Citrus exocortis

Citrus exocortis is a serious plant disease caused by the Citrus exocortis viroid (CEVd). Unlike viruses, viroids are composed solely of a short strand of circular, single-stranded RNA without a protective protein shell. This pathogen forces the plant's own cellular machinery to replicate the viroid, which disrupts essential physiological processes within the host tissue.

The most visible symptom of the disease is the scaling of the bark on the rootstock, particularly in varieties grafted onto susceptible rootstocks such as Poncirus trifoliata. As the disease progresses, the tree exhibits reduced vigor, stunted growth, and canopy dieback. Fruit production is negatively affected, resulting in smaller yields and poor fruit quality that renders the crop commercially non-viable.

The biological cycle involves the systemic infection of the host. The viroid moves through the phloem and can remain latent for years without showing symptoms. However, it remains highly infectious throughout this period. Transmission primarily occurs through contaminated grafting tools, buds, and nursery stock, making the disease difficult to manage once it is established in an orchard.

Development and spread are facilitated by warm climates and horticultural practices that involve repeated contact with plant sap. Because the pathogen is easily transferred through mechanical wounds, contaminated pruning equipment serves as a primary vector. In the absence of insect vectors, human activity in the orchard remains the most significant factor in the disease's distribution.

Management of Citrus exocortis is focused strictly on prevention rather than cure, as there is no chemical control for infected trees. Growers must use certified viroid-free budwood and rootstocks. Strict sanitation protocols, including the sterilization of pruning tools with bleach or alcohol between trees, are critical. Any tree showing symptoms must be removed and destroyed to protect the remaining orchard.

• Use of certified pathogen-free stock.
• Sterilization of pruning tools.
• Selection of resistant rootstocks.
• Regular field inspection.