.
The disease appeared in 1982 in the coastal plain (Tihama) of northern Yemen, very probably as a result of citrus introductions from India. An eradication programme is now under way. In 1983, Aubert and Bové discovered canker-affected trees in Saudi Arabia, north of Jizan. Eradication was immediately recommended.
Reports have recently appeared concerning the presence of canker in the Persian Gulf area. In the United Arab Emirates, the disease is present. It has not been eradicated but is apparently controlled by copper sprays. The author did not see canker in Iran in December 1985.
In the Ibri region of Oman, the disease seemed to be limited to one orchard and a few nearby trees, amounting altogether to less than 60 trees. These trees have been removed and burnt. The young acid lime seedlings with canker seen in 1987 in the Sohar nursery have also been burnt, according to the nursery manager. Nevertheless, as stated above, citrus canker is now (November 1992) present in the Sohar area and will probably spread to the entire region.
TABLE 40 Characterization of Xanthomonas campestris in Oman
| Characters tested | Samples from Oman | Xanthomonas campestris control |
| Gram stain | - | - |
| Polar flagellum | + | + |
| Fluorescence | - | - |
| Nitrate reduction to nitrites | - | - |
| Cytochrome C oxydase | - | - |
| Hypersensibility on tobacco | + | + |
| Esculin hydrolysis | + | + |
| Gelatin hydrolysis | + | + |
| H2S production | + | + |
| Urease | - | - |
| Starch hydrolysis | + | + |
| Carbohydrate utilization: | ||
| Glucose | + | + |
| Galactose | + | + |
| Cellobiose | + | + |
| Sorbitol | - | - |
| Inositol | - | - |
In the Salalah region the trees in the affected orchard have unfortunately not been destroyed. As citrus canker was reported in Salalah as early as 1984, the disease may now have spread. For this reason, it is now imperative that a careful survey for affected trees and orchards be carried out. All affected trees and orchards must be burnt and destroyed in situ. The following recommendations, given as early as 1987 to the authorities of Oman and based on experience in Florida in the eradication and containment of citrus canker, should be followed.
Recommendations
It is clear that citrus canker has been introduced into Oman from abroad, in spite of quarantine procedures. According to El Sheshtawi, about 4 000 citrus trees from India were destroyed in 1985 and 1986 at the Seeb International Airport because of canker. However, quarantine at the airport is not fully effective since, as recounted above, in 1987 the author detected tristeza and citrus canker on hundreds of acid lime seedling trees, as well as tristeza on grapefruit trees just imported from India.
Importation of any citrus material from India should be totally and strictly prohibited, as three major diseases of citrus are widespread in the country: citrus canker (a bacterial disease), citrus greening (an intracellular bacterial disease) and citrus tristeza (a virus disease). All three diseases can be introduced through budwood or trees and canker can also be introduced through fruit. Pakistan, where greening was reported in 1976 and confirmed in 1987, should be subject to the same restrictions as India. The same applies to Saudi Arabia, northern Yemen and the United Arab Emirates. The only good decision that Oman can make is to prohibit all importation of citrus budwood or trees by unauthorized persons. Only countries where no canker is present should be considered as sources of fruit importation for consumption.
The survey undertaken in Oman and the analyses carried out in France have revealed the following salient points regarding the country's citrus industry.
Ahlawat, Y.S. & Chenulu, V.V. 1985. Rubbery wood - a hitherto unrecorded disease of citrus. Curr. Sci., 54: 580-581.
Bové, J.M. 1986. Witches' broom disease of lime. FAO Plant Prot. Pull., 34: 217-218.
Bové, J.M., Whitcomb, R.F. & McCoy, R.E. 1983. Culture techniques for spiroplasmas from plants. In S. Razin & J.G. Tully, eds. Methods in mycoplasmology, vol. 11, p. 225-234. New York, Academic Press.
Bové, J.M., Garnier, M., Mjeni, A.M. & Khayrallah, A. 1988. Witches' broom disease of small-fruited acid lime trees in Oman: first MLO disease of citrus. In Proc. 10th Conf: IOCV, p. 307-309. Riverside, Univ. Calif.
Garnier, M., Zreik, L. & Bové, J.M. 1991. Witches' broom disease of lime trees in the Sultanate of Oman: first MLO disease of citrus. Plant Disease, 75(6): 546-551.
