Heat treatment eliminates ‘Candidatus Liberibacter asiaticus’ from infected citrus trees under controlled conditions

Authors: HOFFMAN, MICHELE - University Of Arizona; Doud, Melissa; WILLIAMS, LISA - Florida Department Of Agriculture And Consumer Services; ZHANG, MU-QING - University Of Florida; DING, FANG - University Of Florida; Stover, Eddie; Hall, David; ZHANG, SHOUAN - University Of Florida; JONES, LISA - Florida Department Of Agriculture And Consumer Services; GOOCH, MARK - Florida Department Of Agriculture And Consumer Services; FLEITES, LAURA - University Of Florida; DIXON, WAYNE - Florida Department Of Agriculture And Consumer Services; GABRIEL, DEAN - University Of Florida; Duan, Ping

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/31/2012
Publication Date: 1/1/2013
Citation: Hoffman, M.T., Doud, M.S., Williams, L., Zhang, M., Ding, F., Stover, E., Hall, D., Zhang, S., Jones, L., Gooch, M., Fleites, L., Dixon, W., Gabriel, D., Duan, Y. 2013. Heat treatment eliminates ‘Candidatus Liberibacter asiaticus’ from infected citrus trees under controlled conditions. Phytopathology. 103:15-22.

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. The objective of this study was to determine if controlled heat treatments of HLB-affected plants would suppress or eradicate Las which is the most prevalent and heat tolerant species affecting citrus crop production. In this study, HLB infected citrus trees and HLB positive periwinkles were placed in a temperature-controlled growth chamber for time periods ranging from 2 to 10 days using different temperature treatments (40°C, 42°C and 45°C). After treatment, the quantity of Las bacterial DNA significantly decreased in the infected citrus and was undetectable in many samples. In addition, healthy vigorous growth was seen in all surviving trees. The results indicate that continuous thermal exposure of 40-42°C for a minimum of 48 hours is sufficient to significantly reduce bacteria in the citrus plant. This treatment approach may be useful to control Liberibacter-infected plants in nursery and greenhouse settings. We are currently developing several heat chamber prototypes suitable for performing heat treatments in field situations.

Technical Abstract: Citrus huanglongbing (HLB) is one of the most destructive diseases of citrus worldwide. The causal agents of HLB are three species of a-proteobacteria, ‘Candidatus Liberibacter asiaticus’ (Las), ‘Ca. Liberibacter africanus’ and ‘Ca. Liberibacter americanus’. Previous studies have found distinct variations in temperature sensitivity and tolerance among these species. Here we describe the use of controlled heat treatments to cure HLB caused by Las, the most prevalent and heat tolerant species. Using temperature-controlled growth chambers, we evaluated the time duration and temperature required to suppress or eliminate the Las bacterium in citrus, using various temperature treatments for time periods ranging from 2 days to 4 months. Results of quantitative polymerase chain reaction (qPCR) after treatment illustrate significant decreases in the Las bacterial titer, combined with healthy vigorous growth by all surviving trees. Repeated qPCR testing confirmed that previously infected, heat treated plants showed no detectable levels of Las, while untreated control plants remained highly infected. Continuous thermal exposure to 40-42°C for a minimum of 48 hours was sufficient to significantly reduce titer or eliminate Las bacteria entirely in HLB-affected citrus seedlings. This method may be useful for the control of Liberibacter-infected plants in nursery and greenhouse settings.