Reliable sampling tissue and seasonality for consistent detection of 'Candidatus Liberibacter asiaticus' by qPCR

Authors: HAJERI, SUBHAS - Central California Tristeza Eradication Agency; Yokomi, Raymond - Ray

Submitted to: Current Agriculture Research Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2020
Publication Date: 2/7/2020
Citation: Hajeri, S., Yokomi, R.K. 2020. Reliable sampling tissue and seasonality for consistent detection of 'Candidatus Liberibacter asiaticus' by qPCR. Current Agriculture Research Journal. 8(1). https://doi.org/10.12944/CARJ.8.1.01.
DOI: https://doi.org/10.12944/CARJ.8.1.01

Interpretive Summary: HLB is associated with 'Candidatus Liberibacter asiaticus' (CLas), a phloem-restricted bacterium transmitted by the Asian citrus psyllid. Early detection of HLB, preferably before symptom appearance, is a critical need in California. Regulatory testing of CLas requires use of a direct pathogen detection method such as PCR and stipulates sampling of mature leaves. Such leaves are good for visual inspection for HLB symptoms but can be unreliable tissue for early detection and/or low titer infections. A data gap exists for sampling mature commercial citrus trees despite reports from other areas. This invitational paper provides information about phloem transport and the source/sink pathway that photoassimalates and CLas follow in citrus phloem which is critical to identify the most reliable tissue and seasonality for early detection of CLas in mature citrus trees in California.

Technical Abstract: Huanglongbing (HLB) is the most serious disease of citrus, putatively caused by the bacterium ‘Candidatus Liberibacter asiaticus’ (CLas), transmitted by the Asian citrus psyllid in the USA. Early detection and prompt response are key factors in the eradication or suppression of HLB epidemics in California. However, due to lack of visible symptoms, low titer and uneven distribution of CLas in a tree, selecting the best sample for early detection by qPCR is a major hurdle. Regulatory testing of CLas by qPCR stipulates sampling of mature leaves. Such leaves are good for visual inspection of HLB symptoms but can be unreliable tissue for early detection and low titer infections. From previous studies involving phloem-limited pathogens such as Citrus tristeza virus, Spiroplasma citri and CLas, sink tissue such as the new flush, fruit and roots showed high titer of these pathogens. However, when it comes to sampling large trees in commercial settings, data gaps exist despite valuable data available from previous studies. There is a need for a systematic sampling study to identify the most reliable tissue matched with its ideal season for early detection of CLas in mature citrus trees.