By Michael Rogers
It has now been two decades since huanglongbing (HLB), or citrus greening, was first detected in Florida. That announcement in August 2005 changed everything for the Florida citrus industry. At the time, Florida citrus was still a global powerhouse. Less than a year earlier, HLB had been confirmed in Brazil, and we were aware of its devastating reputation across Asia. Yet few of us could have anticipated just how transformative HLB would be for Florida citrus production.
Today, Florida’s citrus production is down by more than 90% from pre-HLB levels. The disease has been the single largest driver of this decline, though hurricanes, freezes and urbanization have certainly added to the challenge. Despite these hardships, the industry continues to persevere while our collective knowledge of HLB has grown immensely. The last 20 years of research have deepened our understanding of how HLB affects citrus and has provided practical tools for keeping trees productive while pursuing long-term solutions.
GLOBAL CONTEXT
HLB has long been recognized as the most serious disease of citrus worldwide. Historical records indicate it has existed in Asia for well over a century, possibly two. In those regions, where the disease became endemic generations ago, its effects were often accepted as part of “normal” citrus production losses. The symptoms of yellow shoots, blotchy mottled leaves, poor root systems, premature fruit drop and nutrient deficiencies were often attributed to other problems like soil fertility or fungal root diseases. As a result, relatively little research focused specifically on HLB before it reached the Americas.
When the disease arrived in Florida and Brazil in the mid-2000s, it presented an unfortunate but unique opportunity. For the first time, scientists could observe the before-and-after effects of HLB in a modern, well-documented production system. That comparison has allowed researchers to identify which physiological and yield declines are truly due to HLB, and to focus on interventions that mitigate those effects. In many ways, Florida has become the global laboratory for understanding HLB biology and management, reshaping how we approach both short-term management and long-term genetic solutions.
RETHINKING GROVE MANAGEMENT
If HLB forced one major shift, it was the realization that we can no longer grow citrus the way we did 20 or 30 years ago. Managing HLB requires a holistic focus on tree health, stress reduction and root/soil health. Many of the practices we use today, such as improved soil management, better irrigation and nutrient delivery, and the use of plant growth regulators (PGRs), did not receive much thought before HLB.
We’ve learned that HLB severely impacts the root systems of citrus trees, creating conditions that limit both nutrient and water uptake. Maintaining soil pH near 6 and improving soil organic matter through compost or other amendments can help support healthier roots and stimulate beneficial microbial activity. These soil management practices, in turn, enhance nutrient uptake in trees with phloem, the tissue responsible for transporting sugars and nutrients, compromised by HLB.
Balanced fertilizer programs, frequent low-dose (spoon-fed) nutrient applications and precise irrigation scheduling all help reduce tree stress and support recovery. The careful use of PGRs can help correct hormonal imbalances caused by the disease and improve fruit retention.
More recently, growers have adopted individual protective covers to delay infection in young trees and have begun using trunk injections of antibiotics to improve the health of trees showing HLB symptoms.
Taken together, these changes don’t cure HLB, but they extend the productive lifespan of trees. These practical tools are buying us the time that is needed for breeders and molecular scientists to bring more durable solutions to the field. However, time has already run out for many growers in the industry.
BREEDING FOR THE FUTURE
One of the most encouraging developments has been the identification and breeding of citrus varieties that show greater tolerance to HLB. When the disease first arrived, University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) researchers already maintained a diverse “library” of hybrid citrus genotypes as part of a long-running plant improvement program. As HLB swept through experimental blocks, we observed that some of these hybrids, though not originally bred for HLB resistance, performed markedly better under disease pressure. Those survivors became the foundation for targeted breeding efforts aimed at improving tolerance.
After 20 years of selection and field testing, we are now seeing the fruits of that work. New UF/IFAS releases show improved tolerance to HLB and are beginning to make their way into commercial groves. Growers planting these newer varieties are reporting better yields, improved canopy health and greater longevity compared to traditional cultivars like Hamlin and Valencia.
These new trees are not immune to HLB, but they tolerate HLB better. As more nurseries propagate these tolerant lines, and as management practices improve, growers have more options than ever for maintaining economically viable groves.
BIOLOGY BEHIND TOLERANCE
Much of the recent progress in understanding HLB tolerance comes from studying the physiological and molecular differences between susceptible and tolerant varieties. Research by UF/IFAS scientists has uncovered several key mechanisms.
HLB disrupts the plant’s phloem, blocking the movement of sugars and leading to starch accumulation in leaves. This blockage is often made worse by the plant’s own defense response. When the HLB bacterium infects phloem cells, the plant produces reactive oxygen species (ROS) as a defense signal. In susceptible varieties, ROS levels increase dramatically, triggering the formation of callose, a substance that plugs phloem channels. Unfortunately, this defense overreaction chokes off sugar transport, starving the roots and fruit.
In contrast, tolerant varieties seem to manage ROS more effectively. They maintain lower levels of oxidative stress and avoid excessive callose buildup. As a result, sugars continue to flow through the phloem, keeping the plant’s metabolism and fruit development closer to normal.
One gene in particular, NPR1, has attracted attention. It plays a central role in regulating the plant’s immune system. In transgenic citrus plants where NPR1 expression has been adjusted, the defense response is more balanced — strong enough to minimize HLB symptoms but not so strong that it harms the plant. These modified plants appear to have improved tolerance to HLB without the yield reductions associated with overactive disease response.
NEW TOOLS
Beyond traditional breeding and transgenic research, gene editing technologies such as CRISPR have opened powerful new doors. UF/IFAS scientists are now using CRISPR to make precise, single-gene changes that mimic natural mutations associated with possible HLB tolerance. Because these edits do not introduce foreign DNA, the resulting plants are considered non-transgenic, a significant regulatory and consumer advantage.
Several of these edited citrus lines are already in field trials. Two of the most promising candidates have been transferred to the Florida Department of Agriculture and Consumer Services budwood clean-up program and are being propagated for larger-scale testing. Some of these trials will soon include grower cooperators, allowing evaluation of performance under real-world production conditions.
At the same time, advanced molecular tools are being used to identify which genes are activated during the earliest stages of infection, specifically, the moment the psyllid injects the HLB bacterium into the plant. Understanding those early molecular “signals” are allowing researchers to design even more targeted gene edits, preventing infection before the bacteria ever become established. The ultimate goal is to create varieties that either resist the bacterium entirely or tolerate infection without yield loss.
WHAT’S NEXT
After 20 years of intensive work, we can confidently say that we understand HLB far better than we did in 2005. That doesn’t mean the fight is over, but it does mean that real, practical progress is being made. We’ve learned that there is no single “silver bullet.” The path forward will depend on a combination of approaches. This includes better root and soil management, new tolerant varieties, improved psyllid suppression and, eventually, resistant trees developed through advanced breeding and molecular biology.
The citrus industry’s resilience has been tested before. Just as we overcame freezes, hurricanes and other setbacks, we are steadily adapting to this new era. The next few years hold promise as new varieties, management practices and scientific discoveries converge.
Until then, our job as researchers is clear: to continue pushing forward, providing growers with tools they can use today while building toward durable solutions of tomorrow. And our message to growers remains the same: Don’t give up on citrus. By planting tolerant varieties, optimizing grove care and staying informed about emerging technologies, we can sustain production and prepare for the next generation of Florida citrus.
Michael E. Rogers is the center director and a professor at the UF/IFAS Citrus Research and Education Center in Lake Alfred.
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