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Relationship Between HLB and Virulence Factors and/or Plant Signaling Molecules

At UF/IFAS, we are working on finding solutions for Florida’s citrus growers. This is a summary of one project made possible by state legislative funding for the UF/IFAS Citrus Initiative during the 2018-19 cycle. It documents how we are making progress and  providing Florida growers with reasonable, pragmatic solutions to successfully grow citrus in the new age of citrus greening.

Researcher: Nabil Killiny, Plant Pathology

IMPACT: A key chemical responsible for HLB-disease symptoms in citrus trees was identified. This finding may lead to developing strategies to better manage diseased trees and enhance overall tree health. This information will be used in future studies designed to reverse disease symptoms and improve citrus growth and production.

One obvious symptom of citrus greening disease is leaf chlorosis — yellow blotches appearing on otherwise dark green citrus leaves. Although chlorosis can indicate other health issues, in trees affected by greening, it’s thought to be triggered by the underlying bacterial infection causing the disease.

To explore this possibility, UF/IFAS plant pathologist Nabil Killiny of the UF/IFAS Citrus Research and Education Center conducted a comparison study showing the chemical components of sap taken from healthy Valencia sweet orange trees, and sap from counterpart trees affected by citrus greening, also known as Huanglongbing or HLB. Killiny and his colleagues identified 39 compounds common to healthy and HLB-affected trees, and they noted increased amounts of several acids in sap from the HLB-affected trees. One of these, an organic acid, had increased in both the xylem tissue — which circulates water throughout the tree tissues — and the phloem tissue — which distributes nutrients in a similar manne r. This finding led Killiny to hypothesize that the organic acid may function as a plant signaling molecule in HLB-affected trees, capable of traveling through the xylem into bacteria-free tissue and producing leaf chlorosis there. A follow-up study confirmed the hypothesis, showing that applications of this acid caused healthy Valencia trees to develop leaf chlorosis. This finding suggests that increased organic acid production in HLB-affected trees has a negative effect on the biochemical systems responsible for producing leaf pigment. Also, now that organic acid has been shown to induce some HLB-like disease symptoms in healthy trees,
researchers may use it to investigate factors that contribute to the development of HLB, such as nutritional deficiencies.