Microscopic Atlas Tracks Huanglongbing Disease Progression Through Citrus Trees

Researchers at the University of Florida Citrus Research and Education Center have built a high-resolution microscopic atlas that maps the step-by-step development of citrus huanglongbing (HLB), also called citrus greening disease, long before visible decline takes over. Published in Phytopathology, the work links ultrastructural damage across multiple organs to the same underlying pathogen, Candidatus Liberibacter asiaticus* (CLas), an organism that targets the plant phloem.

HLB is notorious for its slow, uneven progression, but the cellular sequence driving symptom onset and organ failure has been difficult to resolve. Because CLas colonizes phloem-containing tissues, prior research often focused on single organs, leaving uncertainty about how disease mechanics differ between photosynthetic and nonphotosynthetic compartments over time.

To close this gap, the team performed repeated sampling from infected and healthy trees grown in both greenhouse and grove settings. Across four collection times during a year-long study, they applied transmission electron microscopy and light microscopy to leaves, multiple bark layers, stems and trunks, fruit-associated tissues, and reproductive structures, including albedo, pedicels, columella tissues, and seed coats. Root tissues were examined as well, enabling direct comparison with aerial photosynthetic regions.

The atlas shows that CLas infection triggers pronounced phloem cell death and callose deposition in leaves, fruit tissues, and bark of branches and trunks. Callose—a defensive polysaccharide—accumulates in sieve-related regions and can physically impede phloem transport. As transport efficiency drops, starch accumulates in affected photosynthetic tissues.

That metabolic shift is tied to downstream damage. The study reports chloroplast degeneration and the characteristic blotchy mottling and yellowing associated with HLB. In parallel, roots exhibit a contrasting pattern: instead of starch buildup, root tissues show starch depletion, consistent with carbon starvation.

These findings support a sequential model in which phloem shutdown initiates a cascade that drives aerial tissue dysfunction, while root collapse reflects systemic carbon imbalance. The resulting organ-specific damage helps explain why a single phloem-targeting pathogen can produce complex whole-tree decline.

By integrating temporal imaging across many organs, the researchers provide a resource designed to “guide readers through the HLB maze,” including insights for recognizing sieve elements at the ultrastructural level. For the citrus industry, the atlas offers a clearer framework for identifying when and where intervention could be most effective.