Citrus bud-union incompatibility

Citrus bud-union incompatibility is a physiological disorder that occurs at the graft interface between the scion and the rootstock. This condition disrupts the vascular connectivity, effectively strangulating the tree by limiting the transport of water, minerals, and carbohydrates between the roots and the canopy.

The pathology is classified as a graft union failure, which can be strictly physiological or induced by biological factors such as the Citrus Tristeza Virus (CTV). When the scion and rootstock do not possess genetic or immunological compatibility, the tissues fail to integrate, leading to structural weakness.

This disorder affects major commercial citrus species, including oranges, lemons, and grapefruits. It is particularly prevalent in older orchards or those using rootstocks that are highly susceptible to latent viral infections, leading to widespread decline.

Common clinical signs of bud-union disease include:

• A distinct constriction or "bud-union crease" appearing at the graft site.
• Gradual yellowing of foliage and general tree decline.
• Stunted growth resulting in trees significantly smaller than the healthy ones.
• Bark necrosis at the junction, often followed by sudden tree collapse under stress.

Environmental and agronomic conditions, such as poor propagation techniques or improper rootstock-scion combinations, significantly increase the risk of this disorder. Prevention is the only viable management strategy: nurseries must prioritize the use of virus-indexed, certified disease-free propagation materials and select rootstocks that are proven to be compatible with the target citrus varieties.