Cultural management of HLB: current status and on-going research

Huanglongbing (HLB), formerly known as greening, is a bacterial disease restricted to some Asian and African regions until two decades ago. Nowadays, causing bacteria and their vectors have spread to almost all citrus producing regions, and it is currently considered the most devastating citrus disease. HLB management can be approached in terms of prevention, limiting or avoiding pathogen and associated vectors to reach an area, or in terms of control, trying to reduce the impact of the disease by adopting different cultural strategies depending on infestation/infection levels. In both cases, control of psyllid populations is currently the best way to stop HLB spread. Best cultural actions (CHMAs, TPS system) to attain this goal and thus, able to limit HLB spread, and ongoing research in this regard is summarized in this review.

Candidatus Liberibacter: From Movement, Host Responses, to Symptom Development of Citrus HLB

Candidatus Liberibacter spp. are fastidious α-proteobacteria that cause multiple diseases on plants hosts of economic importance, including the most devastating citrus disease: Huanglongbing (HLB). HLB was reported in Asia a century ago, but has since spread worldwide. Understanding the pathogenesis of Candidatus Liberibacter spp. remains challenging as they are yet to be cultured in artificial media and infect the phloem, a sophisticated environment that is difficult to manipulate. Despite those challenges, tremendous progress has been made on Ca. Liberibacter pathosystems. Here, we first reviewed recent studies on genetic information of flagellar and type IV pili biosynthesis, their expression profiles, and movement of Ca. Liberibacter spp. inside the plant and insect hosts. Next, we reviewed the transcriptomic, proteomic and metabolomic studies of susceptible and tolerant plant genotypes to Ca. Liberibacter spp. infection and how Ca. Liberibacter spp. adapt in plants. Analyses of the interactions between plants and Ca. Liberibacter spp. imply the involvement of immune response in the Ca. Liberibacter pathosystems. Lastly, we reviewed how Ca. Liberibacter spp. movement inside and interactions with plants lead to symptom development.

Modelling ‘Candidatus Liberibacter asiaticus’ movement within citrus plants

The phloem-limited ‘Candidatus Liberibacter asiaticus’ (Las) causes huanglongbing (HLB), a destructive citrus disease. Graft-inoculated potted plants were used to assess Las speed of movement in phloem in greenhouse (GH), and the impacts of temperature on plant colonization in growth chambers (GC) experiments. For assessment of Las speed, plants were inoculated at the main stem and assessed over time by qPCR or symptom at various distances from the inoculum. For colonization, the plants were inoculated in one of two opposite top branches, were maintained at 8 to 20, 18 to 30 or 24 to 38ºC daily range, and assessed by qPCR of samples taken from non-inoculated shoots. For all experiments, frequencies of Las-positive sites were submitted to ANOVA and binomial GLM and logistic regression analysis. Probabilities of detecting Las in GH plants were functions of time and distance from the inoculation site, which resulted in 2.9 and 3.8 cm day-1 average speed of movement. In GC, the temperature impacted plant colonization by Las, new shoot emission and symptom expression. After a 7-month exposure time, Las was absent in all new shoot in the cooler environment (avg 3 per plant), and present in 70% at the milder (6 shoots, severe symptoms) and 25% in the warmer (8 shoots, no visible symptoms) environments. Temperature of 25.7ºC was the optimum condition for plant colonization. This explains the higher impact and incidence of HLB during the winter months or regions of milder climates in Brazil.