Abstract
Citrus exocortis viroid (CEVd) and hop stunt viroid (HSVd) are the main viroids circulating in all citrus-growing areas worldwide, and causing two well-known diseases on citrus trees; exocortis and cachexia, respectively. These viroids are small, covalently closed single-stranded RNA, allocated to the Pospiviroidae family. CEVd is the first viroid being described on citrus trees in 1948 in California. It is considered the largest citrus viroid at 371 nucleotides. It causes bark scaling disorder on the rootstock of citrus trees grafted on trifoliate orange and its hybrids and can cause dwarfing of trees grown on these rootstocks. HSVd was first observed in 1945 in Florida. It consists of 299 nucleotides. Stunting, chlorosis, bark gumming, stem pitting, decline, and depressions in the wood are the main symptoms of HSVd in mandarin and its hybrids. The introduction and propagation of infected budwoods are the main causes of viroids spread in citrus orchards. These agents are mechanically sap-transmissible and spread by contaminated tools. Neither seed transmission nor vectors have been reported for both viroids. Root transmission, though possible, would be overshadowed by mechanical transmission. Rapid and sensitive molecular-based detection methods specific to both viroids are available. Both diseases are controlled by using viroids-free budwoods for new plantations, launching budwood certification programs, and establishing a quarantine system for new citrus varieties introduction. The most important achievements in CEVd and HSVd researches are outlined in this chapter. This would help to provide a clearer understanding of the diseases they cause and contribute to the development of better control strategies.
Highlights
Viroids are circular, highly structured, single-stranded RNA phytopathogens
Citrus exocortis viroid (CEVd) and hop stunt viroid (HSVd) are destructive to certain citrus varieties and, can cause yield losses that may be as high as 34 to 76 percent depending on the combination viroid-rootstock-scion [21, 46]
The mechanisms through which CEVd and HSVd interact with their hosts and induce pathogenesis are beginning to be deciphered
Summary
Highly structured, single-stranded RNA (ssRNA) phytopathogens. they do not code for any peptide, these enigmatic pathogens have evolved the capacity to replicate within cellular organella, the nucleus and chloroplast. From the seven known citrus viroids only, two, namely, CEVd and HSVd, have been reported to be associated with citrus diseases that can pose significant economic risks to global citrus production. These diseases are exocortis and cachexia, respectively [8]. Given the importance of and rapid research progress in citrus virology in recent years, this review emphasizes recent findings related to CEVd and HSVd, the most serious viroids associated with citrus. It comprises reviews and research articles covering broad research areas on the characterization of both viroids and their symptoms, the development of reliable and rapid diagnosis methods, and management strategies. A brief snapshot of the present situation of CEVd and HSVd in the Mediterranean region, with an emphasis on their spread in citrus-growing areas of Morocco, is included
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