Objective  ‘Candidatus Liberibacter asiaticus’ (CLas), the main species causing Citrus Huanglongbing (HLB), is also the sole pathogen causing agent of HLB in China. Exploring the temporal and spatial distribution of CLas in citrus fruit branch would help timely and accurate detection of CLas, and is of great significance to the prevention and control of HLB.

Method  The titers of CLas were measured in HLB-affected fruit branches of four traditional citrus cultivars namely Citrus reticulata Blanco cv. Shatangju, (C. reticulata × C. sinensis) cv. W-Murcott, C. maxima cv. Hongxinmi Yu and C. maxima cv. Shatian Yu using real time PCR method. The temporal and spatial distribution of CLas in citrus fruit branches were studied.

Result  CLas was unevenly distributed in fruit branches with enrichment in the fruits (Fruit pith, centrol axis or seed coat). For the mandarin of Shatangju and W-Murcott, CLas titers were the highest in the piths at the fruit mature stage. For the pomelos of Hongxinmi Yu and Shatian Yu, CLas titers were the highest in the fruit central axes at ripe stage. CLas transferred from the source organs (leaves) to the sink organs (fruits) along with the flow of nutrients in the processes of fruit swelling and ripening. CLas titers kept increasing in central axes and fruit pith, especially significantly increased in fruit pith.

Conclusion  CLas is enriched with the fruit developmental stages, and such transfer and distribution pattern of CLas in fruit branches can be explained by the theory of plant source-sink relationship.