Mitigating citrus huanglongbing via effective application of antimicrobial compounds and thermotherapy

Huanglongbing (HLB) is a serious citrus disease that threatens the citrus industry worldwide. HLB is a systemic, infectious disease and the putative causal bacterium Candidatus liberibacter asiaticus (Las) resides in citrus phloem. In this study, the effects of heat treatment, chemical formulations, and application methods on bacterial titer in the citrus host, measured by quantitative PCR, were evaluated. Thermotherapy at 45 °C (Ct = 30.79 ± 6.26) was more effective against Las than thermotherapy at 40 °C (Ct = 28.72 ± 6.00) or 42 °C (Ct = 28.02 ± 6.25), and enhanced the delivery efficiency of effective chemical compounds into the citrus phloem. Ampicillin sodium (Amp) and a combination of actidione and validoxylamine A (Act + VA) were the most effective chemical formulations against Las. When Amp or Act + VA treatment was combined with thermotherapy, the area under disease progress curve standardized (AUDPCs) was significantly lower than the thermotherapy treatments alone. The Las titer of Amp and Act + VA treatment applied by bark paint were significantly reduced, compared with those of Amp or Act + VA applied by root drench. Therefore, we propose a chemo-thermotherapy strategy coupled with a bark paint application method for disease control in HLB-infected citrus plants.