It’s hard not to associate Florida citrus with the devastating citrus greening disease, also known as huanglongbing (HLB). In the 20-plus years the pathogen has been in the state, Florida’s orange production has dropped from 244 million 90-pound boxes in 1998 to 12 million boxes for the 2024-25 season.
While that nearly 95% reduction in orange production has had significant consequences, a team of researchers at the University of Florida’s Institute of Food and Agricultural Sciences are learning more about HLB and its vector, the Asian citrus psyllid, to help growers combat the threat.
Citrus greening is a global issue, with Brazil now at the epicenter, having almost half of the country’s production impacted. The disease also runs rampant in Asia and Africa. As Tripti Vashisth, associate professor of horticultural sciences and citrus extension specialist with UF/IFAS, notes: The weather is similar to India, but growers in Florida have experienced much more significant loses.
“In my opinion, a lot has to do with the soil,” she says. “The soil in Florida, unfortunately, is sand, which has lower water-holding capacity, lower nutrient-holding capacity. But if you look in India, they have a heavy clay soil; so does China, Nepal and then even Brazil has a very-high-in-iron soil.”
Vashisth says along with soils that don’t hold moisture and nutrients, Florida growers have also faced significant damages from major hurricanes. Hurricane Wilma, which hit October 2005, shortly after greening was first detected, caused $180 million in damage; Hurricane Irma in 2017 caused nearly $760 million in damage; Hurricane Ian in September 2022 hit nearly 375,000 acres of citrus groves and caused about $675 million in damage; Hurricane Milton impacted 166,000 acres and caused about $55 million in damage.
“Once a hurricane hits, it’s just not that year, it’s [the] next couple of years that the trees are just getting back to the normal,” she says. “Milton is not isolated; we were still recovering from Ian, and it has been three hurricanes in six or seven years.”
Short-Term Solutions
As a horticulturalist, Vashisth focuses on what growers can do to help strengthen and create more resilient trees. She says growers have to manage irrigation, nutrition, plant-growth regulators, psyllid control and antibiotic injections; it’s a lot to balance.
“For the growers, it becomes really challenging that they have to be on top of each of these things, because once the tree has an infection, it is not as forgiving as a healthy tree,” she says. “If you miss fertilization on a healthy tree, it’s way more forgiving, because it has the reserves to run on ... but when it is sick, it needs everything all the time. It has become cumbersome for the growers, and that’s their biggest challenge — that there is no silver bullet and there are many different things that they can do.”
Vashisth says that when she started in 2014, it was believed that nothing could help the fruit drop associated with citrus greening, adding that growers can lose between 40% to 50% of their crop yearly due to fruit drop. However, she says she discovered growers applied plant growth regulators at the wrong time.
“After almost three years of research, we found that the signal that tells a fruit that it needs to drop arises almost three months before the actual drop happens,” she says. “So, you have to be applying those plant growth regulators three months before the physical fruit drop begins, which changes the complete scenario because most of the time people see the drop and then they want to apply, but that’s too late to apply.
“After that research, now we have better tools for controlling fruit drop, and it is quite successful,” she continues. “There are two plant growth regulators that we can use to control fruit drop, and it works. The timing is the critical part of it.”
Vashisth says she also looked into the application of gibberellic acid to help encourage leaf development in declining trees. A concern, though, is that encouraging a tree to develop leaves might take resources away from flower development and thus cause a reduction in fruit.
“We applied it, and we did see a reduction in flowering, but interestingly that doesn’t affect the yield of the tree,” she says. “In the end, we get the same amount of fruit. Basically, we reduce the number of flowers, we create more leaves. These leaves can now feed the fruit, and they have a better return.”
She says about 60% of the state’s citrus growers now use gibberellic acid.
“It’s not a silver bullet,” she says. “It’s just one more tool that can help the tree.”
Vashisth says Florida’s sandy soils make it difficult for trees to maintain the right nutrient balance. She says soil moisture levels are also critical for trees with citrus greening. She noticed the state’s dry season overlaps when valencia growers experience fruit drop, and it runs through flowering and fruit set for the next season. She says she found productivity improved by more than double when growers irrigate in small doses daily.
