Citrus Virus and Viroid Diseases

Abstract

Citrus are cultivated in a vast area worldwide and many countries grow it. Citrus fruits are delicious and everybody can eat it easily so many farmers like to grow them because of the good market. This plant has many diseases that induce various kinds of agents like fungi, bacteria, nematodes, and viruses. In this chapter, we discussed some citrus viral diseases that are very important and dangerous for fields. First, the Citrus Tristeza virus is explained that exists around the world. After that, you will know about other viruses like the Citrus psorosis virus. Viroids are another agent that causes diseases and reduces the amount of production. You learn some of them in this chapter like Hop stunt viroid, Citrus exocortis viroid, etc. The significant point of knowing citrus viral diseases is in the management of diseases. The control of viral diseases is difficult because there are no poisons or combinations to remove viruses from infected plants. If farmers or experts know about symptoms that cause viruses or viroids, they can report it to the related office and do some work to control it and it is important to the agriculture industry.

1. Introduction

Citrus is a genus of the Rutaceae family that is flowering, small to moderate size of trees. The genotype of citrus is available on the Ensemble website (http://www.ensembl.org) [1]. Citrus usually grows in the tropical and subtropical regions and they produce hesperidium fruit with a leathery rind. Important crops harvested from these plants are Oranges, Lemons, Grapefruits, Pomelo, and Limes. Because of their taste and benefits for humans, most countries cultivate them. The amount of citrus production around the world is 96,812,090 tons in 2020 and the amount of area harvested is 521,000 hectares in 2020 [2]. Like other biotic organisms, plants have various kinds of diseases and pests. So citrus as the same like others can be infected by viruses, nematodes, bacteria, and fungi that can impact the amount of production. One of the agents that induce loss of production is viruses. Viruses are the smallest organism in the world and they have 16–300 nm in diameter. It usually includes nucleic acids and proteins that make a particle of a virus called a virion. For a long time, they are adopted specific organisms and cells and almost all of them require alive cells or organisms for replication. Viruses are intracellular parasites because they used protein synthesis machinery (ribosomes) in their hosts. In the other words, infected cells produce viruses’ proteins and other components that viruses need to infect and live. Furthermore, they achieve their energy from energy-generating metabolic pathways’ host [3].

Plant viruses are the most indispensable agent that causes huge economic losses. They are widespread and almost all plant species can be infected by viruses. Characteristics of plant viruses are the same as animal viruses but their host is different. Plant viruses cannot infect animals and animal viruses cannot cause diseases in plants. Due to these features, recently, biotechnologists use plant viruses for curing some diseases like cancer in humans. Until now, plant pathologists can detect various kinds of plant viruses in plants. Because of the importance of citrus among other crops, various kinds of viruses are introduced that we mention in this chapter. In this chapter, you will read about the most important citrus viral disease (Tristeza) and after that, you know about other virus diseases that are reported. Viroids are another casual disease in plants [4]. Viroids are a subgroup of viruses. Some characteristics of viroids are similar to viruses although they have some differences like coat protein that viroids do not have and their genome is naked. Usually, their genome is circular in host cells [5, 6, 7]and the replication ways are different. They follow a rolling circle mechanism for propagation [8, 9]. The genome of viroids does not translate, so they cannot produce any proteins. The infection of viroids is occurred by interaction with the host factors in order to replicate and exert their pathogenic effects [9]. this casual disorder can cause diseases in citrus and we review it in this chapter. We will read about Exocortis and cachexia both being induced by viroids and they are a significant effect on the production amount. We try to discuss other viroids that recently they are reported by researchers. Briefly, in this chapter, you know about different viruses and viroids that are important in the citrus industry.

