Behavior of ‘Candidatus Liberibacter asiaticus’ in new growth of citrus suggests the application of a small tree system for evaluating anti-microbial compounds

Authors: Hilf, Mark; LUO, WEIQI - North Carolina State University

Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/14/2018
Publication Date: 8/30/2018
Citation: Hilf, M.E., Luo, W. 2018. Behavior of ‘Candidatus Liberibacter asiaticus’ in new growth of citrus suggests the application of a small tree system for evaluating anti-microbial compounds. Journal of Phytopathology. 108(10):1165-1171. https://doi.org/10.1094/PHYTO-12-17-0408-R.
DOI: https://doi.org/10.1094/PHYTO-12-17-0408-R

Interpretive Summary: ‘Candidatus Liberibacter asiaticus’ is a species of bacteria which is thought to be the cause of the citrus disease huanglongbing. In this manuscript we describe the finding that pruning small citrus trees which are infected with this bacteria causes the bacteria to rapidly move into the leaves of the new growth which is induced by the pruning. This is a predictable and repeatable phenomenon and we propose that this is a system which can be used to screen chemical compounds for antimicrobial activity. Application of a compound to the tree which results in fewer or no infected leaves after pruning could be a useful antimicrobial material. In previous studies screening of compounds took several months, whereas this small plant system can provide evidence of antimicrobial activity in 30 days.

Technical Abstract: ‘Candidatus Liberibacter asiaticus’ is a phloem-colonizing intracellular bacterial pathogen of citrus associated with the disease huanglongbing. A study of patterns of colonization and bacterial population growth in new growth of different citrus types was conducted by pruning infected citron, sweet orange, sour orange, mandarin, citrange and Citrus macrophylla trees to force the growth of axillary and adventitious shoots. The first three leaves on newly emerged shoots were harvested at 30, 60 and 90 days to assess colonization and population growth of ‘Ca. L. asiaticus’ using real time PCR (qPCR). Single trials were conducted with mandarin and citron, two trials each for citrange, sour orange and sweet orange, and four trials for C. macrophylla. In citron the proportion of colonized leaves increased significantly over time, with 67, 85 and 96% of leaves colonized at 30, 60 and 90 days, respectively. For the other citrus types the exact proportion of colonized leaves differed, but colonization exceeded 60% in mandarin, sour orange, and citrange, and exceeded 80% at 30 days in two trials with sweet orange and three trials with C. macrophylla, but there was no significant increase in the proportion of colonized leaves at 60 and 90 days. Bacteria were readily detected by 30 days in new leaves of all citrus types. Differences in the growth of the bacterial population between citrus types and at different times of the year were noted, but common trends were apparent. In general, bacterial titers peaked at 60 days, except in leaves of C. macrophylla where bacterial titers peaked at 30 days. The early and consistently high proportion of leaf colonization observed for new growth of sweet orange during two trials and for C. macrophylla during three trials indicates a near synchronous colonization of new leaves by 30 days. The consistency of this behavior and the readily detectable bacterial populations which are present by 30 days suggest that colonization of new growth by ‘Ca. L. asiaticus’ in these small trees could be adapted as a screening system for antimicrobial compounds, since a consistent and significant reduction in leaf colonization of new growth after application of a test compound likely would indicate a degree of antimicrobial activity.