Hope for the citrus industry

Life might be trying to take its lemons back.

Over the past few decades, citrus trees across the country have fallen victim to citrus greening, a disease that has wiped out large swaths of Florida’s once-thriving orange groves.

The disease has made its way to Texas and California, the next two largest citrus-producing states, and now threatens every glass of orange juice and slice of grapefruit along with the industries and growers they support.

From 2008 to 2012, the disease spread to Georgia, South Carolina, Louisiana, Texas and California. While initially only detected in residential areas in California, each passing year means an increased risk to large-scale commercial groves in the vicinity. In Texas, it has spread to both residential and commercial groves over the years.

To protect the economic and cultural impact of citrus, Texas A&M AgriLife Research scientists led by Kranthi Mandadi, Ph.D., are seeking novel solutions to support the citrus industry.

Mandadi, a professor in the Texas A&M Department of Plant Pathology and Microbiology and researcher at the Texas A&M AgriLife Research and Extension Center at Weslaco, has dedicated a decade to studying citrus greening. In the past few years, he and his collaborators have found new potential treatments for citrus greening, offering hope to the producers and consumers alike.

“We’ve seen a nearly 90% loss in citrus production in Florida over the past two decades,” Mandadi said. “Some people think of it as a cancer for citrus trees. If we don’t develop solutions, we’re eventually going to see the same results unfold across the country. But luckily, we’re now much closer to effective, sustainable solutions.”

Citrus greening’s national threat

Citrus greening, a bacterial disease also known as Huanglongbing, established in Florida in the early 2000s. The disease-causing bacterium is spread from tree to tree by the Asian citrus psyllid, a tiny insect found in groves across the state and country.

Once the bacterium infects plants, it spreads through the tree’s vascular system and blocks the flow of nutrients. Mandadi said that by the time symptoms appear, which can be years after the initial infection, it’s too late to recover the tree.

“Citrus greening is a silent killer,” he said. “Trees are often asymptomatic at first, and it’s not feasible for producers to regularly perform molecular diagnostics on all trees in a large orchard. Instead, growers have to apply frequent insecticides and hope their trees won’t be infected.”

After just a few years following its introduction, the U.S. Department of Agriculture estimated that Florida’s citrus production had dropped by 75%, while the cost of production had more than doubled.

Mandadi said Florida’s tropical climate likely supported the bacteria and insect’s ability to thrive, compounded by hurricanes and other environmental conditions that left trees more susceptible to infection and disease. Producers attempted to use more pesticides and remove infected trees to replant, but citrus greening persisted.

Recently, a few antibiotic-based products were approved for emergency use in citrus in Florida and gave some reprieve to growers, but more sustainable products are needed to stop citrus greening in the long term.

Fighting back in Texas

Despite the threat, Texas is in a better position to respond to citrus greening than Florida had been in, said Dale Murden, a citrus grower and president of the Texas Citrus Mutual and Citrus Pest and Disease Management Corporation located in South Texas.

For one, Texas’ drier climate keeps the disease from spreading as quickly. Additionally, our researchers and regulatory bodies in Texas had a head start on preparing for citrus greening’s inevitable arrival. Murden said Texas producers adopted aggressive pest management strategies and quarantine measures to slow the disease’s spread.

“We have a robust citrus industry here in Texas,” he said. “We have a lot at stake when it comes to our citrus, and we were prepared to respond after seeing the impact citrus greening had in Florida.”

Mandadi has been working with the citrus industry and producers to address citrus greening since its arrival, but, like others, he was initially stalled by the fastidious nature of the bacteria that causes it.

For over a decade, researchers had to play defense, using surrogate systems to test potential solutions or therapies against citrus greening — until Mandadi and his research team developed a breakthrough technology that would enhance research efforts not just for themselves, but for citrus greening researchers nationwide.  

Getting to the root of the issue with hairy root technology

While some plant pathogens can be grown as pure cultures in laboratories using a liquid or jelly-like nutrient medium, an estimated 99% of bacteria in the environment are fastidious, or unable to grow outside their native environment.

The bacterium causing citrus greening falls into this fastidious category and was unable to be grown as lab cultures — until 2020, when Mandadi and his AgriLife Research team developed an alternative approach to liquid cultures called the “hairy root” system.

The system works by growing roots from infected citrus leaves or branches, allowing scientists to create a natural growth environment for the bacterium to thrive in the hairy roots, and subsequently use them to screen numerous types of treatments at a speed that was previously not possible.

“The hairy root system allows us to mimic the natural environment the bacterium is adapted for,” Mandadi said. “We were suddenly able to test potential treatments at a much faster rate, between four to six times as fast, and on a larger scale.”

This breakthrough has also helped other collaborating researchers across the country and can be applied to other plant diseases. While Mandadi’s team developed the technique to study citrus greening, they have used a similar process to study other fastidious bacteria, including those that cause zebra chip disease in potatoes and Pierce’s disease in wine grapes.

One of the most encouraging findings from this research has been the identification of natural compounds that could directly counteract the challenging citrus greening pathogen.

The promise of peptides

Plants, animals and all other forms of life use small proteins called peptides for a variety of important roles in maintaining health.

One key function some peptides can perform is acting as a natural defense. Antimicrobial peptides are produced by most multicellular organisms as part of an innate defense system.

Using hairy root technology and the high throughput efficacy screening it offers, Mandadi’s team identified a few natural defensin peptides present in spinach that seem to be effective at killing the citrus greening bacterium.

Following this discovery, Mandadi’s team collaborated with University of Florida scientists who developed a citrus RNA viral vector capable of delivering the peptide directly to the infected trees’ vascular system to test the spinach peptides in infected fields in Florida.

Early results have shown promise at promoting the yield of infected trees by 30% to 50%. Mandadi said this treatment approach is novel, non-gene editing and relatively easy to apply, and it could reduce the reliance on pesticides.

“Peptide therapy takes a new approach to treating citrus greening, and I think it will set a precedent for other products in the future,” Mandadi said.

An important step before the spinach peptides can be available to the citrus growers is to get approval from federal regulators. Southern Gardens Citrus, a subsidiary of U.S. Sugar, and Silvec Biologics, a Maryland-based tech company, have been working with Mandadi’s team toward obtaining the regulatory approvals.

Earlier this year, Silvec Biologics submitted the peptides for regulatory approval with the Environmental Protection Agency, and is making headway in the various milestones.

The future of citrus in the U.S.

The race to save citrus is not just a fight for an industry, but an effort to preserve a vital part of American culture and agriculture. A single silver bullet might not exist for this complex disease, but a combination of innovative and practical solutions like the spinach peptides, along with other integrated pest and disease management approaches, may help keep citrus on the table for years to come. 

“The progress Mandadi and collaborators have made makes me hopeful for the future of our citrus industry,” Murden said. “We will have new, sustainable treatments that could keep citrus trees productive under disease pressure. They aren’t just answering research questions but developing field products to support growers so we can farm profitably.”

Mandadi said he’s hopeful as well. “I think the future of citrus is looking brighter,” he said. “We’re closer than ever to offering our spinach defensin peptides to producers in their ongoing fight against this devastating disease. And if we can protect these trees and the industry that sustains them, it would be a sweet success for us all.”