Citrus greening is the most destructive disease affecting citrus trees worldwide. Caused by a bacterium that clogs a tree’s internal nutrient-transport system, it produces bitter, lopsided fruit and eventually kills the tree. There is no cure, and once a tree is infected, it cannot be saved to full health. The disease has already devastated Florida’s citrus industry, reducing production by tens of millions of boxes over the past two decades, and it is now spreading into California.
What Causes Citrus Greening
The disease, formally called Huanglongbing (HLB), is caused by a bacterium that lives inside the phloem, the network of vessels a tree uses to move sugars and nutrients from its leaves to its roots and fruit. The bacterium doesn’t attack the tree directly the way most plant pathogens do. It lacks the typical molecular weapons that bacteria use to destroy host cells. Instead, it triggers the tree’s own immune system to overreact.
Once inside the phloem, the bacterium provokes a surge of damaging molecules called reactive oxygen species. At the same time, the tree’s ability to neutralize these molecules is suppressed. The result is that the tree’s own immune response kills the very cells that make up its nutrient highway. Within weeks of infection, the tree begins depositing a waxy substance called callose inside its phloem vessels, essentially clogging its own plumbing. Nutrients can no longer reach the fruit, roots, or new growth efficiently, and the tree enters a slow, irreversible decline.
How It Spreads
Citrus greening spreads almost entirely through a tiny winged insect called the Asian citrus psyllid. About the size of an aphid, the psyllid feeds on new leaf growth and picks up the bacterium while feeding on an infected tree. The insect can acquire the bacterium in as little as 15 to 30 minutes of feeding. After an incubation period of 8 to 12 days, during which the bacterium multiplies inside the insect’s body, the psyllid becomes capable of infecting every tree it feeds on for the rest of its life.
The psyllid’s reproductive capacity makes containment extremely difficult. A single female can lay up to 800 eggs over her lifetime of several months. Eggs hatch in two to four days, and the full life cycle from egg to adult takes just 15 to 47 days depending on temperature. In warm climates like Florida, this can mean up to 30 overlapping generations per year. Even late-stage nymphs (not just adults) can acquire and transmit the bacterium, so the window for spreading is wide. Transmission rates of up to 100 percent have been documented under the right conditions.
How to Recognize It
The signature symptom is a blotchy, asymmetrical yellowing pattern on the leaves. If you imagine the leaf’s central vein as a mirror line, the yellow patches on one side won’t match the other. This is the single most reliable visual clue and distinguishes greening from simple nutrient deficiencies, which tend to produce symmetrical yellowing on both halves of the leaf.
Fruit from infected trees is small, lopsided, and often stays partly green even when ripe. Color develops abnormally, yellowing from the top of the fruit downward rather than evenly. The juice tastes noticeably bitter and salty compared to healthy fruit. Seeds are often aborted or dark. These fruit symptoms can take one to several years to appear after the initial infection, which is part of what makes the disease so insidious: a tree can harbor the bacterium and serve as a source of infection for neighboring trees long before anyone notices something is wrong.
Laboratory confirmation requires a DNA test (PCR) on leaf tissue. Because the bacterium is extremely difficult to grow in a lab and can hide inside an asymptomatic tree for years, genetic testing is the most reliable diagnostic method. A grower or researcher can process hundreds of samples in a few days with standard equipment.
The Economic Toll
Florida’s citrus industry offers a stark picture of the damage. The disease was first confirmed in the state in 2005, and production has been falling ever since. In the 2024-2025 season, Florida produced just 14.6 million boxes of citrus, down 28 percent from the previous season alone. Orange production specifically dropped 32 percent to 12.2 million boxes, and grapefruit fell 27 percent to 1.3 million boxes. For context, Florida was producing over 200 million boxes annually in the late 1990s. The collapse is driven by a combination of greening, hurricane damage, and growers abandoning citrus altogether because infected groves are no longer profitable.
The disease is no longer confined to Florida. USDA quarantine zones in California have been expanding as new infections are detected on residential properties in Orange and Riverside Counties. The most recent expansions added 29 square miles in the Capistrano Beach area and 129 square miles near Murrieta. California produces the majority of fresh-market citrus in the U.S., so the stakes of preventing widespread establishment there are enormous.
Managing Infected Trees
No treatment eliminates the bacterium from a tree permanently, but several strategies can slow decline and maintain some productivity. The most promising is trunk injection of an antibiotic called oxytetracycline, which was recently approved for commercial use in Florida. In trials on nine-year-old Valencia orange trees, injected trees produced up to 95 percent more fruit after one year and 121 percent more after two years of annual treatment, compared to untreated trees. Juice quality, fruit size, and color all improved significantly. Antibiotic residues in the harvested fruit remained below the allowed safety threshold.
The catch is that the treatment must be repeated. Bacterial levels in the leaves of injected trees returned to pre-treatment levels within a year, indicating that psyllids reinfected them. Still, economic analysis showed the yield gains more than paid for the cost of injections, making it a viable management tool rather than a cure.
Enhanced nutrition programs also help infected trees hold on longer. Research at the University of Florida found that a specific combination of ground-applied nitrogen at moderate rates (about 224 kg per hectare, split into biweekly applications) combined with foliar and ground applications of manganese and zinc at 9 kg per hectare each produced the best results for maintaining canopy volume and leaf health. Adding nutrients beyond those rates didn’t improve outcomes, suggesting there’s a sweet spot rather than a “more is better” approach.
What Home Growers Can Do
If you grow citrus in your backyard, the most important thing you can do is monitor for Asian citrus psyllids. Focus your inspection on new, tender growth, which is the only part of the tree the insects feed on. Adults are tiny, with mottled brown wings, and they rest on the undersides of leaves at a distinctive 30-degree angle with their heads nearly touching the leaf surface. They leap when disturbed. Nymphs are even smaller and move slowly along new shoots.
Psyllid activity peaks whenever your tree is pushing out new flushes of growth, typically in spring and summer. If you find psyllids or notice the asymmetrical leaf yellowing described above, contact your local agricultural extension office. In quarantine zones, moving citrus plants or plant material is restricted, and reporting suspected infections helps agencies track and slow the spread. Removing a confirmed infected tree, while painful, prevents it from serving as a reservoir that puts neighboring trees at risk.
Breeding for Resistance
The long-term solution likely lies in developing citrus varieties that can resist or tolerate the bacterium. Researchers are using gene-editing tools to modify defense-related pathways in citrus trees, specifically targeting genes that normally suppress the tree’s immune signaling. Early work has focused on a gene involved in a defense mechanism called systemic acquired resistance, which is essentially the plant equivalent of an immune memory system. By disabling the genes that dial this system down, scientists hope to create trees that fight off the bacterium more effectively. Results so far suggest that multiple genes are involved, and editing just one isn’t enough to confer full protection. The work is ongoing, but it represents the clearest path toward citrus varieties that can coexist with the disease rather than succumb to it.