Russel, W.C., Newman, C. & Williamson, D.H. 1975. A simple technique for demonstration of DNA in cells infected with mycoplasmas and viruses. Nature, 253: 461.
Whitcomb, R.F. 1983. Culture media for spiroplasmas. In S. Razin & J. G. Tully, eds. Methods in mycoplasmology, vol. 11, p. 147-159. New York, Academic Press.
Citrus
in Pakistan
Background information on citrus diseases in
Pakistan
Citrus
diseases in India
Decline of citrus trees in Pakistan
Tristeza
Exocortis
Gummy bark of sweet orange and cachexia of
Mandarin
Bud-union crease of sweet orange trees on rough
lemon rootstock
Leaf yellow vein clearing - An unidentified
disorder of lemon trees
Sour orange bark pegging and gumming
Witches' brooms of calamondin trees
Citrus
canker
Horticultural
practices
Conclusions
Bibliography
The major citrus-growing province of Pakistan is the Punjab, with the following cities, from north to south: Islamabad-Rawalpindi, Sargodha, Faisalabad, Lahore, Sahiwal, Multan and Bahawalpur (see Map 8). Some citrus orchards are also found in Peshawar in the North-West Frontier Province (NWFP), and in Hyderabad in the province of Sind. Little citrus is grown in Baluchistan Province. From 1975 to 1985, the area covered by citrus increased from 62 800 ha (Punjab: 58 400 ha) to 144 200 ha (Punjab: 136 400 ha); production rose from 671 100 tonnes (Punjab: 625 700 tonnes) to 1 373 000 tonnes (Punjab: 1 307 800 tonnes). The overall yield is, however, low and represents only 9.5 tonnes per hectare.
The major citrus varieties grown in the Punjab are mandarins, with the two varieties Kinnow and Feutrell representing 80 percent of all citrus. They were imported to Pakistan in 1940 - Kinnow from California and Feutrell from Australia. Kinnow mandarin is a hybrid resulting from a cross between King mandarin and Willowleaf (Mediterranean common) mandarin made in 1915 by Frost in California and released in 1935. Feutrell (the correct spelling is Fewtrell) is an old variety from New South Wales in Australia. It may be a natural tangor (mandarin x sweet orange), with Willowleaf as the mandarin parent. It is an early variety (Fewtrell's early), while Kinnow is mid-season in maturity. Both Feutrell and Kinnow mandarins have a fairly strong tendency to alternate and they have numerous seeds.
Besides these two mandarins, there are only two other major commercial citrus varieties grown in Pakistan: Bloodred (Blood Red Malta) and Musambi (Mosambi, Mozambique) sweet oranges. Bloodred is probably of Mediterranean origin. It is a seedy, light blood orange, widely grown in the Punjab. The development of blood coloration is best in the cool, submontane districts of the NWFP. Musambi may be of East African origin (Mozambique). Early in maturity, very low in acidity and moderately seedy, it is a highly popular variety in India and Pakistan.
The rootstock on which citrus is grown in the Punjab is rough lemon (Citrus jambhiri Lush.) also called Jamberi or Jatti Khatti. In India, another lemon-like rootstock, Citrus karna Raf., is extensively used, second only to rough lemon (locally known as Kharna Khatta, Karna nimbu or Khatta nimbu). In the Peshawar district, sour orange (Citrus aurantium L.) is the only rootstock and is know locally as Khatta Taru Jabba, Seville Kimb and Sylhet lime.
MAP 8 The major citrus-growing region of Pakistan
Pakistan is subject to the summer monsoon system, and the main rainy season occurs in July, August and September. Annual rainfall in the Punjab ranges from 350 to 500 mm. During the dry season from October to January there are only occasional rains and showers. A second rainy season covers February and March, to be followed by the dry and hot "summer" season of April, May and June, when irrigation of citrus (by submersion or flooding) is required.
