Review and prospect of thermotherapy for citrus huanglongbing

Abstract: The citrus Huanglongbing (HLB or greening), caused by the phloem-limited bacterium Candidatus Liberibacter spp, is a highly destructive citrus disease in the world. Nowadays, HLB is widely spread in China, USA, Brazil and other countries. Without effective control of infected citrus 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 infection by the heat treatment with hot water and water vapor saturated hot air. In a greenhouse circumstance, the nursery citrus tree with HLB infection gets healthy and pathogen reduction occurs after the light heat treatment at specific temperature and length of time. Compared with existing treatments for HLB, such as genetic improvement, seedling detoxification, and chemical disease control, the thermotherapy method of the physical disease control becomes the best choice for its treatment effectiveness, environmental friendliness and high efficiency in the field application. Combined with the thermotherapy application in microorganism and insect control, the field thermotherapy is beneficial for HLB control, and fruit production life extension of citrus tree infected with HLB. Based on the analysis of the research status and existing problems of field thermotherapy, several further study suggestions are proposed as follows: 1) Study and confirm the suitable threshold values of heat treatment temperature and timing of typical citrus trees infected with HLB, including the construction of indoor heater and field heat treatment testing system; 2) Establish the rapid HLB diagnosis and treatment effectiveness evaluation system, including establishing the visual diagnosis index and the treatment effectiveness evaluation index; 3) Study the interactions between heat treatment and suppression of HLB, including studying the specific threshold value of heat transfer enough to trigger the beneficial stress, establishing mechanism, confirming heat value for typical citrus, field testing to modify the mechanism; 4) Test and establish the heat transfer principle from different heat transfer mediums to citrus plant, including simulating the heat transfer in the air and soil from hot water and steam to citrus canopy and plant root, testing the temperature change between inside and outside surface of the phloem part, deducing the heat transfer coefficients and formulae, establishing the phloem model of heat transfer during the treatment process; 5) Design, improve and construct the high-efficiency, low-cost and light-weight heat treatment equipment in the field application suitable for the situation of mountain terrain in China; 6) Study the integrated approaches from combination of chemotherapy and thermotherapy, including studying the beneficial interactions to enhance the treatment effectiveness.