2. Citrus viral diseases

2 .1 Citrus tristeza virus

Citrus tristeza Virus (CTV) is the most indispensable virus among plant viruses because it causes the most damaging and economically important citrus disease [10]. It is called tristeza which means “sadness, melancholy” in Portuguese and Spanish [11]. The casual of this disease is a virus that has one of the largest plant RNA viruses. The virus has single-stranded, positive-sense RNA (positive polarity RNA genomes) that is placed inside a flexuous filamentous body that forms a vision of the virus. It is liked a snake. The genome of the virus is not any segment and it is monopartite. The size of the genome is approximately 19.3 k bases (kb) and it can encode 12 open reading frames (ORFs) that have various kinds of features [11, 12]. Totally, CTV has 19 proteins that each of which has a special responsibility [12]. This virus belongs to the family Closteroviridae and the genus closterovirus [11, 12]. This virus infects the member of the citrus genus like sour orange, grapefruit, lime, etc. that belongs to the Rutaceae family [11, 13, 14]. This virus infects phloem-associated cells that are responsible to transport food in the plant. So trees that are infected show a decline and yellowish leaves and as a result, infected trees die (Figure 1). Furthermore, it causes stem pitting in the grafting region and a reduction in fruit size and production [14]. This virus is transmitted by infected propagating material and aphids [2, 5, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27]. Aphids are a significant role in transmission to remote distances. Most species of aphids are Aphis gossypii (the melon and cotton aphid), Aphis spiraecola (green citrus aphid), Toxoptera aurantii (the black citrus aphid), and Toxoptera citricida (the oriental citrus aphid) [2, 28]. The last aphid is the most major aphid to transmit because it can carry the severe strain of Citrus tristeza virus and others cannot do it. When aphids feed extract sap, virus particles penetrate to vectors’ bodies and then viruses replicate in there. If aphids feed the healthy host, they can infect that tree, and virus particles enter the healthy host [28]. CTV has two strains: mild isolates cause only mild or no symptoms in sensitive citrus indicator hosts and usually result in no economic loss. Severe isolates can cause decline, stem pitting, or both and may vary in intensity [14, 19].

Figure 1.

The symptom of Citrus tristeza virus in the infected orchard placed in the Mazandaran province, Iran. Source: Author.

2.2 Citrus psorosis virus (CPsV)

Psorosis is another plant viral disease that is very indispensable in the infected areas. This disease is described by Swingle and Webber (1896) first and after that other researchers reported it in various kinds of citrus cultivation regions. In fact, this disease is an ancient disease among other citrus viral diseases [29]. The casual of this disease is a virus called Citrus psorosis virus (CPsV) [30]. This virus belongs to the genus Ophiovirus, the family Ophioviridae [21, 22, 25, 26, 31]. The virion of CPsV is kinked filaments that are 3–4 nm in diameter [29]. The shape of this virus resembles the elongated twisted and coiled [25, 26, 29].

The genome of this virus consists of three single-stranded RNAs (ssRNAs) of negative polarity [25, 26, 29, 30] So this virus is tripartite in that each RNA encodes some proteins which have various kinds of features. RNA-dependent RNA polymerase and a 24-kDa protein, which is an unknown function, are encoded by RNA 1 which has a 24-kDa weight [29, 30, 31, 32]. Another RNA (RNA 2) is responsible to produce a movement protein and the last RNA (RNA 3) encodes the coat protein that has a 48.6 kDa weight [21, 22, 29, 30, 31]. This virus is usually transmitted by infected graft scion [29, 30] but in some regions reported some insects caused this disease to expand to remote fields. Usually, virions place in the phloem and parenchyma cells of citrus [29].

The most significant symptoms of psorosis are bark scaling in the trunk and main branches and the gum may accumulate below the bark scales that dues to impregnate the xylem and vessel occlusion and fruits bark that formed pustules on the trunks and fruits [21, 22, 29, 31]. Other symptoms that help to detect it in citrus fields are flecking, chlorotic spots, the necrotic shock of young shoots, and ringspot in leaves or fruits [21, 22, 31]. This virus does not cause an infected tree to be killed. It shows slowly decline and a loss of production [29].