Background information on citrus diseases in Pakistan
Prior to the author's survey, information on the situation regarding diseases of citrus in Pakistan was available from several sources. Chapot surveyed Pakistan in March 1970. He found no evidence of the presence of citrus tristeza virus (CTV), but insisted on the wide occurrence of citrus stubborn disease. At the time of his survey he did not know of the work that was going on in Bordeaux (France) and Riverside (United States of America) on stubborn and also in Bordeaux on greening. This work (Igwegbe and Calavan, 1970; Laflèche and Bové, 1970a, b) showed that stubborn and greening were two different diseases. Chapot was of the opinion that the two diseases were "very close, if not identical". He noticed the presence in Pakistan of Diaphorina citri, the Asian psyllid vector of greening, but he was so "stubborn-oriented" that he suggested looking for a relationship between the presence of this psyllid and the frequency of stubborn in Pakistan, and failed to mention the possibility that greening disease might be present in the country. It is well known today that D. citri is not a vector of Spiroplasma citri, the causal mycoplasmal agent of stubborn disease.
The first indication of greening in Pakistan came from Cochran (1976) who observed symptoms of the disease in the citrus collection of the Agricultural Experiment Station in Peshawar and was worried that the disease could become established in the Islamic Republic of Iran, and move from there, via Iraq and Jordan, to the Mediterranean.
The likelihood that greening was present in Pakistan could also be deduced from the situation known to exist in neighbouring India, where greening was observed for the first time in 1960 (Capoor, Rao and Viswanath, 1974). The excellent work carried out by Capoor in Poona, east of Bombay, demonstrated in 1967 that the psyllid, D. citri, was the vector of the greening pathogen (Capoor, Rao and Viswanath, 1967). More recently, Ahlawat and Raychaudhuri (1988) have reported greening to be present in all citrus-growing regions of India. In particular, greening occurs in provinces that border Pakistan, namely, Jammu and Kashmir, Punjab and Rajasthan. These provinces are also infected with CTV (Ahlawat and Raychaudhuri, 1988). It is therefore to be expected that not only greening, but also tristeza, occur in Pakistan. In view of the importance of virus and virus-like diseases of citrus in India, information on these diseases is presented in Tables 41 and 42. Table 41 lists the diseases and supplies the main bibliographical references. Table 42 gives the geographical distribution of the diseases for the various provinces of India. In view of the similarities between the decline problems in Pakistan and India, it seems appropriate to review greening and citrus decline in India before exploring the situation in Pakistan.
TABLE 41 Virus and virus like diseases of citrus in India
TABLE 41 Virus and virus like diseases of citrus in India - continued
TABLE 42 Geographical distribution of virus and virus-like diseases of citrus in India
TABLE 42 Geographical distribution of virus and virus-like diseases of citrus in India - continued
History of citrus decline in India
Citrus decline or dieback disease is reported to have been present in India as early as the eighteenth century (Capoor, 1963). It has taken on alarming proportions since the 1940s. By the 1960s it was recorded in all citrus-growing areas of the country.
As pointed out by Raychaudhuri, Nariani and Ahlawat (1977): "The once flourishing citrus industry in India is, unfortunately, being slowly wiped out by dieback disease. In recent years, dieback has assumed alarming proportions in several States".
In the early 1960s, it was generally accepted that the problem involved CTV, zinc deficiency and some fungal parasites of twigs such as Colletotrichum gloeosporoides, Curvalaria tuberculata, Diplodia natalensis and Fusarium spp. (Capoor, 1963; Reddy, 1965). However, as stated by Fraser and Singh (1968), "none of the above conditions can adequately explain the disease or its spread. Tristeza is widespread in Indian citrus, but is not universal, and is not invariably present in dieback-affected trees. Moreover, many of the species affected in India are tolerant of tristeza in other countries. Deficiencies of zinc and other minor elements have been implicated on a symptom basis, but application of zinc with or without other minor elements failed to cure the condition, although sometimes slight or temporary improvement resulted. The fungi associated with dying-back branches are well known as invaders of moribund or weakened tissues".
In 1966 a study was made of citrus dieback disease in all major citrus areas of India. As a result of the observations made, it was concluded that dieback was caused by "the virus" responsible for greening disease because dieback in India greatly resembled greening in South Africa (Fraser and Singh, 1966, 1968; Fraser et al., 1966).
