2019 ASHS Annual Conference

Huanglongbing (HLB) is rapidly destroying the Florida Citrus industry. To date, plant treatments and attempts to develop sustainable management practices have only provided modest improvements in controlling the disease impact on plant health and yield. Consequently, there is an urgent demand for innovative methods/strategies to look for HLB cures. Since plants produce an array of complex chemical substances (i.e., secondary metabolites) they are considered new resources for producing agents that could act as alternatives to antibiotics in the treatment of the causative agent (Candidatus Liberibacter asiaticus, CLas). Particularly, secondary metabolites form new sources of antimicrobial molecules to use against CLas. Since, the literature is replete with articles on identification and characterization of antimicrobial compounds in oak, the present study was aimed at screening the antibacterial properties oak leaf (Quercus laurifolia) extracts. Nine sweet oranges (Citrus × sinensis ‘Valencia’) grafted on ‘Volkamericana’ rootstock grown in the USDA-ARS greenhouse in Fort Pierce, FL, USA were employed. Six individual 1-year-old healthy plants were graft-inoculated via side-grafting with CLas-positive sweet orange bud sticks while 3 individuals were kept uninfected as control. Ct values corresponding to CLas titer from qRT-PCR were used to confirm that plants were either healthy or CLas-infected. After HLB infection was confirmed 3 individuals were chosen and treated with root drench and foliar intended spray applications (twice per week for 2 months) of oak leaf extract. This extract was prepared by macerating the oak leaves in distillated water and allowing to set over-night. After 6-month treatments a series of physiological parameters (i.e., stomatal conductance, chlorophyll contents, starch accumulation, electrolyte leakage), as well as nutrient content (N, P, K) and CLas titer, were measured. Additionally, a biochemical composition of the oak leaf extracts was also performed. As a result, the oak-treated plants overcome the infection and restored their physiological and nutritional levels which resulted in the same ranges of the healthy plants, while the un-treated plants showed classic HLB symptoms.