Huanglongbing

A hallmark symptom is a blotchy, asymmetric yellow mottling of the leaves that does not match on either side of the midrib, which helps distinguish it from nutrient deficiencies that are usually symmetrical. Affected trees show yellow shoots, thinning canopies, and twig dieback. Fruit are often small, lopsided, and fail to colour properly, remaining green at the bottom, with aborted seeds and a bitter, off taste, which is the origin of the name greening.

The Liberibacter bacteria live in the phloem of citrus trees and are transmitted by citrus psyllids that feed on the sap. A psyllid acquires the bacteria while feeding on an infected tree and can then transmit them to healthy trees. The bacteria multiply and spread within the tree, and infected but symptomless trees can serve as sources for further spread. Grafting or planting infected material also moves the disease over distance.

• Bacteria are confined to the phloem and disrupt nutrient transport
• Citrus psyllids acquire and transmit the bacteria while feeding
• Infected, sometimes symptomless, trees act as sources of the bacteria
• Grafting and movement of infected material spread the disease over distance

Citrus of all kinds, including orange, lemon, lime, and grapefruit, are susceptible, along with some related ornamental plants that can host the psyllid and the bacteria. Disease spread is driven by the presence and activity of citrus psyllids and by the movement of infected nursery stock, rather than by a particular climate, though conditions favouring psyllid populations increase risk.

• Presence of citrus psyllid vectors in the growing area
• Nearby infected trees or ornamental host plants
• Movement or planting of infected nursery material
• Conditions that favour psyllid populations and their flush feeding

Because there is no cure, management centres on keeping the disease out, reducing the psyllid vector, and removing infected trees to protect the wider area. Coordinated, area-wide action is generally more effective than isolated efforts.

• Plant only certified, disease-free nursery stock
• Manage citrus psyllid populations to reduce transmission
• Scout regularly and test suspect trees to detect infection early
• Remove and destroy confirmed infected trees to reduce sources
• Participate in coordinated, area-wide management programmes