At the time the above survey was carried out the so-called greening "virus" had not yet been identified. This was achieved in 1970-71 when Bové and co-workers discovered that the agent of South African greening was not a virus but a micro-organism restricted to the sieve tubes of affected plants (Laflèche and Bové, 1970a, b; Saglio et al., 1971a, b). Furthermore, these workers were able to show that the micro-organism seen in the sieve tubes of South African greening was also present in the phloem of a Musambi sweet orange seedling infected with a Poona (Maharashtra) strain of Indian citrus decline or dieback (Saglio et al., 1971a, b; Bové and Saglio, 1974). This result confirmed the conclusion of Fraser et al. (1966) that Indian citrus dieback was indeed caused by an agent similar to that of greening disease. It has since been shown by the Bordeaux group that the agent of greening is a bacterium with a cell wall of the Gram-negative type (Garnier, Danel and Bové, 1984a, b). This bacterium has not yet been cultured in cell-free medium; it is therefore referred to as a "bacterium-like organism" or BLO.
The work showing that the greening agent was a BLO and not a mycoplasma-like organism (MLO) was carried out with plants infected with different strains of the greening agent (including an Indian strain, the Poona strain)as early as 1971 (Saglio et al., 1971a,b; Bové and Saglio, 1974). Similar results were later obtained by Naidu and Govindu (1981) when they confirmed that a BLO was associated with citrus greening in India.
Psyllid vector of greening in India
In southern Africa, the greening BLO is naturally transmitted by the African citrus psyllid, Trioza erytreae Del Guercio (McClean and Oberholzer, 1965). In India, the greening BLO also has a psyllid vector, the Asian citrus psyllid, D. citri Kuwayama, as reported for the first time by Capoor and co-workers in 1967 (Capoor, Rao and Viswanath, 1967, 1974).
In Asia, including China and Southeast Asia, the greening BLO is also transmitted by D. citri (Celino, Salibe and Cortez, 1966; Martinez and Wallace, 1967; Anonymous, 1977a, b; Ke, Chen and Lin, 1980; Xu, Li and Liao, 1985).
However, it has been shown that T. erytreae, the African psyllid, is able to transmit the Asian form of the greening BLO and that, vice versa, D. citri, the Asian psyllid can also transmit the African form of the greening BLO (Massonie, Garnier and Bové, 1976; Lallemand, Fos and Bové, 1986).
Greening or citrus dieback in India
A brief summary of the major facts related to citrus dieback in India is given here, based on the observations of Fraser and Singh (1968), Capoor, Rao and Viswanath (1967), Bové and Saglio (1974) and Ahlawat and Raychaudhuri (1988).
Citrus dieback or decline in India is caused by the Asian form of the greening BLO, and the vector is the Asian citrus psyllid, D. citri. Until 1966, the true cause of citrus dieback, i.e. greening, was overlooked, and the disease was erroneously thought to involve CTV, zinc deficiency and some fungal parasites of twigs.
Citrus dieback, i.e. greening, and D. citri are present in all citrus-growing districts in India, including provinces that border on or are close to Pakistan: Jammu, Himachal Pradesh, Punjab, Haryana, Rajasthan and Gujarat (see Table 42).
All sweet orange varieties grown are highly susceptible to the disease, and no rootstock exerts a worthwhile ameliorative effect where sweet orange is concerned. Mandarins vary in reaction according to strain, locality and rootstock, while grapefruit appears almost as susceptible as sweet orange. Tangelo varieties are also extremely susceptible.
Citrus hystrix, Meyer lemon, citron, sour orange and Poncirus trifoliata are moderately susceptible species. Lemon, Rangpur lime, sweet lime and Citrus karna are species with some tolerance.
Regarding rough lemon, there are many strains of this species in India, most of which appear fairly tolerant but, in Assam, the Sohmyndong strain is quite susceptible. The strains in use as rootstocks at Coorg (Karnataka) and Nagpur (Maharashtra) show a useful degree of tolerance in that they prolong the productive life of the Coorg and Nagpur mandarins compared with seedling trees of these mandarins.
Young commercial blocks of sweet orange varieties propagated from infected sources usually grow well for four to five years. Major development of symptoms occurs after the first crop of fruit, appearing first in only a few trees and, in the early stages, often in only one part of the tree. However, by the time the block is seven to eight years old, most or all of the trees show severe dieback.
A good percentage of transmission is obtained by bark or leaf-patch grafting. No transmission occurs by mechanical means or through seeds.
