Getting to the Root of the Problem: Fastidious Pathogens [Updated]

UPDATE: Research highlighted in this Breakthrough helped lay the groundwork for a citrus greening treatment approved by the Environmental Protection Agency (EPA) in 2026. The treatment is expected to give citrus growers a new tool to help infected trees remain productive.

Can Researchers Grow Unculturable Pathogens?

Candidatus Liberibacter spp. are fastidious—or unculturable—pathogens that cause billions of dollars of damage to crops annually. Because these pathogens are difficult or impossible to grow in the lab, scientists have a harder time studying how they work and testing possible treatments.

With an initial $299,992 FFAR New Innovator in Food and Agriculture Research grant, matched by Southern Gardens Citrus for a total investment of $599,986, Texas A&M AgriLife Research scientists, led by Dr. Kranthi Mandadi, developed a way to grow fastidious bacteria in a laboratory setting and evaluate new treatments. The AgriLife team is using modified plant host tissues, hairy roots, to mimic the bacteria’s natural environment for its growth and to test different potential treatments to control the pathogens.

This breakthrough offers new tools for disease management strategies that can prevent crop and economic losses.

Growing Answers from the Roots

Because fastidious pathogens cannot be grown in a lab, developing treatments for diseases like citrus greening, which is devastating Florida citrus crops, is challenging. Finding solutions for these diseases requires a ready supply of the pathogens, ideally in a laboratory setting, and the ability to evaluate treatments quickly.

Inspired by seminal work on mammalian virus culturing in animal/host tissues that led to the 1954 Nobel Prize in medicine, Texas A&M University AgriLife researchers and collaborators successfully created a plant/host tissue system via hairy roots for culturing fastidious pathogens such as Candidatus Liberibacter spp. Researchers can produce hairy roots containing the pathogens on demand from host plant tissue and grow or maintain them in laboratory growth chambers to provide a continuous supply of the pathogen.

The hairy root system also enables high-throughput screening of multiple treatments, such as resistance genes, genome-editing/CRISPR targets and antimicrobial chemicals to control Candidatus Liberibacter spp. Researchers identified several potential treatments by screening hundreds of candidate therapies using this method in a fraction of the time it takes for traditional screening methods.

The researchers believe using the hairy root system to screen therapies will open new possibilities to fight several other devastating fastidious plant pathogens and threats.

Where the research led

Early investment in research tools can help pave the way for practical solutions for farmers. The screening system developed through this FFAR-supported research was later used to test spinach-derived antimicrobial proteins against the bacteria that cause citrus greening. Following FFAR’s initial investment, with additional research, field trials and investment from other partners, a citrus greening treatment based on those proteins received EPA approval in 2026. Florida field trials reported that treated trees produced up to 50% more fruit than untreated trees.

More About This Research

FFAR-funded research:

Irigoyen, S., Ramasamy, M., Pant, S., Niraula, P., Bedre, R., Gurung, M., Rossi, D., Laughlin, C., Gorman, Z., Achor, D., Levy, A., Kolomiets, M. V., Sétamou, M., Badillo-Vargas, I. E., Avila, C. A., Irey, M. S., & Mandadi, K. K. (2020). Plant hairy roots enable high throughput identification of antimicrobials against Candidatus Liberibacter spp. Nature Communications, 11, 5802. https://doi.org/10.1038/s41467-020-19631-x

Follow-on research supported by other funders:

Padilla, C. S., Irigoyen, S. C., Ramasamy, M., Damaj, M. B., Dominguez, M. M., Rossi, D., Bedre, R. H., Dawson, W. O., El-Mohtar, C., Irey, M. S., & Mandadi, K. K. (2025). Naturally occurring spinach defensins confer tolerance to citrus greening and potato zebra chip diseases. Plant Biotechnology Journal, 23(5), 1876–1878. https://doi.org/10.1111/pbi.70013

FFAR’s $299,992 investment in this research is part of the New Innovator in Food & Agriculture Research Award.