This virus has two strains: strain psorosis A (PsA) and psorosis B (PsB) [2, 3, 5, 6, 8, 9, 11, 13, 14, 21, 22, 23, 24, 25, 26, 27, 28, 30, 31, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44]. The symptom of psorosis A is bark-scaling in the trunk and limbs of infected field trees and staining of interior wood and leaf symptoms are flecking and spots, and shock reaction in very young shoots, in some isolates (Figure 2). It can also cause dieback and decrease fruit yield [21, 29, 30]. Symptoms caused by psorosis B are severe and include bark scaling even on fine twigs, gummosis, and chlorotic blotching in old leaves with pustules on the underside of leaves [21, 22, 29, 30]. and sometimes ring spots on fruits. Bark scaling often appears in 10–12-year-old trees [29].

Figure 2.

Symptoms of Citrus psorosis virus on (a) the leaf (ring spots) and (b) bark (bark scaling), Mazandaran province, Iran. Source: Author.

If the strain psorosis B does not exist in any fields, you may see some leaves symptoms, and all of them are placed in the “psorosis group” diseases such as chlorotic leaf-flecking, oak-leaf patterns, ring spots, and ring patterns [29].

Totally, this virus infected citrus trees that are 10–15 years of age when they are at maximum fruit production [30] and it causes trees not to produce fruits insufficient that should have.

2.3 Satsuma Dwarf virus or Citrus mosaic virus

Both of them are shown in Japan the first and located in the Secoviridae family. The genome of the causal disease consists of two RNAs that are single-stranded, positive-sense. These viruses are transmitted by grafting infected budwood and mechanical with infected knives or scissors. The symptom of this agent is dwarfing because of short internodes, multiple sprouting and small leaves, and poor growth dues to the limitation of an extensive root system with a gradual decrease of yields. Sometimes, you can see various green patterns on the fruit rind of satsuma mandarin that cause a decline in commercial value which is called citrus mosaic. This virus does not make any symptoms on the trunk of infected trees. The symptom of leaves are small, boat-and spoon-shaped leaves, and the infected fruits show small in size, with a thick rind and poor taste. If trees infect with the Citrus mosaic virus, fruits have rings, mosaic, and blotches patterns on their outer surfaces. During the growth of fruit, these patterns are changed to brown and dented, resulting in a rough fruit surface. In this disease, virus particles are found in leaf cells of satsuma because they fill Intercellular spaces and tubules [33].

2.4 Citrus variegation virus (CVV)

This virus is another plant viral pathogenic that infects citrus cultivars. It belongs to the Bromoviridae family, Ilarvirus genus [37].

This virus is shown in the various kinds of citrus cultivars. Its symptom is variegated, mosaic, malformation on leaves, and dwarfing [2, 37]. The basis of the symptoms on citrus cultivars researchers introduced two strains: A strain can create chlorotic mottle with variable severity on the leaves and crinkles symptoms that are called infectious variegation strain. The other one is crinkly leaf strain causes distorted, puffed, or puckered leaf segments but without variegation. This agent like others transmitted by infected scion and mechanically [37].

2.5 Citrus leaf rugose virus

This virus has a close relationship with the Citrus variegation virus. Both of them place in the same genus [2]. The genome of this virus is tripartite. It means it has three single-stranded RNAs [45].

This virus induces symptoms in some cultivars like lemons, Mexican lime, and grapefruit. The rest of the citrus cultivars do not show obvious symptoms. The symptom of sensitive hosts includes leaf flecking, puckering in leaves, and stunting [2].

