Temporal Progression of ‘Candidatus Liberibacter asiaticus’ Infection in Citrus and Acquisition Efficiency by Diaphorina citri

Abstract

Over the last decade, the plant disease huanglongbing (HLB) has emerged as a primary threat to citrus production worldwide. HLB is associated with infection by phloem-limited bacteria ('Candidatus Liberibacter' spp.) that are transmitted by the Asian citrus psyllid, Diaphorina citri. Transmission efficiency varies with vector-related aspects (e.g., developmental stage and feeding periods) but there is no information on the effects of host-pathogen interactions. Here, acquisition efficiency of 'Candidatus Liberibacter asiaticus' by D. citri was evaluated in relation to temporal progression of infection and pathogen titer in citrus. We graft inoculated sweet orange trees with 'Ca. L. asiaticus'; then, at different times after inoculation, we inspected plants for HLB symptoms, measured bacterial infection levels (i.e., titer or concentration) in plants, and measured acquisition by psyllid adults that were confined on the trees. Plant infection levels increased rapidly over time, saturating at uniformly high levels (≈10(8) copy number of 16S ribosomal DNA/g of plant tissue) near 200 days after inoculation-the same time at which all infected trees first showed disease symptoms. Pathogen acquisition by vectors was positively associated with plant infection level and time since inoculation, with acquisition occurring as early as the first measurement, at 60 days after inoculation. These results suggest that there is ample potential for psyllids to acquire the pathogen from trees during the asymptomatic phase of infection. If so, this could limit the effectiveness of tree rouging as a disease management tool and would likely explain the rapid spread observed for this disease in the field.

Spatiotemporal dynamics of the Southern California Asian citrus psyllid (Diaphorina citri) invasion

“…citri , has been documented in regions throughout the world, including a recent emergence in the Western Hemisphere [10, 56, 57]. In 2012, the first HLB positive test in a hybrid-lemon citrus tree was identified in a residential neighborhood in Los Angeles County [18], confirming the relative importance of residential areas for the likelihood of HLB-related damage resulting from D .…”

“…Given the long latent period, coupled with the ability of D . citri to acquire the pathogen several months before detection in trees is possible [56, 57], it is likely that other cases exist but have yet to manifest. In regions where HLB is more widespread (i.e.…”

Asymptomatic spread of huanglongbing and implications for disease control

“…These delays are as follows: (i) the time from initial infection of young flush on a tree until the onset of disease symptoms and (ii) the time from initial infection of young flush by adult psyllids until the flush become infectious. An experimental study shows a minimum time to symptoms of ∼200 d and a latent period bounded above by 60 d (1).…”

“…This strategy has had only marginal effect because symptoms appear anywhere from months to years after an initial infection, long after the trees have been active in the transmission process (1). Presymptomatic trees can serve as a source of inoculum for psyllids, but the length of time from initial infection of a tree until it is infective as a source of inoculum is known from experimental measurements only to within coarse bounds (within 60 d) (1). An alternative statistical approach to infer latent and incubation periods from successive snapshots of symptom spread in groves has recently been published (2) (SI Appendix, section S.2).…”

Host and environmental factors influencing ‘Candidatus Liberibacter asiaticus’ acquisition in Diaphorina citri

“…the time required for D. citri adults and nymphs confined with CLas‐infected host‐plant tissues to acquire the pathogen. CLas acquisition rates by D. citri are highly variable, and recent investigations using molecular detection of CLas have reported acquisition rates varying from 1% to 90% among D. citri adults when feeding from CLas‐infected plants . Acquisition may be influenced by a number of factors that may include the life cycle stage of the psyllid .…”