One obvious symptom of HLB is leaf chlorosis — yellow blotches appearing on otherwise dark green citrus leaves. Although chlorosis can indicate other health issues, in trees affected by HLB, it’s thought to be triggered by the underlying bacterial infection causing the disease.
University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) plant pathologist Nabil Killiny conducted a study to explore this possibility. The study showed the chemical components of sap taken from healthy Valencia sweet orange trees and sap from counterpart trees affected by 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 disease-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 manner.
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 the disease, such as nutritional deficiencies.
Killiny’s project was made possible by state legislative funding for the UF/IFAS Citrus Initiative during the 2018-19 cycle.
See more HLB-related articles here.