Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/1/2011
Publication Date: 2/1/2012
Citation: Zhang, M., Powell, C., Guo, Y., Doud, M.S., Duan, Y. 2012. A graft-based chemotherapy method for screening effective molecules and rescuing Huanglongbing (HLB)-affected citrus plants. Journal of Phytopathology. 102:564-574. Interpretive Summary: Huanglongbing (HLB) is a devastating bacterial disease of citrus worldwide. There was no cure for this disease, and only a limited number of compounds may be used to eradicate the bacteria in the affected trees. In this study, a method was designed to screen chemicals for their HLB control and evaluate their phytotoxicity to citrus. This screening method used an infected citrus scion which can transmit the disease once grafted to the rootstock. The HLB-affected lemon scion proved to have a high bacterial transmission rate and survival rate while still containing a high amount of bacteria. The effectiveness and phytotoxicity of the chemicals were studied by soaking this infected scion in various antibacterial solutions, grafting it to a healthy rootstock, and then measuring the bacterium in the scion at a later time period. A mixture of penicillin and streptomycin was effective in eliminating the HLB bacterium from the infected citrus scion and thereby preventing the spread of HLB to the rootstock, and rescuing the HLB-affected trees. Using the graft-based screening method, other chemicals that control HLB may potentially be identified. In addition, this method can be used to rescue infected scions that can then be propagated in disease-free citrus nurseries. Lastly, the newly designed chemical screening method can be implemented to establish new controls for similar diseases.
Technical Abstract: Huanglongbing (HLB), the most devastating disease of citrus worldwide, is associated with three species of fastidious and phloem-limited a-Proteobacteria in the genus of ‘Candidatus Liberibacter’. We previously reported a regeneration-based chemical screening system using periwinkle cuttings. In this study, a graft-based method was designed to evaluate the best scion and rootstock combination in order to rapidly screen potential chemical compounds for the control of HLB while simultaneously assess their phytotoxicity to citrus. By comparison, the HLB-affected lemon scions were proved to be excellent scions by having a higher titer of Las bacterium, a greater survival rate, and a higher rootstock transmission rate. Compounds individually or in combination were effectively evaluated via soaking the lemon scions in the antimicrobial solutions and grafting them onto the Las-free citrus rootstocks. As our previous reports, a mixture of penicillin and streptomycin (PS) was effective in eliminating the HLB bacterium from the infected citrus scions. Using this method with PS, the severely HLB-affected citrus germplasms were successfully rescued. This graft-based method will be useful for determining other HLB control compounds, propagating disease-free citrus nurseries, and also has the potential to be used as a screening method to identify chemotherapy treatments for similar diseases.