Citrus greening, officially called Huanglongbing (HLB), is a bacterial disease that slowly kills citrus trees by blocking their internal nutrient transport system. It has no cure and has devastated citrus industries worldwide, dropping Florida’s total citrus production to just 14.6 million boxes in the 2024–2025 season, a fraction of what the state once produced. The disease is spread by a tiny insect called the Asian citrus psyllid, and once a tree is infected, decline is inevitable.
How the Disease Works Inside a Tree
The culprit is a bacterium that lives inside the phloem, the network of tubes a tree uses to move sugars from its leaves to the rest of the plant. Once the bacterium colonizes these tubes, the tree’s immune response backfires. The sieve plates (tiny pore-covered walls inside the phloem tubes) accumulate a sugar-based compound called callose, along with starch. This buildup essentially clogs the plumbing, blocking the flow of nutrients from leaves to roots, fruit, and new growth.
The result mimics severe nutrient deficiency even though the soil may have plenty of minerals. The tree can still photosynthesize in its leaves, but it can’t deliver those sugars where they’re needed. Over time, branches die back, fruit drops prematurely, and the canopy thins. Most infected trees decline over several years, though the timeline varies by variety and growing conditions.
How to Recognize Citrus Greening
The single most reliable leaf symptom is blotchy mottling, an irregular pattern of yellow and green patches that looks asymmetrical when you split the leaf along its center vein. This uneven mottling distinguishes greening from nutrient deficiencies, which tend to produce symmetrical yellowing. You may also notice corky or thickened leaf veins, yellowing between veins, or bright yellow shoots standing out against the rest of a green canopy. Yellow veins or vein corking alone, without the blotchy mottle, are not enough to confirm the disease.
Fruit tells its own story. Infected trees often produce lopsided, small, or oddly shaped fruit. The coloring inverts: the stem end turns yellow-orange while the bottom stays green, the opposite of normal ripening. Seeds inside may be aborted or underdeveloped. The juice tastes noticeably off, more bitter and salty than healthy fruit. Research on orange juice from symptomatic trees found that the sugar-to-acid ratio dropped by as much as 62% compared to healthy fruit. Interestingly, known bitter compounds in citrus were either absent or below taste thresholds. The unpleasant flavor comes mainly from lower sugar levels and higher acid concentrations, not from bitterness in the traditional sense.
The Insect That Spreads It
The Asian citrus psyllid is a tiny, winged insect roughly the size of an aphid. It feeds by piercing young leaves and sucking sap. When it feeds on an infected tree, it picks up the bacterium. When it moves to a healthy tree, it delivers the pathogen directly into the phloem. A single feeding can transmit the disease.
These insects reproduce fast. A full generation, from egg to reproducing adult, takes about three weeks under favorable conditions. Nymphs slow their development above 104°F, and sustained exposure to 108°F for three or more hours per day kills them. But in the humid, warm climates where most citrus grows, conditions are often ideal for rapid population growth. Even a small number of psyllids can spread the bacterium across an entire grove or neighborhood in a single season.
Where Citrus Greening Has Spread
The disease originated in Asia and has spread to most major citrus-producing regions worldwide, including Africa, South America, Central America, and the Caribbean. In the United States, Florida has been hardest hit, with the disease first confirmed there in 2005. Texas, California, and Louisiana have also detected infections. As recently as 2026, federal authorities expanded the quarantined area in Arizona after new detections, reflecting how the disease continues to push into new territory. Quarantine zones restrict the movement of citrus plants and plant material to slow the spread.
Why There’s No Simple Fix
Because the bacterium lives deep inside the phloem, it’s shielded from most treatments. Surface-applied pesticides kill psyllids but can’t reach the pathogen already inside the tree. One approach that has gained traction is trunk injection of an antibiotic, which delivers the compound directly into the tree’s vascular system. Injections work best during spring and early summer when the tree is actively moving fluids. However, the treatment comes with real trade-offs: moderate to severe side effects including leaf yellowing, leaf drop, twig dieback, and fruit drop can occur after injection. Timing matters too, as injections performed too close to harvest can leave residues in the fruit.
Critically, antibiotic injections are not a replacement for psyllid control. Without suppressing the insect population, reinfection is constant. Most management programs combine aggressive insect control (area-wide spray programs, biological controls) with nutritional support to keep infected trees productive as long as possible. Even with the best care, infected trees produce less fruit of lower quality and eventually need to be removed.
Resistant Varieties and Genetic Approaches
The long-term hope lies in developing citrus varieties that can tolerate or resist the bacterium. The Australian finger lime has shown notable tolerance to infection, offering researchers clues about which genetic traits help a tree survive. Gene-editing tools have already produced citrus lines resistant to citrus canker, a different bacterial disease, by modifying a single susceptibility gene in Hamlin sweet orange. All edited plants in that study showed complete resistance. Adapting similar techniques for greening resistance is an active area of work, though the biology of HLB is more complex because the bacterium hides inside the phloem rather than attacking surface tissue.
Some rootstocks and varieties naturally show more tolerance than others, surviving longer and producing more usable fruit even when infected. Growers increasingly select these tolerant rootstocks when replanting, buying time while researchers work toward varieties with true resistance. For backyard citrus growers in affected areas, choosing tolerant varieties and controlling psyllids with horticultural oils or systemic treatments remain the most practical steps to keep trees alive.