2.6 Citrus leprosis Virus

Citrus leprosis is an important disease in the North and South of the Americas and it caused millions of citrus trees to die. For the first time, it was reported from the USA. This disease damages the amount of citrus production annually in the USA. It is a very destructive disease on the Americas continent, especially on oranges and mandarins. This disease can be transmitted by the mites [13]. This disease is non-systemic. A casual disease is a heterogenic group of RNA viruses endemic to the North and South of the American continent. These viruses belong to three genera that include Cilevirus (bipartite positive sense (+) single-stranded [ss] RNA), higrevirus (tripartite (+) ssRNA), and dichorhavirus (bipartite negative sense (−) ssRNA, family Rhabdoviridae, order Mononegavirales). Cileviruses and higreviruses are active in the cytoplasm of infected plant cells but dichorhaviruses exist in the nucleus of infected plant cells. So this disease has two classifications that basis on the place of viroplasms in infected cells: CL-Cytoplasmic (CL-C) and CL-Nuclear (CL-N) [43].

The characteristic of Dichorhaviruses is short rod-like non-enveloped particles (40–50 × 100–110 nm) and encapsidate two ss (−) RNA molecules with 6 ORF. The type of virus in this genus is the orchid fleck virus (OFV). OFV has two types that include Citrus necrotic spot virus and Citrus leprosis virus Nuclear type (CL-N) [43]. The symptoms of this disease are necrotic or chlorotic spots in leaves, branches, and fruits, which progressively leads to the early drop of leaves and fruits, branch dieback, and occasionally to the death, predominantly, of the youngest citrus trees (Figure 3) [43].

Figure 3.

Symptom of citrus leprosis virus on (A and B) fruits, (C) twigs, (D) leaves in the United States. Source: Levy et al. [34].

The casual of the disease is Citrus leprosis virus Cytoplasmic (CL-C) which is the type species of the genus Cilevirus, family Kitaviridae. The members of this family have bacilliform or spherical virions with positive-sense single-stranded RNA genomes [20]. The symptoms of this disease are shown on twigs, leaves, and fruits that are local chlorotic and/or necrotic lesions on the surface of the host’s tissue [13, 20]. If mites feed the lesion, the agent of casual disease can transmit to the healthy plant. In addition, infected shoots induced the disease in healthy plants by grafting [13]. This virus do not move systematically in its host [13, 20].

There are many ways to control disease. First, the control of mite as a vector is important. After that, infestation gardening tools like knives and scissors. The last, remove the infected twigs and control weeds [13].

2.7 Indian citrus ringspot virus (ICRSV)

ICRSV can infect mandarine and expand worldwide. This disease is reported by Wallace and Drake (1968) from California for the first time [46]. This virus has limitations on hosts. It attacks the mandarine. At the first, it is reported on “Kinnow” mandarin (Citrus reticulata cv. “Blanco,” a hybrid between “King” and “Willow” mandarins) from India. This virus has a positive sense, flexuous RNA virus approximately 7.5 kb in size with six open reading frames (ORFs) belonging to the genus Mandarivirus in the Alphaflexiviridae family of order Tymovirales [42]. The shape of the virion looks like capilloviruses [47].

The symptom of this disease is bright yellow ring spots on mature leaves, followed by a rapid decline [42, 46, 54]. Several affected trees show dieback and decline type of symptoms and thus become less productive [46].

2.8 Citrus leaf blotch virus

This virus is placed in the Betaflexiviridae family and it is seed-borne [42]. This virus infects trifoliate rootstocks like Citrange and the symptom is bud union crease when propagated on Troyer citrange. Also sometimes it shows chlorotic blotching in young leaves. This disease detects in France for the first time. The virion of the virus is filamentous particles about 900 × 14 nm in size, with a single-stranded, positive sense, genomic RNA (gRNA) of 8747 nt, and a coat protein of about 41 kDa. The gRNA contains three open reading frames (ORFs) and untranslated regions of 73 and 541 nt at the 5′ and 3′ termini, respectively. Because this virus can be transmitted by grafting, sanitation, and quarantine of scions are very important [48].

