**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, 8–27]. 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.

#### **Figure 1.**

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

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].

#### **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 [29–32]. 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, 29–31]. 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, 21–28, 30, 31, 33–44]. 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].

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.