“Even though we are not applying more water, it’s more frequent,” Vashisth says. “The challenge here, from the grower point of view, is that I’m asking them to irrigate at least every day, even if they cannot do three times a day. I’m saying every day, and it takes human labor to turn on those pumps, get the irrigation going, and that’s a challenge.”
This is where she sees the potential for automatic systems and chemigation to help growers deploy gibberellic acid during months when growers aren’t using sprayers in November and December.
Vashisth says another challenge to research is that each citrus variety responds differently to HLB. Of the sweet orange varieties, hamlin is more susceptible than valencia. Murcott mandarins can drop up to 60% of its fruit if not managed properly. She says rootstock, too, has different responses to greening.
“Every variety has a different response, and it just depends on where they are grown,” she says. “Soil has a lot to do with how the trees respond. The same treatment at one place may result different have different results at different places.”
Vashisth decsribes her work with growers on improving tree health as a partnership, noting she’s trying to help growers manage groves with the best tools available. She says a major focus of her research going forward will be looking at abiotic stress and its impact on fruit quality.
“We are doing things that can be short-to-mid-term goals so we have something that can be adopted right away,” she says.
Whole System Disease
Megan Dewdney, associate professor of plant pathology and Extension specialist with UF/IFAS, says growers in Florida had been lucky up until the onset of citrus greening, as there weren’t many significant diseases or insects to manage. Going from changes in production and income to a different management technique was an adjustment, she says.
“They also had to sort of get their heads around going from that very gentle method of plant management to a much more aggressive, and that’s a very much a mindset change,” she says. “We’re talking about at the same time as a biological problem; we’re also talking about almost a sociological issue.”
Dewdney, who joined UF/IFAS in 2008, says while HLB looked like a rapid onset, she thinks it was a slower progression.
“It looked like it was explosive, but I suspect it was more of a gradual creep, but it was silent,” she says.
Dewdney says she works a lot with her entomological colleagues, as the Asian citrus psyllid is a vector of the disease. She says it’s important to understand as much about the insect and the pathogen.
“One of the huge challenges with this disease is the fact that it is within the vascular system, but the fact that it can kind of hide there for a long time in an older, healthy tree,” she says. “So, a tree can look relatively healthy for three, four, five years before it starts to really succumb. And that whole time, if you’ve got psyllids around, they’re able to feed on it and potentially pick up the pathogen and move it along. And so, it’s sort of sitting there silently.”
Dewdney says trees might not exhibit any symptoms or might just look off, so a grower might think it’s a fertilizer issue or something else other than greening. While a tree infected with HLB looks more stressed, other pathogens might cause additional issues. She says she sees more prevalence in twig dieback and stem end rot in trees infected with greening. While thought of as a postharvest disease, stem end rot now shows up in the field.
“When a weakened fruit starts to detach, it doesn’t fully detach,” she says. “Now you see these organisms going in, and you see the stem end rot on the tree instead of where you’d normally see it in the packinghouse, but that fruit is not heavy enough to drop, because it’s deficient.”
Dewdney says HLB has also complicated growers’ management of phytophthora. She says HLB-infected roots attract phytophthora zoospores. Also, systemic products for phytophthora management don’t work as well, she suspects, in part due to the tree’s weakened vascular system. Greening also affects flowering patterns, Dewdney says, noting that a branch or branches might flower early, which can allow a pathogen that causes post-bloom to build up inoculum.
“If you’re unlucky enough to have the right weather conditions at bloom, which is wet and wet and warm, you see explosive bloom, explosive disease in the field, because it’s been building up over spring on all these weakened trees,” she says. “This has the secondary effect that this disease [has], called post-bloom fruit drops. It makes the little fruitlets fall off, and then you don’t have any fruit this just before you even get to worrying about anything.”
She notes that growers often start young trees, which are very susceptible to HLB, under individual protective covers, or IPCs, which are mesh bags that protect the young citrus trees from pests. However, growers still need to be proactive with insect and disease control. An unprotected tree will die within a year, while an established citrus tree that gets infected at year eight or longer will have a slower decline.
Dewdney says that before the prevalence of citrus greening, it would take growers about seven years to get to a profitable production level.
“Now, I don’t know if it’s even if it’s that short anymore to get to profit, but the trees don’t necessarily last longer than three, four, five years,” she says. “It is a tree canopy disease because it absolutely decimates systems.”