2.9 Bud-union crease of citrus trees

The symptom of this disease is a dotted or continuous line of orange-yellow discoloration associated with projections or bark pegs on the inner bark surface and corresponding pits in the wood at the bud union [24, 15, 27, 49]. Sometimes, this disease causes the infected tree to decline and die or remain stunted. This disease is related to scion-rootstock combinations. Some sweet orange and lemon varieties on trifoliate orange and trifoliate orange hybrids are shown this disease. The basis on experiments, the casual of this disease is not a virus or other infection casual. It is guessed a physiological incompatibility causes this disease [49].

The infection tree shows a pale yellow color on the leaves. This symptom resembles suffering from a nutrient deficiency. Also, these trees have a poor flash that causes them to decline and die. The amount of production in infected trees is too low. In severe infection, a groove is made on the bark at the bud union. Gum-impregnated projections exist Underneath Bark strips [27].

2.10 Citrus yellow vein clearing virus

This disease is showed in Pakistan for the first time in 1988 on sour orange (Citrus aurantium L.) and lemon (C. limon Burm. f.) [35, 36, 50]. The symptom of this disease is strong yellow vein clearing, leaf distortion, and occasionally, ring spots and veinal necrosis [35, 36]. The symptoms of mild or moderate vein clearing or chlorosis have been observed only in young leaves during spring flushes [35]. This virus after inoculation on chenopodium showed local lesions while phaseolus Vulgaris showed systemic chlorosis, severe mosaic, blotching, and necrosis. The casual disease is the Citrus yellow vein clearing virus (CYVCV). The virion of the virus is the flexuous filamentous virion particle is sized between 13 and 14 nm in diameter with a modal length of 685 nm [35]. The place of virions of virus in infected plants is in the phloem [35]. This virus is a single-strand positive-sense RNA virus comprised of 7529 nucleotides (nt) [35, 44, 50] and the genome of the virus has 6 open reading frames (ORF). Citrus yellow vein clearing virus (CYVCV) belongs to Genus (Mandarivirus), family (Alphaflexiviridae) [50].

In China, upon becoming infected with CVYCV, the leaves of young spring or autumn shoots in lemon or sour orange trees display a water-soaked appearance and yellow, clearing veins on their ventral side. The leaves also represent chlorotic with significant crinkling and warping [23, 35].

In India, the symptoms are different and it may be shown, with mosaic and irregular ring spot-like symptoms. In drastic infection, fruits are imperfect and trees die and which changes the amount of production. This virus on herbaceous plants appears different symptoms like mosaic-like patterns, chlorosis, and necrosis on leaves. The infection in weeds is asymptomatic. Also, this virus can infect a wild grapevine and causes different symptoms that include short internodes, reduced leaf size, chlorosis, and necrosis [23].

The symptoms of the disease disappear in summer. The many possibilities are for this reaction. One of them is the high temperature causes the limitation of replication of the virus. Another reason is related to the movement of viruses. The high temperature in summer may inhibit the virus’ movement in the plant, or cause the virus to transfer to other tissues in the asymptomatic tree [35].

This virus can be transmitted by grafting, knife, and scissors gardening, and insects like aphids and white flies [23, 35]. The genus of aphids can infect contain Aphis spiraecola Patch, Aphis craccivora Koch, Aphis gossypii Glover, and citrus whitefly Dialeurodes citri (Ashmead) [23, 35].

Now, this virus is expanded in many citrus-growing areas in Asia like China, Iran, and Turkey [50].

2.11 Leaf variegation with ring spots

The symptom of this disease is yellowish variegation on both the upper and lower surfaces of the leaves, although the yellow color is lighter on the lower surface. The shape and size of yellow areas are different but most of them are small and more or less circular in outline they have a green center and they are more similar to ring spots. Yellow spots can expand on the twigs with variegated leaves. Sometimes leaves that have yellow spots midribs are colored with gum. Fruit on branches with variegated leaves develops slight depressions or furrows, yellow in color.

The rate of transmission of this disease to healthy plants is very low. It is transmitted by grafting infected shoots. It is guessed the casual of this disease is a virus [41].

