Solar thermotherapy reduces the titer of ‘Candidatus Liberibacter asiaticus’ and enhancing canopy growth by altering gene expression profiles in HLB-affected citrus plants

Authors: Doud, Melissa; Latza, Christina; HOFFMAN, M.T. - Former ARS Employee; LUO, WEIQI - North Carolina State University; Gottwald, Timothy; Duan, Ping; Armstrong, Cheryl

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/3/2017
Publication Date: 9/10/2017
Citation: Doud, M.S., Latza, C.L., Hoffman, M., Luo, W., Gottwald, T.R., Duan, Y., Armstrong, C.M. 2017. Solar thermotherapy reduces the titer of ‘Candidatus Liberibacter asiaticus’ and enhancing canopy growth by altering gene expression profiles in HLB-affected citrus plants. Horticulture Research. 4:17054. https://doi.org/10.1038/hortres.2017.54.
DOI: https://doi.org/10.1038/hortres.2017.54

Interpretive Summary: Citrus huanglongbing (HLB) disease has affected all surveyed members of citrus and due to limited control methods has resulted in the rapid decline of mature citrus groves worldwide. This disease is caused by three bacterial species, ‘Candidatus Liberibacter asiaticus’ (Las), ‘Ca. L. africanus’ and ‘Ca. L. americanus’ which have distinct variations in temperature sensitivity and tolerance. A previous study determined that that continuous thermal exposure of 40-42°C for a minimum of 48 hours was sufficient to significantly reduce bacteria in the citrus plant. In addition, healthy vigorous growth was seen in all treated trees. The goal of this research project was to see if it was feasible to expose citrus trees planted in the field to high temperatures and if so, determine if the amount of Las in the tree is affected by the treatment. To accomplish this work, portable clear plastic greenhouses and homemade tents made from opaque plastic sheeting were placed over citrus trees. Within the tent, temperatures increased above 40 °C for 3 to 8.5 hours per day through solarization. As a result of the thermotherapy treatment, bacteria titers were decreased in trees and the citrus trees produced new leaves which resulted in a denser canopy for up to two years. Although solar thermotherapy did not cure residential and commercial HLB-affected citrus trees, the treatment decreased the amount of bacteria in the trees and cause the citrus trees to grow better for a period of time.

Technical Abstract: Huanglongbing (HLB), a systemic and destructive disease of citrus, is associated with three species of a-proteobacteria, ‘Candidatus Liberibacter asiaticus’ (Las), ‘Ca. L. africanus’ and ‘Ca. L. americanus’. Previous studies have found distinct variations in temperature sensitivity and tolerance among these species. Las, the most prevalent and heat-tolerant species, can thrive at temperatures as high as 35 °C. Our earlier work has shown that Las bacteria in potted HLB-affected citrus were significantly reduced or eliminated when continuously exposed to high temperatures of 40 to 42 °C for a minimum of 48 h. To determine the feasibility and effectiveness of solar thermotherapy in the field, portable clear plastic enclosures were placed over commercial and dooryard citrus, exposing trees to high temperatures through solarization. Within 3-6 weeks after a treatment, most trees responded with vigorous new growth. Las titer was greatly reduced for 18 months up to 36 months after the treatment. Unlike with potted trees, exposure to high heat was not sufficient to eradicate the Las population under field conditions. This may be attributed to reduced temperatures at night, rather than continuous high temperatures, and failure to achieve therapeutic temperatures in the root zone. Despite the presence of Las in heat-treated commercial citrus, many trees continued to produce abundant flush and grow vigorously two to three years after treatment. These results indicate solar thermotherapy can be an effective component of an integrated control strategy for HLB.