A review of applied methods for treating HLB-affected tree

Huanglongbing (HLB or citrus greening), caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas), is devastating the citrus industry around the world. HLB is widely spread in China, USA, Brazil and other countries. Without effective control of infected tree in the field, the citrus industry may meet a serious destruction in near future. Some promising progresses have been made in genetic improvement, seedling detoxification, chemical control, physical control and biological control. However, there is no effective field control method and equipment, with high efficiency and low cost, available for HLB control. It was already confirmed that, at suitable temperature and appropriate length of time, infected seedling and grafted branch could be free from HLB-affected tree by the heat treatment with hot water and steam. In contrary to other treatments for HLB, namely; genetic improvement, seedling detoxification, and chemical disease control, the thermotherapy method became the best choice for its treatment effectiveness, environmental friendliness and high efficiency in the field application. Finding the best combination of treatment time-temperature was one of the challenges as the CLas bacteria is not accessible and long time is needed to control the effectiveness of the treatment. Therefore, survival rates of a surrogate bacterium, Klebsiella oxytoca, was analyzed in order to evaluate the effectiveness of a mobile thermotherapy system developed for in-field treatment of HLB-affected trees. A standard temperature-time combination of 54°C for 90 s was chosen based on preliminary studies where heat treatment parameters caused a significant reduction in CLas populations without inflicting permanent damage to the tree. In a greenhouse circumstance, the nursery HLB-diseased tree gets healthy and pathogen reduction occurs after the light heat treatment at specific temperature and time.