2.12 Citrus variegation virus

The symptom of this disease consists of leaf flecking, mosaic, malformation, and dwarfing. The casual of this disease is the Citrus variegation virus (CVV) which is a member of subgroup 2 of the genus Ilarvirus (Bromoviridae). This disease is expanding in the Mediterranean regions and the Americas continent. The symptom of disease on Citrons and lemons is more severe than on oranges and mandarine. It includes a reduction in yield and fruit malformation. This virus has two strains: infectious variegation strain, and crinkly leaf strain. The first strain can cause chlorotic mottle with variable severity on the leaves and sometimes can show crinkle symptoms on infected trees. The second strain induces distorted, puffed, or puckered leaf segments but without variegation. This disease can be transmitted by grafting and mechanical [37].

2.13 Management of citrus viral diseases

The first step for controlling viral diseases is the exclusion of the disease and forbidden to enter the infected propagation scion. The major transmission among citrus viral diseases is grafting infected buds. So, for the propagation of citrus cultivars, it is better to use certificated budwood. If viral diseases exist in a specific area, it is better to use tolerant or resistant cultivars for rootstocks. Other ways to control include cross-protection and cultural practices [2]. Furthermore, Sanitation is very important. After working with scissors and/or knives, they wash with a disinfectant solution.

3. Citrus viroid diseases

3.1 Citrus exocortis viroid (CEVd)

In 1948, this disease is showed with bark scaling on the rootstock of citrus trees grafted on trifoliate orange (Poncirus trifoliata) that is reported by Roistacher (1988) [5, 38, 51].

The agent of this disease is different. These are the smallest pathogen among the pathogenic agents of plants that are known. Usually, they have no coated protein and their body are circular. Their genome is single-stranded RNA [5, 7, 25] and they do not translate. Usually, they have between 246 and 399 or 400 nucleotides [6, 8, 9].

This viroid belongs to the Pospiviroidae family, the Pospiviroid genus. Members of this family have a rod-like or quasi-rod-like secondary structure with five structural domains (terminal left, pathogenic, central, variable, and terminal right) and a central conserved region (CCR) within the central domain (C). they do an asymmetric rolling-circle pathway for replication which occurs in the nucleus [5, 7, 8].

The significant symptoms that are caused by this viroid are bark scaling and cracking, general decline, decreased size and stunting, and leaf epinasty and cracks in the petiole (Figure 4) [10, 19]. This viroid can infect various kinds of citrus cultivars like P. trifoliata and its hybrids, rangpur lime (Citrus limonia), lemon (C. limon), and citron (C. medica) [10].

Figure 4.

Symptoms of Citrus exocortis viroid (a) decline of plant, (b) leaf epinasty as seen on grafting infected scion on Etrog citron rootstock, indicator host of CEVd, (c) bark scaling in the United States. Sources: Roy et al. [52].

This agent of the disease is the same as viruses and is transmitted by grafting and infected trimming and cutting instruments [10].

3.2 Cachexia or xyloporosis (Hop stunt viroid (HSVd))

This disease was shown in 1950 on Parson’s Special mandarin [19] and is reported by Roistacher (1983) [53]. This viroid the same as exocortis belongs to the Pospiviroidae family but in the classification, its genus is different and placed in the Hostuviroid genus. Because of a specific 6-nucleotide motif located in the Variable (V) domain of Hop stunt viroid that is responsible for the induction of cachexia symptoms in cachexia-sensitive hosts, this viroid was placed in the group [24, 38, 51]. In fact, those nucleotides express the symptom of Citrus cachexia viroid because the variable domain is responsible for pathogenicity [38]. The secondary structure of this viroid is a rod-like structure with five domains, a “central conserved region” (CCR) and a “terminal conserved hairpin” (TCH). The variable domain located at the Right of the central conserved region and basis on two strains is introduced. “pathogenic strains” that show cachexia symptoms in sensitive hosts, and “non-pathogenic strains” that infect the same sensitive hosts without inducing symptoms. Six nucleotides in this region show different symptoms. At the left of the central conserved domain is the pathogenicity domain that is responsible induce symptoms in sensitive hosts [24, 53].

