ABSTRACT
Huanglongbing, or citrus greening, is a devastating disease of citrus plants recently spreading worldwide, which is caused by an uncultivable bacterial pathogen, “
Candidatus
Liberibacter asiaticus,” and vectored by a phloem-sucking insect,
Diaphorina citri
. We investigated the infection density dynamics of “
Ca
. Liberibacter asiaticus” in field populations of
D. citri
with experiments using field-collected insects to address how “
Ca
. Liberibacter asiaticus” infection density in the vector insect is relevant to pathogen transmission to citrus plants. Of 500 insects continuously collected from “
Ca
. Liberibacter asiaticus”-infected citrus trees with pathological symptoms in the spring and autumn of 2009, 497 (99.4%) were “
Ca
. Liberibacter asiaticus” positive. The infections were systemic across head-thorax and abdomen, ranging from 10
3
to 10
7
bacteria per insect. In spring, the infection densities were low in March, at ∼10
3
bacteria per insect, increasing up to 10
6
to 10
7
bacteria per insect in April and May, and decreasing to 10
5
to 10
6
bacteria per insect in late May, whereas the infection densities were constantly ∼10
6
to 10
7
bacteria per insect in autumn. Statistical analysis suggested that several factors, such as insect sex, host trees, and collection dates, may be correlated with “
Ca
. Liberibacter asiaticus” infection densities in field
D. citri
populations. Inoculation experiments with citrus seedlings using field-collected “
Ca
. Liberibacter asiaticus”-infected insects suggested that (i) “
Ca
. Liberibacter asiaticus”-transmitting insects tend to exhibit higher infection densities than do nontransmitting insects, (ii) a threshold level (∼10
6
bacteria per insect) of “
Ca
. Liberibacter asiaticus” density in
D. citri
is required for successful transmission to citrus plants, and (iii)
D. citri
attaining the threshold infection level transmits “
Ca
. Liberibacter asiaticus” to citrus plants in a stochastic manner. These findings provide valuable insights into understanding, predicting, and controlling this notorious citrus pathogen.