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
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. — By Ashley Vargo, Texas A&M Agrilife