The symptom of this disease includes discoloration, gumming, and browning of phloem tissue, wood pitting, and bark cracking [6, 19, 24, 38, 51, 53, 54]. If you remove the outer bark layers, you can see the discoloration of the bark. As a result, infected trees are stunted and chlorotic, and may decline and die [53]. The sensitive citrus cultivars are alemow (Citrus macrophylla Webster), clementines (C. clementina Hort. Ex. Tan), mandarins (C. reticulata Blanco), satsumas (C. unshiu (Macf.) Marc.), “Rangpur” lime (C. limonia Osb.), kumquats (Fortunella spp.), and hybrids like tangelos that if they can be infected, you will see the severe symptoms (Figure 5) [24].

Figure 5.

Severe symptoms of cachexia, including gumming and pitting of bark and wood (A) of declining Minneola tangelo orchard from Jiroft, Kerman Province (B) in comparison with mild symptoms of cachexia in the same variety from Mazandaran Province (C). Source: Banihashemian et al. [17].

3.3 Citrus bentleaf viroid (CBLVd) or citrus viroid I

This viroid belongs to Pospiviroidae family, Apscaviroid genus and it has 318 nucleotides. Its structure is rod-like. Because of the resemblance of a part of the central conserved region (CCR) with the apple scar skin viroid (ASSVd), this viroid is located in this genus, although this viroid infected citrus cultivars. The left terminal region of citrus bentleaf viroid is as the same as citrus exocortis viroid (CEVd) [16]. The symptom of this disease is just on the leaves as curling.

3.4 Citrus gummy bark viroid (CGBVd) or Citrus viroid II

This viroid is a strain of Hop stunt viroid and so it places in Hostuviroid genus [55]. This viroid infects the sweet orange tree and causes spots or a line of reddish-brown, gum-impregnated tissue around the scion circumference especially visible near the bud union. In a severe infection, discoloration and gumming are extended and maybe to reach the main branches [39, 40, 55]. For the first time, this disease with discoloration phloem by Nour-Eldin in Egypt [39, 40]. After that, this disease is recorded in many countries such as North African and Near East countries including Saudi Arabia, Sudan, Lybia, Iran, Turkey, and Greece [40].

If this viroid infects Dortyol and Washington navel trees, they show bark scaling resembling mild psorosis scaling. It is suggested that a variant of CVd-I1 may be either the causal agent or a factor involved in gummy bark symptoms [40].

3.5 Citrus dwarfing viroid or Citrus viroid III

The length of this viroid genome is 294–297 nucleotides. This viroid is placed in Apscaviroid genus, Pospiviroidae family. This viroid causes citrus trees not to grow at suitable sizes and they are shorter than healthy trees [18].

3.6 Citrus bark cracking viroid (CBCV) or Citrus viroid IV

This viroid belongs to Cocadviroid genus, Pospiviroidae family. This genus has 246–301 nucleotides and its secondary structure is rod-like or quasi-rod-like with five domains the same as other genera. These viroids induce symptoms in citrus plants but recently this viroid causes severe symptoms on the hop. Although dues to this viroid, Etrog citron shows leaf drooping and necrosis of the leaf midribs, it does not have a minus effect on growth and yield in citrus. If CBCV and HSVd infected one tree, the symptom is severe and you will have observed yield reduction. These viroids have a synergistic effect [18, 51].

3.7 Citrus viroid V

This viroid like CBLVd belongs to Pospiviroidae family, Apscaviroid genus. The genome of this viroid has 293–294 nucleotides and its conformation, like others, is rodlike. This viroid induces mild but typical symptoms on citrus leaves. This viroid causes reduced tree size and yield, and on stems, you can see very small necrotic lesions and cracks, sometimes filled with gum insensitive hosts like Etrog citron [56].

