Citrus exocortis

Citrus exocortis is a debilitating plant disease caused by the Citrus exocortis viroid (CEVd). This pathogen is not a typical virus but a small, single-stranded circular RNA molecule that does not encode any proteins, yet it causes significant disruption to the host's physiological processes.

The disease primarily affects a wide range of Citrus species. It is particularly problematic in orchards where susceptible rootstocks, such as Poncirus trifoliata, are utilized. Once a tree is infected, the viroid colonizes the vascular tissues, becoming a permanent resident of the plant.

The most diagnostic symptom of the disease is the scaling and peeling of the bark on the tree trunk, usually located below the graft union. These long, vertical strips of exfoliating bark give the trunk a ragged appearance. Infected trees show significant stunting, reduced canopy density, and yellowing foliage, which eventually leads to a dramatic drop in fruit production.

Transmission occurs primarily through the mechanical movement of infected sap. In agricultural settings, the main vectors are pruning tools, grafting knives, and harvesting equipment that have not been properly disinfected between trees. Contaminated nursery stocks also serve as a major source for long-distance dissemination of the viroid.

Since there is no curative treatment for trees infected with CEVd, prevention is the only viable strategy. Management practices include:

• Utilizing certified disease-free rootstocks and scion wood.
• Sterilizing all cutting tools between plants using chlorine bleach or high-concentration alcohol.
• Implementing strict quarantine measures for any new plant material introduced to the orchard.
• Systematic roguing of infected trees to minimize the inoculum reservoir in the field.