Influence of temperature on infection and establishment of 'Candidatus Liberibacter americanus' and 'Candidatus Liberibacter asiaticus' in citrus plants

The objectives of this work were (i) to determine the influence of temperature on infection of citrus by Candidatus Liberibacter asiaticus and Candidatus Liberibacter americanus, the two bacterial species associated with citrus huanglongbing (HLB) in Brazil, and (ii) to determine the influence of temperature on citrus colonization by Ca. L. asiaticus, which has taken over from Ca. L. americanus as the predominant species in Brazil since 2008. Two experiments were carried out with graft-inoculated Valencia oranges on Rangpur lime rootstocks. Immediately after inoculation the plants were maintained for 423 days in growth chambers under the following night/day temperature conditions: 17/22, 22/27 or 27/32 degrees C, with a dark/light photoperiod of 8/16 h. Infection and colonization of plants were determined using quantitative PCR (qPCR). Candidatus Liberibacter americanus did not infect the plants maintained at 27/32 degrees C; however, infection by Ca. L. asiaticus occurred at all studied temperatures. Two months after inoculation, Ca. L. asiaticus was distributed throughout the inoculated plants, with mean Ct values in the range of 3031 for leaves and 2528 for roots. Over time, Ca. L. asiaticus reached the highest titres in mature leaves (mean Ct value = 26.7) of citrus plants maintained at 22/27 degrees C. Candidatus Liberibacter asiaticus colonization of citrus plants was negatively affected by the daily temperature regime of 27/32 degrees C (mean Ct value in mature leaves = 33.6).