Dewdney says she and a research team will look at trunk injections of oxytetracycline and its alternatives, as well as the optimal timing of applications for young trees.
“I don’t think we’ve seen too much that looks much better than the oxytetracycline so far,” she says. “There’s always hope that something a little less difficult to work with and less controversial.”
All the while, growers face tough decisions on whether to stay in the citrus business.
“We’re talking about family farms, businesses that have been in the families for multiple generations,” Dewdney says. “It’s just heartbreaking to watch this. I’ve had growers that are practically in tears on the phone with me, saying, ‘I don’t know how much longer I can go.’ That’s really hard to listen to. We’re not going to hang up on that poor person, because you know that they’re going through an absolute crisis.”
A Look Ahead
While seedless membranes and flavor are obviously important, the ultimate goal is to have HLB-resistant cultivars and rootstock, says Manjul Dutt, assistant professor of horticultural sciences with a focus on citrus and subtropical fruit breeding and genetics at UF/IFAS.
“Genetic resistance or genetic tolerance to HLB and other diseases remains the only long-term solution,” he says. “We need all those strategies to ensure that the grower is able to have a profitable crop in the short term, but the bottom line is you need to have tolerance and, if you get very lucky, resistance.”
Traditional breeding used to be a nearly 20-year process, but thanks to modern technology, Dutt says it’s closer to 10 years. A challenge to developing new cultivars is that sweet oranges and grapefruits have limited genetic diversity, which also limits genetic resistance.
In grapefruits, Dutt says he modifies citrus embryonic callus cells, which he says are similar to human stem cells, through conventional methods or using biotechnology. These then get placed in growing mediums with plant hormones to develop into new citrus plants.
“Using plant biotechnology, plant genetics, you can actually accelerate the process of hopefully trying to find that one needle in the haystack,” he says.
Wild strains could be a source of new genetic diversity, but they often have unpleasant tastes, so Dutt sees wild strains as a good source of resistant and tolerant rootstock. He says having both resistance and tolerance in the scion and the rootstock will be the best defense against citrus greening.
“We are screening large populations of seedlings to identify natural variation, because natural variation happens,” he says. “We are also using many tools to actually induce other kinds of variation, such as genome editing and the development of GMO plants.”
And he says an HLB-tolerant rootstock developed through using CRISPR could support a tolerant non-GMO sweet orange variety.
“Citrus breeding is a very slow and long-term process,” he says. “You have to use multiple strategies. We have all these different strategies, conventional breeding, GMO, CRISPR, you name it. We’re all doing that at the same time. Now it’s more work for my folks, more work for me, but there is an urgent need to get that done.”
He says his team focuses primarily on sweet oranges and grapefruits but also does a little research in mandarins.
“The first objective is for us to stabilize the industry,” he says. “Even if I have a breakthrough in a mandarin that’s not going to stabilize our industry, we need to have breakthroughs with sweet oranges with grapefruits, because that’s the bulk of our industry.”
But a happy accident did happen along the way in Dutt’s research. As his team incorporated finger limes into the research program due to its high tolerance to citrus greening, Dutt developed two red-pigmented finger lime varieties. He says, along with its promising potential for growers, he sees finger limes as a potential source for HLB-tolerant rootstocks.
Dutt says his team is also looking into the potential of finger limes in a breeding program.
“We know it has good genetic potential, but what can we do with it?” he says. “We did an experiment to see how it works as a rootstock. It worked well. Will it work well as a parent? We don’t know, and that’s too early, but we do have hybrid populations with UF SunLime as one of the parents. So, you know, science never stops.”
He says he and his research team have hundreds of trees being evaluated for tolerance and resistance to HLB, and there’s a team comparing the resistance and susceptibility of related cultivars to learn what triggers the tolerance and resistance.
“And if you can identify those differences, can we utilize those to build better citrus?” he says.
Dutt says, as a citrus breeder, he and his team deploy a combination of breeding techniques developed more than 200 years ago and modern breeding strategies such as CRISPR and artificial intelligence.
“It’s an exciting time to be a plant breeder, I think, because there’s a huge potential in actually making a significant contribution to the field, given what you’ve seen with HLB,” he says.
