Liberibacters Associated with Citrus Huanglongbing in Brazil: 'Candidatus Liberibacter asiaticus' Is Heat Tolerant, 'Ca. L. americanus' Is Heat Sensitive

In Sao Paulo State, Brazil, 'Candidatus Liberibacter americanus' and 'Candidatus Liberibacter asiaticus' are associated with huanglongbing (HLB). Affected municipalities occur mainly in the central and southern regions, where the annual number of hours above 30 degrees C is two to five times lower than that in the extreme northern and western regions. The influence of temperature on sweet orange trees infected with 'Ca. L. asiaticus' or 'Ca. L. americanus' was studied in temperature-controlled growth chambers. Symptom progression on new shoots of naturally infected and experimentally graft-inoculated symptomatic sweet orange trees was assessed. Mottled leaves developed on all infected trees at 22 to 24 degrees C, but not on any 'Ca. L. americanus'-infected trees at 27 to 32 degrees C. Quantitative, real time-PCR was used to determine the liberibacter titers in the trees. After 90 days, 'Ca. L. asiaticus'-infected trees had high titers at 32 and 35 degrees C, but not at 38 degrees C, while 'Ca. L. americanus'-infected trees had high titers at 24 degrees C, but at 32 degrees C the titers were very low or the liberibacters could not be detected. Thus, the multiplication of 'Ca. L. asiaticus' is not yet affected at 35 degrees C, while a temperature of 32 degrees C is detrimental to 'Ca. L. americanus'. Thus, 'Ca. L. americanus' is less heat tolerant than 'Ca. L. asiaticus'. The uneven distribution of these two liberibacters in Sao Paulo State might be in relation with these results.