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.