**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].

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

#### **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].*

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].
