Beneficial Horticultural Responses from the Application of Solar Thermotherapy to Mature Huanglongbing-affected Citrus Trees

Authors: Armstrong, Cheryl; Doud, Melissa; LUO, WEIQI - North Carolina State University; Zhao, Wei; Plotto, Anne; Bai, Jinhe; BALDWIN, LIZ - Retired ARS Employee; Manthey, John; RAITHORE, SMITA - Symrise Ag; Stover, Eddie; Duan, Ping

Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/13/2021
Publication Date: 5/5/2021
Citation: Armstrong, C.M., Doud, M.S., Luo, W., Zhao, W., Plotto, A., Bai, J., Baldwin, L., Manthey, J.A., Raithore, S., Stover, E.W., Duan, Y. 2021. Beneficial horticultural responses from the application of solar thermotherapy to mature HLB-affected citrus trees. HortScience. https://doi.org/10.1016/j.hpj.2021.04.008.
DOI: https://doi.org/10.1016/j.hpj.2021.04.008

Interpretive Summary: The devastating effects of the bacterial disease, huanglonging (HLB) have severely crippled the citrus industry in Florida. Since 2006, HLB has been the driving force behind the loss of more than $7.8 billion in revenues and > 7,500 jobs. Production costs have risen steeply as a result of insecticide programs used to reduce ACP populations and nutrient applications aimed at extending the production life of HLB-affected citrus groves. Thus, the development and full deployment of any cost-effective strategies that can control the Las bacterium or mitigate the effects of HLB in the United States are critically needed for the industry to survive and profit. One of the management strategies was developed by the practice of heat-treatment to HLB-affected trees, we first demonstrated in 2013 that a continuous thermal exposure for 2 days was sufficient to significantly reduce Las titer or eliminate Las bacteria entirely in potted HLB-affected citrus plants under controlled condition. To further develop thermotherapy into a field-applicable control strategy, we devised practical methods for treating mature trees in a field setting. Our initial study utilized a solar thermal technology, whereby a portable greenhouse structure was placed over the trees to harness the sun’s heat energy and expose residential and commercial citrus to temperatures >40°C. This method of solar thermotherapy not only reduced the titer of Las for 18 to 36 months, but also increased the vigor of HLB-affected trees. Furthermore, we investigated the effects of thermotherapy treatments on the plants beyond the scope of the associated pathogen titer. Herein, we report on the horticultural performance of HLB-affected commercially grown Valencia trees treated with solar thermotherapy. We not only monitored tree canopy density and Las titer in leaves but also determined fruit yield and juice quality after treatment. The results showed that solar thermotherapy resulted in improved juice quality and flavor in well-managed groves. The outcome of this study emphasizes the utility of solar thermotherapy as a part of an integrated management strategy for HLB-affected citrus.

Technical Abstract: The detrimental effects of Huanglongbing (HLB) on citrus are well known and so is the need for effective methods to combat this disease. Solar thermotherapy (ST), one of the management methods to help alleviate some of the negative effects of HLB, is the process of heating trees to therapeutic temperatures by encompassing the tree within a plastic structure (visqueen) to harness the sun’s natural energy. ST was applied to mature ‘Valencia’ citrus trees in three locations. Tree vigor, yield, and Candidatus Liberibacter asiaticus (Las) titer in leaves were monitored for two consecutive seasons post ST treatment, while fruit and juice quality were evaluated at the end of the second season. ST promoted an increase in canopy density in most groves tested and did not have a significant effect on fruit quantity, despite the prolonged exposure of the trees to increased temperatures. Moreover, Las titer was reduced in both leaves and juice processed from affected trees post solar thermotherapy. With respect to fruit quality, the ratio of total soluble solids to titratable acidity was higher post solar thermotherapy as was the sucrose content of the juice in most treated groves, whereas limonin was higher in juice from control trees. Principal components analysis of aroma volatiles showed significant differences between juice from treated versus control trees with volatiles imparting top-note and freshness to orange juice, such as acetaldehyde, hexanal, Z-3-hexenol and linalool, being higher in juice from treated trees. Taste panels confirmed that flavor differences existed, ultimately showing that solar thermotherapy resulted in improved juice flavor in well managed groves. Although the effects of solar thermotherapy on the trees appear temporary, such outcomes emphasize its utility when used as part of an integrated management strategy for HLB-affected citrus, imparting beneficial horticultural aspects with minimal to positive effects on subsequent juice flavor.