Efficacy of In-field Thermotherapy in Comparison and Combination of Defoliation for Mitigating Huanglongbing in Sweet Orange

Abstract Huanglongbing (HLB), a bacterial disease, is one of the most destructive citrus diseases. For many decades, it has been known that using heat/high temperatures (thermotherapy) is effective in suppressing plant diseases, particularly the suppression of Candidatus Liberibacter (CLas, casual agent for HLB) when the treated plants are grown in pots (allowing treatment of roots) under controlled conditions. However, in-field thermotherapy has yielded inconsistent results: the tree shows vigorous and symptomless growth for a brief period and then relapses with HLB symptoms. To understand why, this 2-year study was conducted to evaluate the efficacy of in-field thermotherapy and its comparison with defoliation. A significant reduction in visible tree health, foliage, and yield was observed over time in all the treatments. The quantitative real-time polymerase chain reaction (qRT-PCR) results showed that in-field thermotherapy and other treatments were not effective in reducing CLas titer. Interestingly, the performance of thermotherapy and partial defoliated trees were comparable throughout the course of the experiment, indicating that the short-term effects and vigorous growth after thermotherapy are likely an artifact of defoliation and should not be confused with or considered recovery from disease. In an in-depth molecular and biochemical analysis, we found a few subtle transient (up to 15 days) differences occurring in the in-field thermotherapy trees. Genes associated with stress and plant defense response were observed to be altered by in-field thermotherapy. Overall, our results indicate that in-field thermotherapy is not an effective and reliable strategy for mitigating HLB in commercial citrus production and that its efficacy within in-field conditions is similar to manual defoliation. It is critical that any strategy aimed at mitigating HLB should target the whole plant, including the roots, as the CLas colonizes in shoots and the root system; therefore, it can possibly translocate within the entire plant with the bulk phloem flow.