As a result, Typical viroid symptoms on this host include leaf tip browning, petiole wrinkle, and browning, mid-vein necrosis, leaf epinasty, leaf bending, bark cracking, gumming, and tree stunting [7].

3.8 Management of viroid diseases

The best work is prevention and it includes (1) using a viroid-free budwood as propagation material; (2) The appropriate sanitation at cutting and trimming time; (3) treatment of cutting tools with disinfectants; and (4) adequate indexing methods [24].

4. Conclusion

Viruses and viroids have an indispensable role in plant infection. In this chapter, we discussed the most major diseases that are caused by viruses and viroids. Citrus is the most important commercial crop for many countries and people who work in fields and nurseries that cultivate citrus should know about various kinds of citrus diseases especially virus diseases because most of them are controlled easily. The prevention is the first step to manage viral diseases. Furthermore, when experts have suitable information about various kinds of symptoms, they can rapidly detect a virus disease and control it and it is a very necessary requirement for each region. Table 1 shows a summary about citrus viruses and viroids and their detection ways.

Diseases	Causal agent	Main symptoms	Diagnostic methods	Control measures
Tristeza	Citrus tristeza virus	Yellowish leaves; Decline	ELISA; PCR	Using healthy scion; The control of vector
Psorosis	Citrus psorosis virus	Ring spot; Chlorotic spots; Bark scaling	ELISA; PCR	Using healthy scion; The control of vector
Citrus mosaic	Satsuma dwarf virus or Citrus mosaic virus	Dwarfing; Short root system	PCR	Using healthy scion
Crinkly leaf	Citrus variegation virus (CVV)	Variegation and mosaic on leaves; Dwarfing	PCR	Using healthy scion
Citrus leaf rugose	Citrus leaf rugose virus	Leaf flecking; Stunting	PCR	Using healthy scion
Citrus leprosis	Citrus leprosis Virus	Branch dieback; Local chlorotic and/or necrotic lesions on leaves	PCR	Using healthy scion
Indian citrus ringspot	Indian citrus ringspot virus	Yellow ring spots on leaves; Decline	PCR	Using healthy scion
Citrus leaf blotch	Citrus leaf blotch virus	Chlorotic blotching in young leaves	PCR	Using healthy scion
Bud-union crease	Bud-union crease of citrus trees	Bark pegs on the inner bark surface; Orange-yellow discoloration underneath of bark	PCR	Using healthy scion
Citrus yellow vein clearing	Citrus yellow vein clearing virus	Strong yellow vein clearing, leaf distortion, veinal necrosis	PCR	Using healthy scion
Leaf variegation	Leaf variegation with ring spots	Yellowish variegation on leaves	PCR	Using healthy scion
Citrus variegation	Citrus variegation virus	Leaf flecking, mosaic, dwarfing	PCR	Using healthy scion
Exocortis	Citrus exocortis viroid	Bark scaling, cracking, decline	ELISA; PCR	Using healthy scion
Cachexia or xyloporosis	Hop stunt viroid	Discoloration, bark cracking, gumming	PCR	Using healthy scion
Citrus bentleaf	Citrus bent leaf viroid (CBLVd) or Citrus viroid I		PCR	Using healthy scion
Citrus gummy bark	Citrus gummy bark viroid (CGBVd) or Citrus viroid II	Discoloration phloem, gumming		Using healthy scion
Citrus dwarfing	Citrus dwarfing viroid or Citrus viroid III	Decline	PCR	Using healthy scion
Citrus bark cracking	Citrus bark cracking viroid(CBCV) or Citrus viroid IV	Leaf drooping	PCR	Using healthy scion
Citrus viroid	Citrus viroid V	Leaf epinasty, bark cracking	PCR	Using healthy scion

Table 1.

Summary about viruses and viroids that can infect citrus trees with the way of detection and symptoms induced on citrus plants.