A common symptom of Huanglongbing (HLB) in citrus trees is leaf chlorosis, where the leaves develop yellow blotches. HLB is caused by a bacterium that resides in the plant phloem, disrupting the flow of nutrients. While chlorosis also can indicate other plant health issues, it’s thought to be triggered by the presence of the HLB bacterium causing the disease.
To study the causes behind leaf chlorosis, UF/IFAS plant pathologist Nabil Killiny and a research team looked at 39 common chemical compounds found in both healthy and HLB-infected citrus trees. The samples studied were taken from tree phloem and xylem sap. Disease-infected trees showed an increase in several acids in the sap. One of these organic acids increased in both the xylem and phloem tissues of the HLB infected trees, leading Killiny to further study why this was happening. A later study proved that this organic acid was working as a plant signaling molecule in HLB-infected trees.
These results suggest that the organic acid has a negative effect on the biochemical systems that produce leaf pigment. Further research is needed to study other factors that contribute to the citrus greening disease development. This knowledge can also be used to determine whether other nutritional deficiencies or biochemical processes can be changed or treated to encourage better growth and health in HLB-infected trees.
Another area of research is the effect of oak leaf extract on HLB-infected trees. Travis Murphy and Tom Thayer observed a phenomenon in nature that citrus growing under oak trees are free of HLB. Based on this observation, Murphy and Thayer set up experiments with containerized citrus trees under oak trees and observed reduced HLB infection. They then set up field experiments to test the effect on HLB-infected trees where they observed that oak extract formulations they developed had a very positive effect on tree health and fruit yield and quality. Mr. Murphy shared these observations and efforts with UF/IFAS and USDA who then conducted a small collaborative greenhouse study.
The UF/IFAS/USDA team observed that HLB-infected Valencia greenhouse plants treated with oak leaf extract had lower levels of the HLB bacteria, increased chlorophyll (were greener), and increased leaf mineral nutrient levels. The USDA has recently setup a field trial of one hundred twenty-two mature trees that replicates Murphy and Thayer’s field experiment. Murphy and Thayer are currently conducting large-scale field trials with grower cooperators. These trials involve thousands of trees that are being treated with Murphy’s and Thayer’s oak extracts by the grower cooperators. Murphy and Thayer have found that the effects of their oak leaf extracts are sufficiently impactful to help with new management treatment programs to protect trees that are infected with HLB.