Lasering Leaves Improves Treatment of Citrus Greening

A treatment involving laser marking has improved the success of antibiotics in treating a disease that has been ravaging Florida’s citrus crops for a decade.

Huanlongbing, also known as citrus greening, is caused by caused by a bacterium that destroys fruit production and eventually kills the tree.

Until now, cures have been ineffective. Now researchers from the University of Florida have developed a treatment involving lasers could greatly improve the success of antibiotic treatments under investigation.

The bacterium starves trees by attacking phloem — the water-transporting vascular tissue in the inntermost layer of bark — responsible for transporting sugars to developing roots and fruits, resulting in bitter, green, inedible fruits. Once it has fruited, the tree soon dies.

"Effective treatment of this disease has largely been limited by the inability to deliver antibacterial substances to the phloem," said Ed Etxeberria, a professor at The University of Florida's Citrus Research and Education Center. "Penetration of externally applied substances into trees is generally prevented by the presence of protective layers on leaves."

Etxeberria had previously used lasers in his research when he helped develop laser etching as an ecofriendly, paper-free way to label fruit, and was drawn back to them for their potential to allow direct contact of antibiotics with infected tissue.

The researchers found that laser marking leaves with 250-μm-diameter indentations significantly enhanced the penetration of foliar-applied substances across the cuticle of the leaves, into the phloem and throughout the tree. Depending on exposure, the indentation was as little as a single cell layer deep.

A citrus leaf was lasered with a dot matrix perforation pattern to increase penetration of antibiotics. Courtesy of E. Exteberria and P. Gonzalez, et al.

The researchers tested two-year-old Valencia orange trees grown in large pots in a greenhouse. They applied test solutions immediately after laser treating the leaves and examined how far the solutions traveled within each plant using specific fluorescent dyes. Applications of oils on leaf tissue after laser exposure prevented damage.

The team is developing a large-scale field application method using a model of the marking instrument that contains multiple nozzles for the laser light, antimicrobial spray and oil application. The overall system would offer the added advantage of lower application frequencies and hence reduction in chemical use, a condition that would lessen environmental impact, Etxeberria said.

The laser perforation method could also be applied to other botanical research, such as studying the velocity of phloem sap flow and for the delivery of other agrochemicals such as fertilizers, systemic fungicides and insecticides. The research was published in Applications in Plant Sciences (doi: 10.3732/apps.1500106).