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Citrus leprosy is caused by Citrus Leprosy Virus (CiLV), which is transmitted by mites of the genus Tetranychus. This disease directly reduces the yield and lifespan of citrus fruits. The main symptoms include the appearance of disease spots on fruits, leaves, and tender or small branches, leading to premature fruit shedding and leaf shedding, ultimately causing the death of tender or small branches, resulting in severe tree weakness. Regardless of where it occurs, citrus leprosy is a highly destructive citrus disease. The citrus leprosy virus karyotype (CiLV-N) exists in the nucleus and cytoplasm of infected cells, while the citrus leprosy virus cytoplasmic type (CiLV-C) exists in the endoplasmic reticulum. In 2012, a new virus causing similar symptoms was discovered in Colombia and named Citrus Leprosy Virus Cytoplasmic Type 2 (CiLV-C2) due to its close similarity to CiLV-C. Cytoplasmic viruses are the most prevalent and widely distributed among the three types of viruses.
Figure 1. CiLV-C infection in Arabidopsis thaliana plants. (Arena G D,et al, 2020)
Localised leprosis lesions are found on fruits, leaves and twigs. The severity of symptoms varies with the type of citrus. The overall lesion size varies from 10–20 mm.Fruit lesions are round, dark brown dead spots 10–20 mm in diameter on the skin. Occasionally gum may exude from the lesion.On green unripe fruit the lesions are initially yellow, but become more brown or black over time. Green fruit lesions may appear sunken.Stem lesions are grey or brown in colour and may be raised above the surface of the stem. Stem lesions may appear to be joined together when vectors are present in large numbers. Death of the twig may occur. In extreme cases severe defoliation and fruit fall may occur. Leprosis leaf symptoms may be confused with pesticide injury or insect damage. Fruit symptoms may be confused with citrus canker (Xanthomonas citri subsp. citri). On the trunk of an infected citrus tree, leprosis bark scaling symptoms may appear similar to citrus psorosis virus (bark scaling) which is present in Australia. The psorosis virus causes wood staining while leprosis does not.
CiLV-N has short rod-shaped particles with a length of 120 to 130 nanometers (nm) and a width of 35 to 40 nanometers, which appear in the nucleus or cytoplasm of infected cells and are related to the presence of viral cytoplasm in the nucleus. The CiLV-N genome is a binary negative sense single stranded RNA ((-) ssRNA). Both types of RNA have a 3 '- terminal poly (A) tail. CiLV-N RNA1 (6268 nucleotides (nt)) contains five open reading frames (ORFs) encoding nucleocapsid protein (N), putative phosphoprotein (P), intercellular motor protein (MP), matrix protein (M), and glycoprotein (G). CiLV-N RNA2 (5847nt) contains an ORF encoding an RNA dependent RNA polymerase (RdRp) replication module. The size and structure of the CiLV-N genome are very similar to the genome organization of orchid spotted virus (OFV), and it is likely to be a member of the newly proposed Dichorhavirus genus.
| Viral Protein | Key Cellular Localization / Interaction | Function in Pathogenesis |
| p29 (Capsid Protein) | Actin filaments; Endoplasmic Reticulum (ER); Integral membrane protein. | RNA silencing suppressor (RSS). Facilitates viral spread and enhances pathogenicity of other viruses. |
| p15 | Nucleus; Golgi complex; ER; Actin filaments; Peripheral membrane protein. | RNA silencing suppressor (RSS). Unique ability to restore cell-to-cell movement for a defective virus. May regulate host gene expression. |
| p61 (Putative Glycoprotein) | Golgi complex; ER. | Elicitor of Hypersensitive Response (HR). RNA silencing suppressor (RSS). Its expression triggers a defense-related cell death. |
Citrus leprosy is mainly detected by observing local lesions with characteristic symptoms. The presence of CiLV-N and CiLV-C particles in the affected tissues can be confirmed by transmission electron microscopy (TEM). Other laboratory testing methods can also be used, such as double antibody sandwich enzyme-linked immunosorbent assay (DAS-ISA), indirect ELISA, dot blot immunoassay, and the use of monoclonal and polyclonal immune capture reverse transcription polymerase chain reaction; and reverse transcription polymerase chain reaction (RT-PCR).
CL mainly controls mites through management. Although mite resistance has been detected, most of the currently available acaricides are effective. There are reports that biological alternatives such as mite predators and insect pathogenic fungi have achieved promising results. By implementing cultural practices such as removing affected tree branches, using windbreaks to reduce vector wind transmission, controlling weeds (as a substitute for mite hosts), using healthy plant sources, and controlling the movement of people and materials in orchards, the amount of virus inoculation can be reduced. Although resistance has been observed between different citrus varieties, few studies have been conducted in this field to determine commercially resistant varieties. A study developed using hybrid populations suggests that the inheritance of resistance to leprosy may only be controlled by a few genes.
Reference
| Target | Cat. No. | Product Name | Host | |
| CiLV | DAG-WT3469 | Recombinant Citrus leprosis virus C p29 | E. coli | Inquiry |
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