Quercus leaf extract contains and suppresses HLB disease

Authors: PITINO, MARCO - University Of Florida; STURGEON, KASIE - University Of Florida; Dorado, Christina; SHATTERS, ROBERT - US Department Of Agriculture (USDA); CANO, LILIANA - University Of Florida; Manthey, John; ROSSI, LORENZO - University Of Florida

Submitted to: Nature Plants
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/9/2020
Publication Date: 1/12/2020
Citation: Pitino, M., Sturgeon, K., Dorado, C., Shatters, R.G., Cano, L.M., Manthey, J.A., Rossi, L. 2020. Quercus leaf extract contains and suppresses HLB disease. Nature Plants. 148:70-79. https://doi.org/10.1016/j.plaphy.2020.01.013.
DOI: https://doi.org/10.1016/j.plaphy.2020.01.013

Interpretive Summary: Huanglongbing (HLB) also known as citrus greening disease is transmitted by a small insect and currently impacts the major citrus producing areas in the United States and around the world. Since HLB was initially encountered in Florida in 2005, there has been a 70% reduction in orange production. There is currently no cure of HLB and there are no resistant varieties of citrus that exist to date. Once a tree is infected fruit production and quality are negatively impacted until ultimate tree death. Symptom mitigation methods including improved tree nutrition, aggressive pesticide and bactericide use and pulling of infected or dead trees have led to a 200% increase in grove maintenance costs leading to abandoned groves and loss of agricultural lands to sale for other use. These methods do not cure the tree and at best delay the inevitable tree death. There are also concerns over the environmental impact of long-term use of these methods. A low cost, environmentally friendly and effective method for treating HLB infected citrus trees is desperately needed. Oak trees are common in the state of Florida and citrus trees growing under their cover have been found to have little to no HLB infection or observable symptoms. Experiments in which water extracts of oak leaves were applied to HLB infected citrus trees have been found to reduce HLB infection and improve tree health. The results of this work suggest that the water extract of oak leaves could be used as an effective treatment for HLB infected citrus trees.

Technical Abstract: Huanglongbing (HLB) is one of the most destructive citrus diseases worldwide. It is caused by the bacterium Candidatus Liberibacter asiaticus (CLas) and vectored by the Diaphorina citri psyllid. Currently, there is no cure for HLB, no compounds have been successful in controlling HLB, and no sustainable management practices have been established for citrus greening disease. Thus, searching for alternative disease mitigation strategies is considered an urgent priority for the citrus industry. In this study, application of oak leaf extract showed substantial inhibitory effects against CLas in citrus plants. Significant differences were also observed in plant phenotypic and physiological traits after treatments. Typical HLB symptoms, stomatal conductance, chlorophyll contents, electrolyte leakage, starch content and nutrients uptake diminished concurrently with a reduction of CLas titer. The information provided from this study suggests a new management treatment program to effectively deal with the HLB disease.