Citrus exocortis

Citrus exocortis is a serious plant disease caused by the viroid Pospiviroid exocortiscitri. Being a small, circular, non-coding RNA molecule, this pathogen acts as a parasite that utilizes the host's machinery to multiply, leading to systemic physiological disruption and stunted growth in various citrus species.

The host range is primarily limited to Citrus species, with notable sensitivity observed in lemons, limes, and grapefruits, especially when grafted onto susceptible rootstocks such as Poncirus trifoliata. This combination triggers a severe hypersensitive-like response in the bark tissue, which is the hallmark of the infection.

The primary symptom of the disease is the scaling and peeling of the bark on the rootstock portion of the tree. The bark becomes brittle, cracked, and vertical strips peel away. As the disease progresses, the tree exhibits significantly reduced vigor, smaller canopy size, and reduced fruit yields compared to healthy trees.

Transmission of the viroid occurs primarily through mechanical means. Pruning tools, grafting knives, and harvesting equipment contaminated with the sap of infected trees are the most common vectors. Because the viroid is highly stable, it can remain infectious on tools for extended periods if they are not properly sanitized.

Management of Citrus exocortis focuses entirely on prevention, as there is no cure for an infected tree. Recommended practices include:

• Utilizing only certified disease-free propagation material.
• Implementing rigorous sanitation protocols for all pruning and grafting tools using bleach or alcohol.
• Early identification and rogueing of infected trees to prevent secondary spread within the orchard.
• Selecting virus-indexed rootstock-scion combinations that are known to be tolerant or resistant.